mimtifttHmmi biologM * > k / BK ®Ijc ®, ^. ^m pbm-g ^"ortlj Carolina ^tate CoUegs QH5IG niTTpTiMH]jpir.iiii^ 'ION. li.'uiikiiiiillkuiiJAiiuiiia NX. STATE UNIVERSITY D.H. "ILL LIBRARV S00202185 I 19127 This book may be kept out TWO WEEKS ONLY, and is subject to a fine of FIVE CENTS a day thereafter. It is due on the day indicated below: 27Mar'58l* 50M— 048— Form 3 BIOLOGY INTRODUCTION To THE STUDY OF BIOLOGY. BY H. ALLEYNE NICHOLSON, M. D., D. Sc, M.A., Ph.D., F.R.S. E., F. G. S., etc., PROFESSOR OF NATURAL HISTORY AND BOTANY IN UNIVERSITY COLLEGE, TORONTO, FOR.'MERLY LECTURER ON NATURAL HISTORY IN THE MEDICAL SCHOOL OF EDINBURGH, ETC., ETC.; AUTHOR OF "tEXT-EOOKOF GEOLOGY," "text-book OF ZOOLOGY," "manual OF ZOOLOGY." NEW YORK: D. APPLETON AND COMPANY, 549 & 551 BROADWAY. 1872. f^ PREFACE. The present work is based chiefly upon the Intro- duction to the author's * Manual of Zoology,' much of which is given here in an unaltered form. A con- siderable portion, however, of the Introduction has been here recast, whilst fully two-thirds of the work consists of new matter. Illustrations also have been introduced wherever such appeared to be necessary. Many important subjects have, of course, been ne- cessarily treated very superficially, or altogether omit- ted, as unsuitable for a merely elementary work. It is hoped, however, that most of those subjects are touched upon, a knowledge of which would be useful to the student of living or extinct forms of life, or to the general reader. Toronto, Canada, December \y 1871. 191 • CONTENTS. CHAPTER I. PAGE Definition of Biology — Differences between dead and living bodies — Nature and conditions of life — Physical basis of life — Pro- toplasm — Connection between life and the matter of life — Or- ganisation — Light — Air — Temperature — Death — Use of the term " vital force," 1-18 CHAPTER n. Differences between animals and plants — Regnum Protisticum of Plaeckel — Higher plants distinguished from higher animals — Comparison of the lower animals with the lower plants — Form — Internal structure — Chemical composition — Power of locomotion — Nature of the food, . . . . . ^9'^S CHAPTER III. Differences between different organisms — Morphology — Physiology — Specialisation of functions — Morphological type — Synopsis of the main divisions of the animal and vegetable kingdoms, 26-43 CHAPTER IV. Analogy — Homology — Serial Homology — Lateral Homology — Homogeny and Homoplasy — Homomorphism — Mimicry- Correlation of growth, ....... 44-5 Vlll CONTENTS. . CHAPTER V. Principles of classification— Definition of Species — Genus — Family — Order — Class — Sub-kingdom — Impossibility of a linear clas- sification, ......... 56-63 CHAPTER VI. Elementary chemistry of animals and plants — Chemistry of animals — Fats — Albumen — Fibrine — Caseine — Proteine of Mulder — Chemistry of vegetables — Starch — Cellulose — Sugar — Albuminous compounds of plants, .... 64-69 CHAPTER VII. Elementary structure of living bodies — Protoplasm or Bioplasm — Molecules — Cells — The cell-wall — The cell-contents — The nucleus — Cell-multiplication, ..... 70-76 CHAPTER VIII. Physiological functions of animals and plants — Animal and vege- table functions — Unicellular plants — Foraminifera — Vital force as manifested in the digestive process of plants, . . 77-83 CHAPTER IX. General phenomena of nutrition — Assimilation — Death — Growth — Development — Transfornr.ation — Metamorphosis — Law of Quatrefages — Provisional organs of young animals — Absence of sexual reproduction in larval forms — Von Baer's law of development — Retrograde development, . . . 84-96 CHAPTER X. Reproduction — Sexual reproduction — Non-sexual reproduction — General phenomena of gemmation and fission — Gemmation in the Foraminifera — Gemmation in the Sea-mat — Gemmation in Hydra — Fission in Paramoecium — Definition of the zoologi- ,cal individual — Zooids — Internal gemmation of Polyzoa — Al- ternation of generations— Reproduction of Hydractinia — Re- production of Clytia — Structure of free medusiform zooids — Reproduction of the Lucemarida — Parthenogenesis— Of Aph- ides—Of Bees — Law of Quatrefages — Antagonism between sexual reproduction and nutrition, .... 97-I18 CONTENTS. ix CHAPTER XI. Reproduction in plants — Gemmation and fission — Resemblances be- tween plants and Hydroid zoophytes — Reproduction of Angio- spermous flowering plants — Reproduction of ferns, . 1 19- 1 26 CHAPTER Xn. Spontaneous generation — Development of living beings in organic infusions — Experiments of Dr Bastian, .... 127-133 CHAPTER Xni. Origin of species — Doctrine of Special Creation — Doctrine of Evol- ution — Views of Lamarck — The Darwinian hypothesis — Theory of natural selection — Sexual selection — Leading objec- tions to the theory of the evolution of species by natural selection, ......... 134-143 CHAPTER XIV. Distribution in space — Geographical distribution — Zoological pro- vinces — Bathjmietrical distribution — Discoveries in the deep sea — Conditions of life in the deep-sea animals, . . 144-150 CHAPTER XV. Distribution in time — Laws of geological distribution— Chief divi- sions of the stratified series — Contemporaneity of strata— Geo- logical continuity— Imperfection of the palceontological re- cord, • • 15^"'^ LIST OF ILLUSTRATIONS. FIG. I. 2. 3- 4- 5- 7- 8. 9- lO. ir. 12. 13- 14. 15- 16. 17- 18. 19. 20. 21. 22. 23- 24. 25- 26. 27. 28. Nonionina and Gromia, .... Wheel-animalcule, ..... Ciliated spores of Plants, Volvox globaior, and Euplotes Charon Amceba, ...... Actinia mescmbryanthemuvi, and diagrammatic section of the same, ...... Diagrammatic section of a Whelk, Gregarine, Rhizopod, and Infusorian, Hydra vulgaris, and diagrammatic section of the same, Holothurian and lan-a, .... Diagram of Annulose animal, Section of a Cephalopod, .... Skeleton of the Common Perch, . Fore-limb of Man, Fore-leg of Dog, and Wing of Bird, Fairy Shrimp, . . . . . « Centipede, ...... Arm of Chimpanzee, .... Leg of Chimpanzee, .... Phyllium siccifoliuni, .... Yeast-plant, ...... Cells of notochord of Lamprey, . . Ovum of Ascaris nigove/tosa. Germinating cells of Yeast-plant, . Metamorphoses of Butterfly, Young of water-breathing Gasteropod and adult Pteropod Yonng oi A chtheres, znd 3.diVi\i LerncBa, . Diagram of Foraviinifera, F lustra hispid a, ..... Paramc^ciiim multiplying by fission, . . PAGB 13 15 21 29 30 31 35 36 37 33 40 41 45 46 46 47 47 53 72 73 75 76 90 95 96 99 100 lOI Xll LIST OF ILLUSTRATIONS. 29. Group of Hydractinia echinaia, with gonophores 30. Trophosome of Clytia Johnstoni, 31. Free Medusoid of Clytia yohnstoni, 32. Development of ^«r^//a, . 33. Generative zooid of Chrysaora, 34. Bean Aphis, .... 35. Male organs and pollen of Flowering Plants, 36. Female organs of Flowering Plants, 37. Fructifying Frond, Spore- cases, Spore, and Fern, ..... 38. Molecules and bacteria of organic infusions, 39. Ideal section of the Crust of the Earth . s. 105 • • 108 • • 109 • • no • • III « • 114 • • 121 • • 122 Prothallus of a • 125 • • 128 • • 153 \\ ELEMENTS OF BIOLOGY. CHAPTER I. DEFINITION OF BIOLOGY. All natural objects admit of an obvious separation into two primary groups, according as they are dead or alive — according as they exhibit no phenomena except such as can readily be referred to the working of known physical and chemical laws, or as they present, in addition, the phe- nomena which we are accustomed to group together under the name of " vital." The studies which occupy themselves with, dead bodies concern the physicist, the chemist, the geologist, and the mineralogist. The study of living be- ings, irrespective of the exact nature and position of these, is the province of Biology (Gr. bios, life ; logos, a discourse). All living beings, however, may be divided into the two kingdoms of animals and plants, the study of the former constituting the department of Zoology, whilst Botany is exclusively concerned with the latter. In accordance with this division, Biology splits up into the kindred sciences of Zoology and Botany, and properly includes both of these in all their details. Here, however, nothing more is aimed at than the presenting to the student, in a concise form some 2 ELEMENTS OF BIOLOGY. of the leading principles upon which the sciences of Zoology and Botany are based. With this view, technicalities will be as far as may be avoided; and on all matters which are still undecided the evidence on both sides will as far as possible be given, so that the reader may be enabled to form his own judgment as to the questions at issue. DIFFERENCES BETWEEN DEAD AND LIVING BODIES. In marking out the boundaries which limit the province of Biology, the first point is obviously to arrive at a clear conception as to the differences which separate all living bodies from those that are dead. The leading characters by which living bodies are distinguished from dead bodies may be summed up as follows : Firstly, Every living body possesses the power of taking into its interior certain ma- terials foreign to those composing its own substance, and of converting these into the materials of which its body is built up. This constitutes the process of "assimilation," and it is in virtue of this that living bodies grow. In all cases alike, the materials to be assimilated are taken into the interior of the body, and the process of growth is, therefore, one depending upon the "intussusception" of foreign matter in contradistinction to its mere addition from the outside. When, on the other hand, dead bodies increase in .size, as crystals do, the increase is produced simply by the addi- tion of fresh particles from the exterior, or, as it is techni- cally called, by the ''accretion" of matter. This process cannot properly be considered as one of " growth," as being wholly destitute of the essential element of a pre- vious " assimilation." Secondly, All the actions of living beings are accompanied by a corresponding destruction of the matter by which these actions are manifested. In other words, partial death is a constant accompaniment of life ; and the inces- sant loss of substance eaused by vital action has to be DEAD AND LIVING BODIES. 3 compensated for by the simultaneous assimilation of an equivalent amount of fresh matter. Thirdly, If our observation be continued for a sufficient length of time, every living body has the power of repro- ducing its like. That is to say, every living body has the power, directly or indirectly, of giving rise to minute germs, which, under proper conditions, will be developed into the likeness of the parent. Fourthly, Dead bodies are subject to the physical and chemical forces of the universe, and have no power of suspending these forces, or modifying their action, even for a limited period. On the other hand, living bodies, whilst subject to the same forces, are the seat of something in virtue of which they can override, suspend, or modify the actions of the physical and chemical forces by which dead bodies are exclusively governed. Dead matter is com- pletely passive, unable to originate motion, and equally un- able to arrest it when once originated. Living matter, so long as it is living, is the seat of energy, and can overcome the primary law of the inertia of matter. However humble it may be, and even if permanently rooted to one place, every living body possesses, in some part or other, or at some period of its existence, the power of independent and spontaneous movement — a power possessed by nothing that is dead. Similarly, the chemical forces, which work unresisted amongst the particles of dead matter, are in the living organism directed harmoniously to given ends, their action regulated under definite laws, and their natural work- ing often strikingly modified, or even temporarily suspended, and this as effectually and as perfectly in the humblest as in the highest of created beings. As a result of this, dead bodies exhibit nothing but re- actions, and these purely of a physical and chemical na- ture, whilst they show no tendency to pass through periodi- cal changes of state. On the other hand, living bodies exhibit distinct actions, and are pre-eminently characterised by their 4 ELEMENTS OF BIOLOGY. tendency to pass through a series of cyclical changes, which follow one another in a regular and determinate sequence. The above points are the leading characters by which living bodies are fundamentally separated from dead matter. There are, however, a few subordinate points in which some or all living bodies differ from those which are dead : — a. Chemical Composition. — Dead bodies are composed of numerous elements, which exist either in an uncombined condition, or in a state of union. The combinations of these elements may be said to be naturally in a state of stable equilibrium, and they show no tendency to spon- taneous decomposition. Further, the combining elements unite with one another in low combining proportions, and the resulting compounds for the most part consist of no more than two or three elements. Living bodies, on the other hand, are composed of few chemical elements, and these are almost always in a state of combination. Furthermore, the combinations are always complex, consisting of three or four elements, and these elements are united with one another in high combining proportions. Finally, the chemical compounds of living bodies are invariably characterised by the presence of water, and are prone to spontaneous decomposition. Thus, the great organic compound, albumen, is composed of 144 atoms of carbon, no of hydrogen, 18 of nitrogen, 42 of oxygen, and two atoms of sulphur. Iron, however, exists in the blood, possibly in its elemental condition, and cop- per has been detected in the liver of certain of the Mam- malia, and largely in the colouring-matter of the feathers of certain birds. It is to be remembered, also, that certain mineral salts, well known as occurring in dead nature, are apparently absolutely indispensable to living bodies, at any rate as a general rule. Living bodies, therefore, whilst certainly presenting us with a peculiar group of chemical compounds, are to a certain extent built up of substances which commonly occur dissociated from vitality. NATURE AND CONDITIONS OF LIFE. 5 b. Arrangeinent of parts. — Dead bodies, when unmixed, are composed of an aggregation of similar and homogeneous parts which bear no definite and fixed relations to one another. Living bodies, on the other hand, are in the great majority of cases composed of dissimilar and heterogeneous parts, the relations of which amongst themselves are more or less definite. In other words, most living bodies are " organ- ised," being composed of separate parts or " organs," which have certain definite functions in the general economy, li must, however, be borne in mind, that organisation, though in the vast proportion of cases a concomitant of vitality, is not necessarily present in living bodies. Some living beings (such as the minute organisms known as the Foraminifera) ex- hibit no distinct parts or organs, and cannot therefore be said to be "organised" in any proper sense of the term, whilst they, nevertheless, exhibit all the essential phenomena of vitality. c. Form. — Dead bodies are either of no definite shape — when they are said to be " amorphous "—or they are cr)'s- talline, in which case they are almost invariably bounded by straight lines and plane surfaces. Living bodies are almost always of a definite shape, presenting convex and concave surfaces, and being bounded by curved lines. Some living bodies, however, cannot be said to have any fixed form, but are extremely mutable in figure. In no case, however, could such be confounded with either the amorphous or the crystalline forms of dead matter. NATURE AND CONDITIONS OF LIFE. Life has been variously defined by different writers. Bichat defines life as " the sum total of the forces which resist death;" Treviranus, as "the constant uniformity of phenomena with diversity of external influences ; " Duges, as "the special activity of organised bodies;" and Beclard, as "organisation in action." All these definitions, how- ever, are more or less objectionable, either because they really express nothing, or because the assumption under- 6 ELEMENTS OF BIOLOGY, lies them that life is inseparably connected with organisa- tion. More recently attempts have been made to prove that life is merely a form of energy or motion, in which case no difficulty should be found in giving it an exact defini- tion. In the meanwhile, however, this view certainly can- not be said to have been satisfactorily proved, and it does not appear that any rigid definition of life is possible. We may therefore employ the name life as a collective term for the tendency exhibited by certain forms of matter, under certain conditions, to pass through a series of changes in a more or less definite and determinate sequence. As regards the conditions under which alone life or vital activity can be manifested, we have to consider two sets of conditions : the intrinsic or indispensable conditions, \vith- out which no vital phenomena are possible; and the extrin- sic conditions, which are generally present, but which do not appear to be actually essential to living beings. Under the first head, we have only to consider the presence of a " physical basis ; " under the second head, we may briefly look to the presence of organisation, light and air, and the necessity for a certain temperature. a. Protoplasm. — The first of the questions as to the con- ditions of life which it. is necessary to consider, is whether the phenomena of vitality are necessarily associated with any particular form of matter, or with any special " physical basis," as it has been aptly termed. The answer to this question may with little hesitation be given in the affirma- tive. It does not at all appear that the phenomena of life can be manifested by any and every fomi of matter ; and a very little reflection ought to convince us that it would be very surprising if the reverse of this were the case. There is no physical or chemical force which can be rendered manifest to us by all and sundry forms of matter, and it would be indeed remarkable if the case were otherwise with the forces of the living organism. When, for example, we say that certain forms of matter, such as the metals, are NATURE AND CONDITIONS OF LIFK. 7 conductors of electricity, and certain other forms, such as glass, are non-conductors, we are in truth saying that elec- tricity requires for its manifestation a certain " physical basis." Upon merely theoretical grounds, therefore, we might have assumed the existence of a "matter of life," or a physical basis absolutely necessary for the manifestation of vital phenomena. This physical basis of life is known by the now notorious name of "protoplasm," or, as it is better termed by Dr Beale, "bioplasm." As regards its nature, protoplasm, though capable of being built up into the most complex structures, does not necessarily exhibit anything which can be looked upon as organisation or differentiation into distinct parts ; and its chemical composition is the only constant which can be approximately stated. It consists, namely, of carbon, oxy- gen, nitrogen, and hydrogen, united into a proximate com- pound to which Mulder applied the name of "proteine," and which is very nearly identical with albumen or white-of- egg. It further appears probable that all forms of proto- plasm can be made to contract by electricity, and "are liable to undergo that peculiar coagulation at a temperature of 4o°-5o° centigrade, which has been called ' heat-stiffen- ing ' " (Huxley). Protoplasm, therefore, may be regarded as a general term for all forms of albuminoid matter ; and, in this general sense, we may safely assert that protoplasm is the " physical basis " of life ; or, in other words, that vital phenomena cannot be manifested except through the me- dium of a protoplasmic body. It is to be borne in mind, however, that it has not yet been shown that all the forms of matter which we include under the conveniently loose term of " protoplasm," have a constant and undeviating chemical composition. It must also be remembered that there are certain other substances, such as some of the mineral salts, which, though only present in small quantity, nevertheless appear to be absolutely essential to the maintenance of life, at the same time that their exact use is not at present known. 8 ELEMENTS OF BIOLOGY. It seems certain, then, that no body is capable of niani- festing the marvellous phenomena of life, unless it be com- posed of some form or other of albuminous or protoplasmic matter. We know, at any rate, of no such body at present, and we are therefore justified in asserting that the presence of an albuminous basis is an essential condition of vitality. Most naturalists probably would subscribe to this state- ment ; but there are two different senses in which it would be received. Some eminent authorities insist that albumin- ous matter or protoplasm is not only a condition of vitality, but that it is its cause ; or, in other words, that life is one of the properties of protoplasm. It is asserted, namely, that life is the result of the combined properties of the elements which form albuminous matter, just as the proper- ties of water are the resultant of the combined properties of its constituent hydrogen and oxygen ; and it is alleged that it is just as absurd to set down the phenomena of life to an assumed '' vital force," as it would be to ascribe the properties of water to an assumed " aquosity." On the other hand, equally eminent philosophers would assert that the view just mentioned is one which confounds effect with cause, and that albuminous matter is at best but a conditio7i of vitality, just as the presence of a conductor may be said to be an essential condition of electricity. The question as to which of these two opposing views has most in its favour is one of sufficient importance to warrant a brief exposition of the grounds upon which a decision may be arrived at. In the first place, when we come to sum up the actual data upon which such a decision should be formed, it is clear that we know two factors only of the case. We recog- nise certain phenomena which we call " vital," as being exclusively manifested by living beings. We recognise, further, that these phenomena are never manifested except by certain forms of matter, or, it may be, by but a single form of matter. We conclude, therefore, that there must be NATURE AND CONDITIONS OF LIFK. 9 an intimate connection between vital phenomena and the "matter of Hfe;" but we can go no further than this, and the premisses do not in any way warrant the assertion that life is the result of living matter, or one of its properties. We know the succession of phenomena, but we know no more, and it is not possible to decide dogmatically which phenomenon precedes the other in point of time. It is therefore just as reasonable to believe that the matter of life is the result of vital forces as the reverse ; and, as far as mere logic is concerned, neither view can claim the smallest advantage over the other. If we take such a microscopic animalcule as the Amoeba, or, still better, one of the yet more humble organisms which are known as Fora77iinifera, we are presented with a little speck of animal matter, a little particle of albumen, almost or quite destitute of structure, and yet exhibiting all the essential phenomena of vitality. Such a particle of living matter is undoubtedly the seat of certain forces which render it different from any and every collocation of mere dead particles. Whether we call these forces " vital " or not matters little ; but we certainly are not at present justified, by any evidence in our hands, in asserting that they are merely a form of energy or motion. No one has hitherto succeeded in demonstrating how any form or any combina- tion of any of the known physical or chemical forces should produce the vital phenomena which are seen to occur in the albuminous matter of even the most humble of animals. Until such a demonstration can be brought forward, we are not only justified, but we are bound, to look at the forces at work in living matter as something outside* and beyond the mere physical forces. We may call these forces "vital " or not, as we choose, but the fact will either way remain the same. Again, every one will willingly admit that all compound substances possess certain properties which are the result of the combined properties of their component elements. Water, for example, is composed of hydrogen and oxygen, lO KLLMENTS OF BIOLOGY. and its properties are the resultant of tlie combined proper- ties of these two gases. It is a definite chemical compound, having definite and constant properties, and there is no kind of necessity for ascribing the properties of water to any- assumed principle of "aquosity." It is to be remembered, however, that there is only one kind of water, and its pro- perties are universally the same. In the same way, albumi- nous matter, or protoplasm, is a chemical compound which unquestionably possesses certain properties as the result- ant of the combined properties of its component elements. But this is dead protoplasm of which this is true, and unless this be granted it is difficult to see how to avoid having to deny that dead protoplasm can exist at all. It is conceiv- able — nay, more, it is one of the splendid possibilities of the future — that the chemist should succeed in forcing the ele- ments of albuminous matter to combine with one another, and thus in manufacturing protoplasm artificially in the laboratory. But this would be dead albuminous matter; and it is wholly inconceivable that the utmost advances of con- structive chemistry should ever lead to the manufacture of living protoplasm. Dead albuminous matter may be re- garded as a tolerably definite and uniform chemical com- pound, and its properties are, beyond doubt, the resultant of those of its component elements. Like water, therefore, dead protoplasm has universally the same physical and chemical properties. Living protoplasm, on the other hand, though still unchanged in chemical composition and physi- cal characters, exhibits the most varied properties, accord- ing as its forms enter into the composition of different ani- mals. If, then, .we are to ascribe vital phenomena to the inherent constitution of living matter — in the sense that the properties of water are those of its component gases— we are left to account, as best we may, for the utterly immea- surable differences between the vital phenomena of a man and of a sponge, both of which may be regarded as com- posed fundamentally of the same materials. NATURE AND (OXDiTlONS OF^I.TPE. II The more philosophical view, tlicn, as to the nature of the connection between Hfe and its material basis, is the one which regards vitality as something superadded and foreign to the matter by which vital phenomena are manifested. Protoplasm is essential as the physical medium through which vital action may be manifested ; just as a conductor is essential to the manifestation of electric phenomena, or just as a paint-brush and colours are essential to the artist. Because metal conducts the electric current, and renders it perceptible to our senses, no one thinks of therefore assert- ing that electricity is one of the inherent properties of a metal, any more than one would feel inclined to assert that the power of painting was inherent in the camel's hair or in the dead pigments. Behind the material substratum, in all cases, is the active and living force ; and we have no right to assume that the force ceases to exist when its physical basis is removed, though it is no longer perceptible to our senses. It is, on the contrary, quite conceivable theoreti- cally that the vital forces of an organism should suffer no change by the destruction of the physical basis, just as elec- tricity would continue to subsist in a world composed uni- versally of non-conductors. In neither case could the force manifest its presence, or be brought into any perceptible relation with the outer world; but in neither case should we have the smallest ground for assuming that the power was necessarily non-extant. b. Organisation. — Having decided that the presence of a certain physical basis or peculiar form of matter is essential to the manifestation of vital phenomena, we may next pass on to consider whether organisation, or the presence of a certain de- finite structure, is one of the essential conditions of vitality. It is a very common thing to speak of animals as if they were so many machines, ap.d from one limited point of view the comparison is a fair anJ useful one. Eveiy machine, however simple, is composed of certain definite parts which have cer- tain definite relations to )ne another; and every machine, 12 ELEMENTS OF l)IOLO(}Y. therefore, has what in the case of an annual would be spoken of as "organisation." Each part or "organ" of the machine has certain definite functions, and the machine carries out its appointed work, when suppHed with the necessar}^ force, in virtue of the harmonious combination and interaction of its several parts. Most animals, in the same way, consist of definite parts or organs, with fixed relations to one ano- ther, and each discharging its own work or function in the general economy. So far the comparison is a good one, but it may be, and has been, carried too far. It is the very essence of a machine that it should consist of definite parts. It does not matter whether we are dealing with a toasting- fork or a steam-engine, we have in all cases a body com- posed of different parts performing different functions ; and no work can be got out of the machine unless by the invo- cation of a separate factor to supply the necessary force. It has been hastily assumed that the case is the same with animals, and the common simile has gone far to foster and diffuse this belief It has, in fact, been unhesitatingly laid down that life is inseparably connected with organisation; nay, more, it has even been asserted that life is the result of organisation. The falsity of this belief, however, is conclu- sively shown by the study of the minute creatures known as the Foraminifera (fig. i). These little animals possess the power of secreting a very beautiful and elaborate external envelope or shell, and they thus obtain a spurious kind of complexity which is very strikingly at variance with their real simplicity. In point of fact, the bodies of the Foram- inifera exhibit nothing which could truly be termed "organ- isation." They consist simply of formless and structureless albuminous matter. They are not composed of definite parts or organs, and they are in no proper sense to be com- pared to machines. Nevertheless, they live^ assimilate nourishment, grow, maintain their existence against hostile forces, have certain relations with the outer world, and reproduce their like. The highest animal, regarded merely NATURE AND CONDITIONS OF LIFE. 13 as an animal, can do no more than this ; and yet the Foram- inifera attain this end without possessing a single organ of Fig. I. — Foraminifera. a The animal o^ Noniontna, after the shell has been removed by a weak acid ; b Gromia (after Schultze), showing the shell surrounded by a network of filaments derived from the body-substance. any kind. These minute animalcules, therefore, show in an extremely beautiful and instructive manner, that organisa- tion is only a result of life, and not even a necessary result. In other words, we learn that an animal is organised, or possesses structure, because it is alive ; it docs not live because it is organised. c. Light. — In one sense light may be regarded as one of 14 ELEMENTS OF P.IOLOGY. the essential conditions of vitality ; but from another point of view it is wholly unnecessary. Light, namely, is neces- sary for animated nature as a whole, but is by no means essential to all living beings regarded as individuals. Many animals spend a great part of their existence in total dark- ness, and some pass their entire life without access to the rays of the sun. Regarded, however, from a deeper point of view, light is seen to be absolutely essential to life, since vegetable life can only be carried on under the influence of sun-force. All animals, as we shall subsequently see, are dependent, mediately or immediately, upon plants for their food ; since plants alone possess th» power of building up organic compounds out of inorganic materials. Plants, however, can perform this feat of vital chemistry only when supplied with the light-giving and chemical rays of the sun, so that light is an absolute prerequisite for life. The im- portance of light as one of the conditions of life, will, how- ever, be spoken of at greater length in treating of the food of animals and plants, and the distribution of animal life at great depths in the ocean. d. Air. — The presence of atmospheric air, or rather of free oxygen, appears to be essential to animal life, and a supply of oxygen may therefore be regarded as one of the extrinsic conditions of vitality. It would seem, however, that certain low vegetable organisms (vibriones and bacte- ria) flourish in an atmosphere of carbonic acid ; so that free oxygen cannot be looked upon as being an indispensable requisite of life. e. Temperature. — In a general way, the higher manifesta- tions of life are only possible between certain limited ranges of temperature, which may be stated as varying from near the freezing-point to 120'^ or 130° Fahrenheit. Some of the lower forms of life, however, can unquestionably endure temperatures much more extreme than these ; and it would appear that life in its lowest grades is not impossible at tem- peratures considerably below the freezing-point, and rising DEATH. 15 far above the boiling-point of water (from 20° up to 300'' F.) This subject, however, will be treated of at greater length in speaking of the alleged development of living beings de novo (Spontaneous Generation). f. Water. — Lastly, it may be remarked that no vital pro- cesses can be carried on except in the presence of water. This, however, truly depends upon the fact that water is an essential constituent of protoplasmic or albuminous matter in its living state. The necessity, therefore, for a "physical basis " of life, carries with it the necessary presence of water. Life, however, may remain in a dormant condition during long ^.^j periods, even in the total ab- sence of water. DEATH. The non-fulfilment of any of the above-named conditions for any length of time, as a rule, causes death-, or the cessation of vitality ; but, as just remarked, life may sometimes remain in a dormant or "potential" condi- tion for an apparently indefinite length of time. An excellent illustration of this is afforded by the eggs of some animals, and the seeds of many plants ; but a more striking example is to be found in the Rotifera or " Wheel-animalcules " (fig. 2). The Rotifers are minute, mostly microscopic creatures, which inhabit almost all our ponds and streams. Diminu- tive as they are, they are nevertheless, comparAtively speak- Fig. 2. — Rotifera. Epsf>hom nun /a, one of the Wheel animalcules. Kii- larged about 250 diameters. (After Gosse.) 1 6 ELEMENTS OF BIOLOGY. ing, of a very high grade of organisation. They possess a mouth, masticatory organs, a stomach, and aHmentar)'- canal, a distinct and well-developed nervous system, a differen- tiated reproductive apparatus, and even organs of vision. Repeated experiments, however, have shown the remarkable fact, that, with their aquatic habits and complex organisa- tion, the Rotifers are capable of submitting to an apparently indefinite deprivation of the necessary conditions of their existence, without thereby losing their vitality. They may be dried and reduced to dust, and may be kept in this state for a period of many years ; nevertheless, the addition of a little water will, at any time, restore them to their pristine vigour and activity. It follows, therefore, that an organism may be deprived of all power of manifesting any of the phenomena which constitute what we call life, without los- ing its hold upon the vital forces which belong to it. It seems, however, hardly necessary to add that this is a mere instance of revival and not of revitalisation. The desiccated Rotifers are not truly dead, but are merely in a state of sus- pended animation. % USE OF THE TERM VITAL FORCE. If, in conclusion, it be asked whether the term ''vital force " is any longer permissible in the mouth of a scientific man, the question must, I think, be answered in the affirma- tive. Formerly, no doubt, the progress of science was retarded and its growth checked by a too exclusive reference of natural phenomena to a so-called vital force. Equally unquestionable is the fact that the development of Biological science has progressed contemporaneously with the succes- sive victories gained by the physicists over the vitalists. Still, no physicist has hitherto succeeded in explaining any fundamental vital phenomenon upon purely physical and chemical principles. The simplest vital phenomenon has in it something over and above the merely chemical and physical forces which we can demonstrate in the laboratory. USE OF THE TERM VITAL FORCE. 1 7 It is easy, for example, to say that the action of the gastric juice is a chemical one, and doubtless the discovery of this fact was a great step in physiological science. Neverthe- less, in spite of the most searching investigations, it is cer- tain that digestion presents phenomena which are as yet inexplicable upon any chemical theory. This is excmphficd in its most striking form, when we look at a simple organism like the Amoeba. This animalcule, which is structurally little more than a mobile lump of jelly, digests as perfectly — as far as the result to itself is concerned — as does tlie most highly organised animal with the most complex diges- tive apparatus. It takes food into its interior, it digests it without the presence of a single organ for the purpose ; and still more, it possesses that inexplicable selective power by which it assimilates out of its food such constituents as it needs, whilst it rejects the remainder. In the present state of our knowledge, therefore, we must conclude that even in the process of digestion as exhibited in the Amoeba there is something* that is i]Ot merely physical or chemical. Simi- larly, any organism when just dead consists of the same protoplasm as before, in the same forms, and with the same arrangement ; but it has most unquestionably lost a some- thing by which all its properties and actions were modified, and some of them were produced. What that something is we do not know, and perhaps never shall know ; and it is possible, though highly improbable, that future discoveries may demonstrate that it is merely a subtle modification of some physical force. In the meanwhile, as all vital actions exhibit this mysterious something, it would appear unphilo- sophical to ignore its existence altogether, and the term " vital force " may therefore be retained with advantage. In using this term, however, it must not be forgotten that .we are simply employing a convenient expression for an unknown quantity, for that residual portion of every vital action which cannot at present be referred to the operation of any known physical force. 1 8 ELEMENTS OF BIOLOGY. It must, however, also be borne in mind that this residuum is probably not to be ascribed to our ignorance, but that it has a real existence. It appears, namely, in the highest degree probable that every vital action has in it something which is not merely physical and chemical, but which is conditioned by an unknown force, higher in its nature and distinct in kind as compared with all other forces. The presence of this '' vital force " may be recognised even in the simplest phenomena of nutrition ; and no attempt even has hitherto been made to explain the phenomena of reproduc- tion by the working of any known physical or chemical force. CHAPTER n. DIFFERENCES BETWEEN ANIMALS AND PLANTS. Having row arrived at some definite notion as to live essen- tial characters of living beings in general, we have next to consider what are the characteristics of the two great divi- sions of animated nature. What are the characters which induce us to place any given organism in either the animal or vegetable kingdom ? What, in short, are the differences between animals and plants ? It is generally admitted that all bodies which exhibit vital phenomena are capable of being referred to one of the two great kingdoms of organic nature. At the same time it is often extremely difficult in individual cases to come to any decision as to the kingdom to which a given organism should be referred, and in many cases the determination is purely arbitrary. So strongly, in fact, has this difficulty been felt, that some observers have established an intermediate king- dom, a sort of no-man's-land, for the reception of those debatable organisms which cannot be definitely and posi- tively classed either amongst vegetables or amongst animals. Thus, Dr Ernst Haeckel has proposed to form an intermediate kingdom, which he calls the Regniim Protisticum^ for the reception of all doubtful organisms. Even such a cautious observer as Dr Rolleston, whilst questioning the propriety of this step, is forced to conclude that " there are organisms 20 ELEMENTS OF BIOLOGY. which at one period of their life exhibit an aggregate of phenomena such as to justify us in speaking of them as animals, whilst at another they appear to be as distinctly vegetable." In the case of the higher animals and plants there is no difficulty; the former being at once distinguished by the possession of a nervous system, of motor power which can be voluntarily exercised, and of an interna] cavity fitted for the reception and digestion of solid food. The higher plants, on the other hand, possess no nervous system or organs of sense, are incapable of independent locomotion, and are not provided with* an internal digestive cavity, their food being wholly fluid or gaseous. These distinctions, however, do not hold good as regards the lower and less highly organised members of the two kingdoms, many ani- mals having no nervous system or internal digestive cavity, whilst many plants possess the power of locomotion; so that we are compelled to institute a closer comparison in the case of these lower forms of life. a. Form. — As regards external configuration, of all char- acters the most obvious, it must be admitted that no abso- lute distinction can be laid down between plants and ani- mals. Many of our ordinary zoophytes, such as the Hydroid Polypes, the sea-shrubs and corals — as, indeed, the name zoo- phyte implies — are so similar in external appearance to plants that they were long described as such. Amongst the Mol- luscoida, the common sea-mat (Flustra) is invariably re- garded by sea-side visitors as a sea-weed. Many of the Protozoa are equally like some of the lower plants (Proto- phyta); and even at the present day there are not wanting those who look upon the sponges as belonging to the vege- table kingdom. On the other hand, the embryonic forms, or "zoospores," of certain undoubted plants (such as the Protococcus nivalis, Vaucheria, &c.), are provided with ciliated processes with which they swim about, thus coming so closely to resemble some of the Infusorian animalcules as DIFFERENCES BETWEEN ANIMALS AND I'l.ANTS. 21 to have been referred to that division of the Protozoa. This is also the case with some adult plants, such as Volvox globator (fig. 3). <^^1\\V Fig. 3. — Algae and Infusoria, a Ciliated zoospores of C<';//<^>ty/' ; h Ciliated zoospore of Vauclieria; c Volvox globator, nir^giiified ; d Euplotes Charon, one of the Infiisoria, magnified. b. Inte7'nal Structure. — Here, again, no line of demarcation can be drawn between the animal and vegetable kingdoms. In this respect all plants and animals are fundamentally similar, being alike composed of molecular, cellular, and fibrous tissues. c. Chei7iical Composition. — Plants, speaking generally, ex- hibit a preponderance of ternary compounds of carbon, hydrogen, and oxygen — such as starch, cellulose, and sugar — whilst nitrogenised compounds enter more largely into the composition of animal* Still bbth kingdoms contain identical or representative compounds, though there may be a difference in the proportion of these to one another. Moreover, the most characteristic of all vegetable com- pounds—viz., cellulose — has been detected in the outer covering of the sea-squirts or Ascidian Molluscs ; and the so-called "glycogen," which is secreted by the liver of the Mammalia, is closely allied to, if not absolutely identical with, the hydrated starch of plants. As a general rule, how- 22 ELEMENTS OF BIOLOGY. ^ ever, it may be stated that the presence in any organism of an external envelope of cellulose raises a strong presumption of its vegetable nature. In the face, however, of the facts above stated, the presence of cellulose cannot be looked upon as absolutely conclusive. Another highly characteristic vege- table compound is chlorophyll^ the green colouring-matter of plants. Any organism which exhibits chlorophyll in any quantity, as a proper element of its tissues, is most pro- bably vegetable. As in the case of cellulose, however, the presence of chlorophyll cannot be looked upon as a certain test, since it occurs normally in certain undoubted ani- mals {e.g.^ Stentor, amongst the Infusoria^ and the Hydra viridis^ or the green Fresh-water Polype, amongst the Caleft- terata), d. Motor Power. — This, though broadly distinctive of ani- mals, can by no means be said to be characteristic of them. Thus, many animals in thoir mature condition are per- manently fixed, or attached to some foreign object; and the embryos of many plants, together with not a few adult forms, are endowed with locomotive power by means of those vibratile, hair-like processes which are called " cilia," and which are so characteristic of many of the lower forms of animal life. Not only is this the case, but large num- bers of the lower plants, such as the Diatoms and Desmids, exhibit throughout life an amount and kind of locomotive power which does not admit of being rigidly separated from the movements executed by animals, though the closest re- searches have hitherto failed to«show the mechanism where- by these movements are brought about. e. Nature of the Food. — ^AVhilst all the preceding points have failed to yield a means of invariably separating ani- mals from plants, a distinction which holds good almost without exception is to be found in the nature of the food taken respectively by each, and in the results of the conver- sion of the same. The unsatisfactory feature, however, in this distinction is this, that even if it could be shown to be, DIFFERENCES BETWEEN ANIMALS AND PLANTS. 23 theoretically, invariably true, it would nevertheless be prac- tically impossible to apply it to the greater number of those minute organisms concerning which alone there can be any dispute. As a broad rule, all plants are endowed with the power of converting inorganic into organic matter. The food of plants consists of the inorganic compounds, carbonic acid, am- monia, and water, along with small quantities of certain mineral salts. From these, and from these only, plants are capable of elaborating the proteinaceous matter or protoplasm which constitutes the physical basis of life. Plants, there- fore, take as food very simple bodies, and manufacture them into much more complex substances. In other words, by a process of deoxidation or unbuming, rendered possible by the influence of sunlight only, plants convert the inorganic or stable elements — ammonia, carbonic acid, water, and certain mineral salts — into the organic or unstable elements of food. The whole problem of nutrition may be narrowed to the question as to the modes and laws by which these stable elements are raised by the vital chemistry of the plant to the height of unstable compounds. To this gene- ral statement, however, an exception must seemingly be made in favour of certain fungi, which require organised compounds for their nourishment On the other hand, no known animal possesses the power of converting inorganic compounds into organic matter, but all, mediately or immediately, are dependent in this respect upon plants. All animals, as far as is certainly known, require ready-made proteinaceous matter for the mainten- ance of existence, and this they can only obtain in the first instance from plants. Animals, in fact, differ from plants in requiring as food complex organic bodies which they ultimately reduce to very much simpler inorganic bodies. The nutrition of animals is a process of oxidation or burn- ing, and consists essentially in the conversion of the energy of the food into vital work ; this conversion being effected 24 ELEMENTS OF BIOLOGY. by the passage of the food into living tissue. Plants, there- fore, are the great manufacturers in nature, animals are the great consumers. Just, however, as this law does not invariably hold good for plants, certain fungi being in this respect animals, so it is not impossible that a limited exception to the universality of the law will be found in the case of animals also. Thus, in some recent investigations into the fauna of the sea at great depths, a singular organism, of an extremely low type, but occupying large areas of the sea-bottom, has been dis- covered, to which Professor Huxley has given the name of Bathybius. As vegetable life is extremely scanty, or is altogether, wanting, in these abysses of the ocean, it has been conjectured that this organism is possibly endowed with the power — otherwise exclusively found in plants — of elaborating organic compounds out of inorganic materials, and in this way supplying food for the higher animals which surround it. The water of the ocean, however, at these enormous depths, is richly charged with organic matter in solution, and this conjecture is thereby rendered doubtful. Be this as it may, there remain to be noticed two distinc- tions, broadly though not universally applicable, which are due to the nature of the food required respectively by animals and plants. In the first place, the food of all plants consists partly of gaseous matter and partly of matter held in solution. They require, therefore, no special aper- ture for its admission, and no internal cavity for its recep- tion. The food of almost all animals consists of solid particles, and they are therefore usually provided with a mouth and a distinct digestive cavity. Some animals, however, such as the tape-w^orm and the Gregarinae, live entirely by the imbibition of organic fluids through the general surface of the body, and many have neither a distinct mouth nor stomach. Secondly, plants decompose carbonic acid, retaining the carbon and setting free the oxygen, certain fungi forming DIFFERENCES BETWEEN ANIMALS AND PLANTS. 2$ an exception to this law. The reaction of plants upon the atmosphere is therefore characterised by the production of free oxygen. Animals, on the other hand, absorb oxygen and emit carbonic acid, so that their reaction upon the atmosphere is the reverse of that of plants, and is charac- terised by the production of carbonic acid. Finally, it is worthy of notice that it is in their lower and not in their higher developments that the two kingdoms of organic nature approach one another. No difficulty is ex- perienced in separating the higher animals from the higher plants, and for these universal laws can be laid down to which there is no exception. It might, not unnaturally, have been thought that the lowest classes of animals would exhibit most a^nity to the highest plants, and that thus a gradual passage between the two kingdoms would be estab- lished. This is not the case, however. The lower animals are not allied to the higher plants, but to the lower ; and it is in the very lowest members of the vegetable kingdom, or in the embryonic and immature forms of plants little higher in the scale, that we find such a decided animal gift as the power of independent locomotion. It is also in the less highly organised and less specialised forms of plants that we find the only departures from the great laws of vegetable life, the deviation being in the direction of the laws of animal life. CHAPTER III. DIFFERENCES BETWEEN DIFFERENT ORGANISMS. Morphology and Physiology. — The next point which demands notice relates to the nature of the differences by which one organism may be separated from every other, and the question is one of the highest importance. Every Hving being, whether animal or vegetable, may be regarded from two totally distinct, and, indeed, often apparently opposite, points of view. From the first point of view we have to look solely to the laws, form, and arrangement of the struc- tures of the organism ; in short, to its external form and internal structure, wholly irrespective of the manner in which it discharges its vital work. This constitutes the science of Morphology (Gr. morJ)he, form ; togos^ a discourse). From the second point of view we have to study the vital actions performed by living beings, and the functions dis- charged by the different parts of the organism, separately or collectively. This constitutes the science of Physiology. Morphology not only treats of the structure of* living beings in their fully-developed condition (Anatomy), but is also concerned with the changes through which every living being has to pass in reaching its mature or adult condition (Embryology or Development). The term " Histology," again, is further employed to designate that branch of Mor- phology which is specially occupied with the investigation DIFFERENCES BETWEEN DIFFERENT ORGANISMS. 27 of minute or microscopical tissues (Gr. histos^ a web ; logos a discourse). Physiology treats of all the functions exercised by living , bodies, or by the various definite parts or organs of which most living beings are composed. All these various func- tions, however, may be considered under three heads : — I. Functions of Nutrition, divisible into functions of Absorp- tion and Metamorphosis, and comprising all those functions by which an organism is enabled to live, grow, and main- tain its existence as an individual. 2. Functions of Repro- duction, comprising all those functions whereby fresh indi- viduals are produced and the perpetuation of the species is secured. 3. Functiofis of Relation or Correlation, compris- ing all those functions (such as sensation and voluntary motion) whereby the outer world is brought into relation with the organism, and the organism in turn is enabled to act upon the outer world. Of these three, the functions of nutrition and reproduc- tion are often spoken of collectively as the " organic " or " vegetative " functions, as being essential to bare existence, and as being common to animals and plants alike. The functions of relation, again, are often spoken of as the " animal " functions as being most highly developed in animals. These functions, however, though more highly characteristic of, animals, are not peculiar to them, but are manifested to a greater or less extent by various plants. All the innumerable differences which subsist between different organisms may be classed under two heads — morphological and physiological — corresponding with the two aspects of every living being. One organism differs from another either morphologically, in the fundamental points of its structure and the plan upon which it is built, ox physiologically, in performing a different amount of vital work, in a different manner, or with different instruments, or both morphologically and physiologically. These con- stitute the only modes in which any one organism can 28 ELEMENTS OF BIOLOGY. differ fruxi. i\ny another ; and they may be considered re- spectively under the heads of " Specialisation of Functions" and " Morphological Type." a. Specialisation of Fimdioiis. — All animals alike, what- ever their structure may be, perform the three great phy- siological functions ; that is to say, they all nourish them- selves, reproduce their like, and have certain relations with the external world. They differ from one another physio- logically in the manner in which these functions are per- formed. Indeed it is only in the functions of correlation that it is possible that there should be any difference in the amount or perfection of the function performed by the organism, since nutrition and reproduction, as far as their results are concerned, are essentially the same in all ani- mals. In the manner, however, in which the same results are brought about, great differences are observable in dif- ferent animals. The nutrition of such a simple organism as the Amoeba is, indeed, performed perfectly, as far as the result to the animal itself is concerned — as perfectly as in the case of the highest animal — but it is performed with the simplest possible apparatus. It may, in fact, be said to be performed without any special apparatus, since any part of the surface of the body may be extemporised into a mouth, and there is no differentiated alimentary cavity. And not only is the nutritive apparatus of the simplest character, but the function itself is equally simple, and is entirely divested of those complexities and separations into second- ary functions Avhich characterise the process in the higher animals. It is the same, too, with the functions of repro- duction and correlation ; but this point will be more clearly brought out if we examine the method in which one of the three primary functions is performed in two or three examples. Nutrition, as the simplest of the functions, will best answer the purpose. In the simpler Protozoa^ such as the Proteus animalcule or Aniceha (fig. 4), it can hardly be said that there are any DIFFERENCES BETWEEN DIFFERENT ORGANISMS. 29 nutritive organs at all, at any rate of a permanent nature. The prehension of food is effected entirely by the inter- Fig- 4- — A, Amoebze developed in organic infusions, vtry greatly magnified (after Beale) ; B, Amcria princeps (after Carter). vention of temporary fingers or processes of the body-sub- stance, which can be thrust out at will from any point of the surface of the body, and which, when retracted, melt into the protoplasmic body without leaving a trace behind. There is no mouth, and any particle of food seized by one of these temporary arms is simply engulfed in the soft body, as one might thrust a stone into a lump of dough. There is no digestive cavity, and there are no digestive organs of any kind. Nevertheless the Amceba possesses to the full the power of assimilating the materials which it takes as food, of making out of these the substances which it needs for its growth and nourishment, and of rejecting all that may be useless. The fluid which is manufactured out of the food, and which may, in a general sense, be said to correspond to the blood of the higher animals, is probably propelled to all parts of the body by means of a little con- tractile bladder, which dilates and closes at regular intervals. If this interpretation of the facts be correct, the Amceba is furnished with what may be regarded as a very rudimentary 30 ET.EMENTS OF BIOLOGY. form of heart ; but it is not quite dear that the function of this Httle hollow sphere is as above stated. Organs by which the injurious products of the death of the tissues may- be eliminated, are absolutely wanting ; and respiration, if it can be said to exist at all as a distinct function, is simply effected by the general surface of the body, and not by any distinct breathing organ. It follows from this that not only is the entire process of nutrition in the Amoeba of the very simplest character, but that the process is carried out with the utmost possible absence of complication, and with the very simplest machinery. In a Ccelenterate animal, such as one of the sea-anem- ones (fig. 5), the function of nutrition has not increased much in complexity, but the means for its performance are somewhat more specialised. A distinct and permanent Fig. 5. — A, Actift'a mesernbryanthemum, one of the Sea-anemones Rafter JoVinston); B, Section of the same, showing the mouth («), the stomach (b), and the body- cavity (<:). mouth is now present, and this is surrounded by a number of prehensile processes or *' tentacles," which are of a perma- nent nature, and are not produced for the occasion as in the case of the temporary arms of the Ainceba. The mouth DIFFERENCES BETWEEN DIFFERENT ORGANISMS. 3 1 opens into a permanent digestive cavity or stomach ; but this, in turn, opens directly into the body-cavity or general chamber enclosed by the walls of the body (fig. 5, 13). As a result of this, the nutritive fluid prepared from the food, which we may call the blood, gains direct access to the body-cavity, where it is largely diluted with the sea-water, which is also freely admitted to this cavity. The nutritive fluid, thus weakened, is kept in constant circulation by means of innumerable little vibrating hair-like processes or " cilia," with which the lining membrane of the body-cavity is furnished ; and this constitutes the only representative of the circulatory apparatus of the higher animals. As in the Amoeba, there are no distinct respiratory organs, and no special apparatus by which effete matters may be got rid of. Pig. 6 — Diagrammatic section of a Whelk, a Month, with masticatory apparatus ; Z' Salivary glands ; f Stomach ; o'rt' Intestine, siirrnunded by the liver, and ter- minating in the anus (?); g Gill ; h Heart ; /Nervous ganglion. In a Mollusc, again, such as the Whelk (fig. 6), nutrition 32 ELEMENTS OF BIOLOGY. is a much more complicated process. There is now a dis- tinct mouth (a) provided with a masticatory apparatus, and opening into a gullet which is furnished with salivary glands {b). The gullet conducts to the stomach, which, in turn, opens into a long and convoluted intestine {dd), w^hich is completely shut off from the general cavity of the body, and which terminates in a permanent aperture {e), by which the indigestible portions of the food are got rid of. A well- developed liver is also present. The nutritive products of digestion are now propelled through all parts of the organ- ism by a permanent contractile organ or heart {h). Lastly, the function of respiration is carried on by distinct and com- plex organs or gills (^), whereby the blood is submitted to the action of the oxygen contained in the surrounding water. It is not necessary here to follow out this comparison further. In still higher animals the function of nutrition becomes still further broken up into secondary functions, for the due performance of which special organs are pro- vided, the complexity of the organism thus necessarily increasing J>ari passu with the complexity of the function. This gradual subdivision and elaboration is carried out equally with the other two physiological functions — viz., reproduction and correlation — and it constitutes what is technically called the " specialisation of functions," though it has been more happily termed by Milne-Edwards "the principle of the physiological division of labour." As has, however, been already remarked, in any physiological com- parison of organisms one with another, it is at once seen that the functions of relation stand in quite a different posi- tion to that occupied by the functions of nutrition and re- production. As far as these last are concerned, there can be no difference in the amount or perfection of the function discharged by the organism. The simplest and most degraded of animals — say a sponge — nourishes itself as perfectly, as far as the result to itself is concerned, as does DIFFERENCES BETWEEN DIFFERENT ORGANISMS. ^;^ the higliest of animals. Nutrition can do no more tlum maintain the body of any anmial in a hcaUhy and vigorous condition. This is the highest possible perfection of tlie function, and it is attained as fully and perfectly by the sponge as it is by man himself. The same holds good of reproduction. Whilst the functions of nutrition and repro- duction are thus, as regards their essence and results, the same in all animals, it must be remembered that there are enormous diff(frences in the majincr in which the functions are discharged. The result attained is in all cases the same, but it may be arrived at in the most different ways and with the most different apparatus. As regards the functions of relation, on the other hand, we have'every pos- sible grade of perfection exhibited as we ascend from the lowest members of the animal kingdom to the highest. So numerous, in fact, are the changes in these functions, and so great the additions which are made in the higher organ- isms, that it may be doubted if there exists any common element by which a comparison can be drawn on this head between the higher and lower animals. It may reasonably be doubted whether in this respect a horse or a dog has anything in common with a sponge. b. Morphological Type. — The first point in which one ani- mal may differ from another is the- degree to which the principle of the physiological division of labour is carried. The second point in which one animal may differ from another is in its "morphological type;" that is to say, in the fundamental plan upon which it is constructed. By one not specially acquainted with the subject it might be readily imagined that each species or kind of animal was con- structed upon a plan peculiar to itself and not shared by any other. This, however, is far from being the case ; and it is now universally recognised that all the varied species of animals — however great the apparent amount of diversity amongst them — may be arranged under no more than half- a-dozen primary morphological types or plans of structure. 34 ELEMENTS OF BIOLOGY. Upon one or other of these five or six plans every known animal, whether living or extinct, is constructed. It follows from the limited number of primitive types or patterns, that great numbers of animals must agree with one another in their morphological type. It follows also that all so agree- ing can differ from one another only in the sole remaining element of the question — namely, by the amount of speciali- sation of functions which they exhibit. Every animal, there- fore, as Professor Huxley has well expressed it, is the resul- tant of two tendencies, the one morphological, the other physiological. The six types or plans of structure, upon one or other of which all known animals have been constructed, are techni- cally called " sub-kingdoms," and are known by the names Protozoa, Coelenterata, Annuloida, Annulosa, Mollusca, and Vertebrata. We have, then, to remember that every mem- ber of each of these primary divisions of the animal kingdom agrees with every other member of the same division in being formed upon a certain definite plan or type of struc- ture, and differs from every other simply in the grade of its organisation ; or, in other words, in the degree to which it exhibits specialisation of functions. It is to be remembered, also, that whilst all naturalists recognise distinct plans of structure or "morphological types" in both the animal and vegetable kingdoms, all are not agreed as to the number of these types. In other words, all zoologists are not yet agreed as to the characters which should be regarded as constituting a distinct " mor- phological type." The result of this is that different authori- ties divide the animal kingdom into a different number of "sub-kingdoms.^' Most modern naturahsts, however, are agreed as to the morphological distinctness of the Protozoa, Caknierata, Annulosa, Mollusca, and Vo'tehraia. There thus remains the sub-kingdom oi Xht Amwloida alone, about which any serious divergence of opinion is entertained. Following Huxley, this division is here regarded as having SUB-KINGDOM I. — rKOTOZOA. 35 the rank of a "sub-kingdom." It should not be forgotten, however, that this is a provisional arrangement, and that future researches may demonstrate the propriety of a redis- tribution of the somewhat heterogeneous group of organisms at present included under this head. Subjoined is a brief synoptical view of the primary divisions of the animal and vegetable kingdoms, with the characters of the leading groups comprised in each. ANIMAL KINGDOM. , Sub-Kingdom I. — Protozoa. Animal, simple or compound, usually very minute. Body composed of the contractile, structureless, albuminoid substance termed " sar- code ; " showing no composition out of definite segments ; having no nervous system, no regular circulatory system, no definite body-cavity, and either no digestive apparatus, or at mo^ a mouth and short gullet. Reproduction sexual and non-sexual (fig. 7). Fig. 7. — Protozoa. A Gregarlne ; B Rhizopod ; C Infusorian. Class A. Gregarinida. — Protozoa which live parasitically in the interior of insects and other animals, which are destitute of a mouth, and have no power of throwing out prolongations or processes of the body- substance ("pseudopodia"). Class B. Rhizopoda (Root-footed Protozoa). — Protozoa which are 36 elp:mei\ts of biology. simple or compound, and have the power of throwing out and retracting temporary prolongations of the body-substance (" pseudopodia "). A mouth generally, if not universally, absent. Ex. Sponges. Class C. Infusoria (Infusorian Animalcules). — Protozoa mostly with a mouth and short gullet ; destitute of the power of emitting pseudopodia ; furnished with vibrating hair-like processes (cilia) or con- tractile filaments ; the body composed of three distinct layers. Ex. — Bell-animalcule. Sub-Kingdom II. — Ccelenterata. Animals whose alimentary canal communicates freely with the general space included within the walls of the body, so that the "body-cavity" comes to communicate with the outer medium through the mouth. Body composed of two fundamental layers or membranes, an outer layer or " ectoderm," and an inner layer or " endoderm." No central organ of the circulation or distinct blood-system ; in most no nervous system. Skin furnished with microscopic stinging organs or "thread- cells." Reproductive organs in all, but multiplication often by non- sexual methods (figs. 5 and 8). A B Fig 8. — Ccelenterata. A Hydra Tule^nris. the common fresh-water Polj'pe (after Hincks). R Diagrammatic section of a Hydra. Class A. Hydrozoa. — Walls of the digestive sac not separated from SUB-KINGDOM III. -^ANNULOIDA. 37 those of the general body-cavity, the two coinciding with one another. Reproductive organs external. Ex., Fresh-water Polypes, Sca-firs, Portuguese Man-of-War, Jelly-fishes, Sea-blubbers. Class B. Actinozoa — Stomach opening below into the body-cavity, which is divided into a number of compartments by vertical partitions or "mesenteries." Reproductive organs internal, Ex., Sea-anemones, Star -corals, Brain-corals, Sea-pens, Sea -shrubs. Red-coral, Venus's Girdle. Sub-Kingdom III.— Annuloida. • Animals in which the alimentary canal (when present) is completely shut off from the general cavity of the body, and in which there is a peculiar system of canals, distributed through the body, usually communicating with the exterior, and termed the " water-vascular " system. A distinct nervous system, and sometimes a true blood-vascu- lar system. The body of the adult never composed of a succession of definite rings or segments, nor provided with successive pairs of append- ages disposed symmetrically on the two sides of the body. Reproduc- tion rarely asexual. pjg. 9.— Annuloida. n Holothuria tidudosn, one of the Sea-cucumbers ; b and c Young stages of the same (after Jones). Class A. Echinodermata. — Integument composed of numerous calcareous plates jointed together, or leathery, and having grains, spines, or tubercles of calcareous matter deposited in it. Water-vascular system 3 38 ELEMENTS OF BIOLOGY. generally communicating with the exterior, and often employed in loco- motion. Nervous system radiate. Adult generally more or less star- like or ** radiate" in shape, young usually showing more or less distinct " bilateral symmetry " — that is, showing similar parts on the two sides of the body. Ex. Sea-urchins, Star-fishes, Brittle-stars, Sea-lilies, Sea-cucumbers. Class B. Scolecida. — Integument soft, and destitute of calcareous matter. Water-vascular system not assisting in locomotion. Nervous system consisting of one or two ganglia, not disposed in a radiating manner. Body of the adult sometimes flattened, sometimes rounded and wormlike. Ex. Tapeworms, Flukes, Haii-Avorms, Roundworlns, Wheel-.inimalcules. Sub-Kingdom IV. — Annulosa. Animal composed of numerous definite segments or " somites," ar- ranged longitudinally one behind the other. Nervous system consisting in its typical form of a double chain of ganglia, which are placed along the ventral surface of the body, are united by longitudinal cords, and form a collar round the gullet, a pair of ganglia being primitively de- veloped in each segment. Limbs (when present) disposed in pairs, and turned towards that side of the body on which the main masses of the nervous system are situated (fig. lo). / Fig. lo. — Annulosa. A, Diagram of Annulose animal : a Digestive tube, h Heart, c Nerve-chain. B Diagram of the nervous system of one of the Annulosa. Division I. Anarthropoda. — Locomotive appctidages {when present) not distinctly jointed or articulated to the body. Class A. Gephyrea. — Body cylindrical, not definitely segmented. IMouth usually with a circlet of tentacles. Ventral cord of the nervous system not furnished with ganglia. Ex. Spoonworms. Class B. Annelida. — Body cylindrical, definitely segmented. ,A special system of vessels connected with respiration ("pseudohsemal" • SUB-KINGDOM V. — MOLLUSCA. 39 vessels). A gangliatcd ventral nerve-chain. Ex. Leeches, Earth- worms, Tubeworms, Sanchvorms. Class C. Ch^etogxatha. — Head furnished witli rows of bristles. Nervous system consisting of a cephalic and a ventral ganglion united by cords which form a collar round the gullet. Ex. Sagitta. Division II. AKTiiKOVODk.—LocomoiivcaJfJ'i'tidcigcsjoifitcd or arti- cidated to the body. Class D. Crustacea. — Respiration aquatic, by the general surAice of the body or by gills. Two pairs of antennas. Locomotive appen- dages more than four pairs in number, carried upon the thorax, and mostly the abdomen also. Ex. Crabs, Lobsters, King-crabs, Wood- lice. Class E. Arachnida. — Respiration aerial, by the surface of the body, by pulmonary chambers, or by air-tubes (" trachece "). Antennae converted into jaws. Head and thorax amalgamated. Four pairs of legs. Abdomen destitute of limbs. Ex. Spiders, Scorpions, Mites, Ticks. Class F. Myriapoda. — Respiration aerial, by air-tubes (tracheae) or by the skin. Head distinct ; remainder of the body composed of nearly similar segments. Legs more than eight pairs in number, and borne partly by the al)domen. One pair of antennce. Ex. Centipedes and Millipedes. Class G. Insecta. — Respiration aerial, by air-tubes (tracheae). Head, thorax, and abdomen distinct. One pair of antennae. Three pairs of legs borne on the thorax. No locomotive limbs on the segments of the abdomen. Ex. Beetles, Flies, Butterflies. Sub-Kingdom V. — Mollusca. Animal soft-bodied, usually with, a hard covering or shell. Not exhibiting distinct segmentation. Nervous system consisting of a single ganghon or of scattered pairs of ganglia. A distinct heart and breath- ing organ, or neither (fig. 11). Division I. Molluscoida. — Nervous system consisting of a sins^le ganglion or a principal pair of ganglia. N'o hearty or an iniperfxt one. Class A. Polyzoa. — Animal always forming compound growths or colonies. No heart. The mouth of each member of the colony sur- rounded by a circle or crescent of ciliated tentacles. Ex. Sea-mat. Class B. Tunicata. — Animal simple or compound, enclosed in a leathery or gristly case. An imperfect heart. jE";!:. Sea-squirt. Class C. Brachiopoda. — Animal simple, enclosed in a bivalve shell. IMouth furnished with two long fringed processes or "arms." Ex. Lamp-shells. 40 ELEMENTS OF BIOLOGY. Division II. Mollusca Proper. — Nervous system consisting of three j)rincipal pairs of ganglia. Ueart well developed, of at least two chambers. Fig. II. — Mollusca. Diagram of a Cuttle-fish. (Altered from Huxley.) Class D. Lamellibranchiata. — No distinct head or teeth. Body- enclosed in a bivalve shell. One or two leaf-like gills on each side of the body. £x. Oyster, Mussel, Cockle. Class E. Gasteropoda. — A distinct head and toothed tongue. Shell, when present, univalve or multivalve, never bivalve. Locomo- tion effected by creeping about on the flattened under-surface of the body (" foot"), or by swimming by means of a fin-like modification of the same. Ex. Whelk, Periwinkle, Snail. Class F. Pteropoda. — Animal oceanic, swimming bv means of two wing-like appendages, one on each side of the head. Size minute. £x. Cleodora. Class G. Cephalopoda. — Animal with eight or more processes or " arms" placed round the mouth. Mouth armed with jaws and a SUB-KINGDOM VI. — VERTEBRATA. 41 toothed tongue. Two or four plume-like gills. In front of the body a muscular tube ("funnel"), through which is expelled the water wliich has been used in respiration. An external shell in some, an internal skeleton in others. Ex. Cuttle-fishes, Nautilus. Sub-Kingdom VI. — Vertebrata, Body composed of a number of definite segments placed one behind the other in a longitudinal series. The main masses of the nervous system are placed upon the dorsal aspect of the body, and are shut off from the general body-cavity. The limbs (when present) are turned away from that side of the body on which the main masses of the nervous system are placed, and are never more than four in number. In most cases a backbone or ''vertebral column" is present in the fully-grown animal. (Fig. 12.) Fiff. 12.- - Vertebrata. Skeleton of the common Perch {rercajluviatilis). Class A. Pisces (Fishes).— Breathing organs in the form of gills ; heart, when present, usually of two chambers, rarely of three ; blood cold ; limbs, when present, converted into fins. Class B. Amphibia (Amphibians).— Breathing organs of the young, gills ; of the adult, lungs, either alone or associated with gills. Heart of the young of two chambers, of the adult of three cliambers. Blood cold. Skull jointed to the backbone by two articulating surfaces (" condyles "). Limbs never converted into fins. Class C. Reptilia (Reptiles).— Breathing organs in the form of hmgs, never in the form of gills. Heart three-chambered, rarely four- chambered, the pulmonary and systemic circulations connected together, either in the heart or in its immediate neighbourhood. Blood cold. Skull jointed to the backbone by a single articulating surface or condyle. 42 ELEMENTS OF BIOLOGY. Each half of the lower jaw composed of several pieces. Appendages of the skin in the form of homy scales or bony plates. Class D. Aves (Birds). — Respiratory organs in the form of lungs. Lungs connected with air-receptacles placed in various parts of the body. Heart four- chambered. Blood warm. Skull connected with the backbone by a single articulating surface or ' ' condyle." Each half of the lower jaw composed of several pieces. Appendages of the skin in the form of feathers. Fore-limbs converted into wings. Animal oviparous. Class E. TvLvmmalia (Quadrupeds). — Breathing organs in the form of lungs, which are never connected with air-receptacles placed in different parts of the body. Heart four- chambered. Blood \varm. Skull connected with the backbone by two articulating surfaces or "condyles." Each half of the lower jaw composed of a single piece. Appendages of the skin in the form of hairs. Young nourished by means of a special fluid — the milk — secreted by special glands — the mammary glands. Animal viviparous. VEGETABLE KINGDOM. Sub-Kingdom I.— Cryptogams, Plants destitute of true flowers with stamens and pistils. No tru^ seeds, but simple cellules or " spores," in which there is no embryo prior to germination. Class I. Thallophyta.- — Stem and foliage undistinguishable, com- posed of cellular tissue only. Ex. Lichens, Algae, and Fungi. Class II. Anophyta. — Stem and foliage distinct or confluent, of cellular tissue only. ^x. Mosses and Liverworts. Class III. Acrogens. — Stem with woody tissue and vessels, growing at its summit, and usually with distinct foliage. Ex. Horse-tails, Club- mosses, Ferns. Sub-Kingdom II. — Phanerogams. Plants producing true flowers with stamens and pistils. True seeds containing an embr}'o. Section A. Monocotyledones. — Seeds with one cotyledon or seed- leaf. Stems '■'endogenous,^'' toith no manifest distinction into bark, wood, aiid pith. Class I. Endogens. — Leaves parallel- veined, permanent. Root like the stem internally. Ex. Palms, Lilies, Grasses. Class II. Dictyogens. — Leaves net-veined, deciduous. Root with the wood in a solid concentric circle. Ex. Sarsaparilla. SUB-KINGDOM 11. — rilANEROGAM.K. 43 Section B. Dicotyledones. — Seeds zvith hco or more car pds. Stem *■'■ exogenous,^' with bark, wood, and pith. Leaves nettcd-veined. Class III. GYMNOSPERMi^. — Seeds naked, the pollen acting directly upon their surface. Ex. Pines and Cycads. Class IV. Angiosperm^, — Seeds enclosed in seed-vessels, the pollen acting through their tissues, Ex. Oak, Beech, and most ordinary trees and shrubs. CHAPTER IV. ANALOGY, HOMOLOGY, HOMOMORPHISM, MIMICRY, AND CORRELATION OF GROWTH. I. Analogy. — The term "analogue" was defined by- Owen to be "a part or organ in one animal which has the same functions as another part or organ in a different ani- mal." In other words, those parts or organs are analogous which resemble one another physiologically and discharge the same fimctions, wholly irrespective of what their funda- mental structure may be. In most cases the organs which would ordinarily be called " analogous " are such as differ from one another in structure, at the same time that they discharge the same duties. Thus the wings of a bird and the wings of an insect are analogous organs, since they are both organs of flight, and serve to sustain their possessor in the air. They are, however, in no way similar to one an- other except when regarded from this physiological point of view; and they differ altogether from a morphological aspect, being in no way formed on the same fundamental plan. It often happens, however, that " analogous " organs have the deeper relation to one another of being constructed upon the same morphological plan, in which case, in addition to their analogy, we have to consider the relationship which is known by the general name of " homology." II. Homology. — According to Owen, a ''homologue" is " the same organ in different animals under every variety of HOMOLOGY. 45 form and function." In other words, those organs or parts in different animals are ho7nologous^ which agree with one another morphologically in their fundamental sinicture^ quite irrespective of what functions they discharge in the economy. Thus the arm of man, the fore-leg of the dog, and the wing of a bird, are constructed upon the same morphological type, and are therefore homologous (fig. 13). They are not, however, analogous, since they perform wholly different functions, the first being an organ of prehension, the second devoted to terrestrial progression, and the third an organ of flight. There are, however, many cases in which organs in r .J ABC Fig. 13. — A Arm of Man ; B Fore-leg of Dog ; C Wing of Bird : h Humerus ; r Radius ; « Ulna ; c Carpus ; ttic Metacarpus ; / Phalanges. different animals are not only constructed of the same essential parts, but also discharge the same functions, thus coming to be both homologous and analogous. Besides the homologies which subsist between organs in different animals, there are two kinds of homology which may be present in the different parts of the same animal, 46 ELEMENTS OF BIOLOGY. and which are known as " serial homology " and *' lateral homology." Serial homology is established by the presence in a single animal of a succession of two or more parts which are placed in a longitudinal series one behind the other, and which have the same fundamental structure. In no animals is this phenomenon better seen than in the A?tfmlosa, such as the great majority of the Crustaceans, in which it is easy to see that the body is composed of a longitudinal succes- sion of rings or segments, placed in a row one behind the other, and essentially alike in their structure (fig. 14). In Fig. 14. — Fairy Shrimp {Chirocephahis diaphamts). After Baird. the majority of cases, however, whilst these serial parts have a fundamentally identical structure, and are clearly built upon a common plan, they are not all alike ; but they are modified in different regions of the body to fit them for the fulfilment of special functions. Certain of the segments, therefore, differ physiologically from certain others, and thus come to differ morphologically as well. There are other cases, however, as the Centipedes (fig. 15), for instance, in Fig. 15. — Centipede (.S"ri7//7/i?«). In this quiescent, motionless, and apparently dead condition it remains for a longer or shorter time, during which develop- mental changes are going on rapidly in its interior. Finally, the chrysalis ruptures, and there escapes from it the perfect winged insect*or 'Mmago" (fig. 23, r). To these changes the term vida??torJ>/iosis is rightly applied. These changes, however, do not differ in kind from the changes undergone by a Mammal ; the difference being that in the case of a Mammal the ovum is retained within the body of the parent, where it undergoes the necessary developmental changes, so that at birth it has little to do but grow, in order to be converted into the adult animal. From these considerations we arrive at the generalisation laid down by Quatrefages : " Those creatures whose ova — owing to an insufficient supply of nutritious contents, and an incapacity on the part of the mother to provide for their complete development within her own substance — are ra- pidly hatched, give birth to imperfect oftspring, which, in proceeding to their definitive characters, undergo several alterations in structure and form, known as metamorphoses." When the young organism, therefore, is thrown upon the world at a very early period of its development, it generally differs much from the adult in its external characters, and its mode of life is mostly quite diliferent to that of the latter. As a result of this, it commonly happens that the young animal possesses some of the structures of the adult in a very much modified form, whilst it may possess others which ar^ of a merely provisional nature, and arc altogether wanting in the fully-grown organism. Thus the caterpillar has to feed upon hard substances, whilst the butterfly lives upon vegetable juices. The caterpillar, therefore, is fur- 92 ELEMENTS OF BIOLOGY. nished with masticatory organs adapted for the division ot leaves, and the like. The parts of the mouth in the butter- fly, on the other hand, whilst morphologically identical with those of the larva, are so modified that they form a tubular organ, fitted for the suction of fluids, whilst the biting jaws of the caterpillar are aborted. The caterpillar, again, carries three pairs of legs in the front part of its body (fig. 23, «), which correspond with, and are ultimately converted into, the three pairs of legs possessed by the adult insect. The cater- pillar, however, has an additional series of locomotive pro- cesses developed upon some of the hinder segments of the body (fig. 23, a), which processes are merely of a provisional nature, and are not present in the adult even in a rudimen- tary form. In some cases, however, not only does the young form exhibit provisional structures, but there is what may be called a "provisional larva," out of a portion of which, and only a portion, the adult animal is developed. Thus, in the sea-urchins the ^gg gives rise to an actively locomotive larva, which is furnished with a mouth and alimentary canal of its own, and leads a completely independent existence. After a while, however, there is formed upon one side of the stomach of the larva a mass of growing material, which appropriates the stomach, and is gradually developed into a young sea-urchin. Only the stomach, however, of the ori- ginal "provisional larva" is thus retained to form part of the adult organism ; and the remainder of this temporary form, having served its purpose, is either absorbed, or is cast off as useless. There is one respect, however, in which the adult animal is always the superior of the young form, or at any rate almost always ; and that is in its possession of generative organs, and the power thereby conferred on it of producing fresh individuals by a true sexual process. Cases are not unknown in which young and immature forms can. produce fresh beings like themselves, but this is, in the great majority DEVELOPMENT. 93 of cases, by fwn-scxual methods of reproduction, which will be subsequently pointed out. The incapacity for sexual procreation displayed by young animals is in accordance with an important and well-established law, the exposition of which we owe to Dr W. B. Carpenter, that the process of generation is one opposed to that of nutrition, and, a fortiori, hostile to growth and development. The nutritive processes of the young animal are much more active than those of the adult, and so long as this remains the case, the generative functions remain in abeyance. It is not till the organism has reached the point of nutritional equilibrium, that it be- comes capable of exercising the function of reproduction in its highest and most genuine phase. Von Baer's Law of Development. — As the study of living beings in their adult condition shows us that the dif- ferences between those which are constructed upon the same morphological type depend upon the degree to which special- isation of function is carried, so the study of development teaches us that the changes undergone by any animal in passing from the embryonic to the mature condition are due to the same cause. All the members of any given sub-king- dom, when examined in their earliest embryonic condition, are found to present the same fundamental characters. As de- velopment proceeds, however, they diverge from one another with greater or less rapidity, until the adults ultimately be- come more or less different, the range of possible modifica- tion being apparently almost illimitable. The differences are due to the different degrees of specialisation of function necessary to perfect the adult, and therefore, as Von Baer put it, the progress of da'eloptncut is from the general to the special. It is upon a misconception of the true import of this law that the theory arose, that every animal in its development passed through a series of stages, in which it resembles, in turn, the different inferior members of the animal scale. With regard to man, standing at the top of the whole 94 ELEMENTS OF BIOLOGY. animal kingdom, this theory has been expressed as follows : — " Human organogenesis is a transitory comparative ana- tomy, as, in its turn, comparative anatomy is a fixed and permanent state of the organogenesis of man " (Serres). In other words, the embryo of a Vertebrate animal was believed to pass through a series of changes corresponding respect- ively to the permanent types of the lower sub-kingdoms — namely, the Protozoa, Ccelenterata, Annuloida, Annu- losa, and Mollusca — before finally assuming the true ver- tebrate characters. Such, however, is not truly the case. The ovum of every animal is from the first impressed with the power of developing in one direction only, and very early exhibits the fundamental characters proper to its sub- kingdom, never presenting the structural peculiarities be- longing to any other morphological type. Nevertheless, the differences which subsist between the members of each sub- kingdom in their adult condition are truly referable to the ^egree to which development proceeds, the place of each individual in his own sub-kingdom being regulated by the stage at which development is arrested. Thus, many cases are known in which the younger stages of a given animal represent the permanent adult condition of an animal some- what lower in the scale. Thus, to give a single example, the young of the water-breathing Univalve Shell-fish {Gas- teropoda) transiently present all the essential characters which distinguish the adult condition of the minute oceanic Molluscs known as the Pteropods. The young Gasteropod, namely, swims about freely by means of two lobes or fins attached to the sides of the head (fig. 24, A), and similar fins are present in the Pteropods in their adult condition (fig. 24, B), enabling the animal to swim actively at the surface of the open ocean. The development of the Gas- teropod, however, proceeds beyond the point, and the adult is much more highly specialised than is the adult Pteropod. Upon the theory of " Evolution " such facts as the above would be explained simply by the law of hereditary trans- DEVELOPiMENT. 95 mission. Upon this theory, the Pteropods and the Gastcro- pods have proceeded from a common progenitor, and have ■ B Fig. 24. — A, Young oi EolLs, a water-breathing Gasteropod, showing the provi- sional buccal lobes. B, Adult Ptcropod (Z./w{ Clytin (Cnvt/'afiularia) Johnsioni, magnified ; p Nutritive zooid ; ^Capsules in which the reproductive zooids are produced. " medusoid" (fig. 31) consists of a little transparent glassy disc or bell, from the under surface of which there is sus- pended a modified zooid or " polypite," in the form of a REPRODUCTION. 109 central process, which is known by llie name of the '^ manu- brium." The whole organism s\\ims gaily through the water, pro- pelled by the contractions of the bell or disc {gonocalyx) ; and no one would now suspect that it was in any way related to the fixed plant -Hke zoophyte from which it was originally budded off. The central polypite is fur- nished with a mouth at its distal end, and the mouth opens into a digestive sac. From the proxi- mal end of this stomach proceed four radiating canals which ex- tend to the circumference of the disc, where they all open into a single circular vessel surrounding the mouth of the bell. From the margins of the disc hang also a number of delicate extensile filaments or tentacles; and the circumference is still further adorned with a series of brightly- coloured spots, which are pro- bably organs of sense. The mouth of the bell is par- tially closed by a delicate transparent membrane or shelf the so-called -veil.'' Thus constituted, these beautiful httle beings lead an independent and locomotive existence for a longer or shorter period. Ultimately, the essential elements of reproduction are developed in special orirans, situated in the course of the radiating canals of the disc! The resulting embryos are ciliated and free-swimming, but ultimately fix themselves, and develop into the plant-like colony from which fresh medusoids may be budded off. 6 ^'.?- 3'- — Free mediisiform gono- pliorc of Clytin Johns toni (after Hinuks). a Central polypite or mainibrium; b b Radiating gas- tro- vascular canals; c Circular canal; vi Marginal bodies; / Tentacles. no ELEMENTS OF BIOLOGY. For these phenomena we can find no parallel amongst plants. If we imagine, however, a tree which could detach its flowers, and if we suppose these to be organised for an independent existence, and to be capable of increasing in size after their liberation, we should have very much the state of things which we observe in Clytia. Still more extraordinary phenomena have been observed in some others of the Hydrozoa, as in the Lncer7iarida. In these, the egg gives rise to a minute, free-swimming, ciliated body (fig. 32, «), which consists of two layers enclosing a central cavity. Soon it becomes pear-shaped, fixes itself to some solid body by its tapering extremity, and develops a mouth and tentacles at the other extremity. It is now known as the Hydra-tuba (fig. 32), from its resemblance in form to the fresh-water polype or Hydra. The Hydra-tuba has the power of multiplying itself by gemmation, and it a Fig. 32. — Development of one of the Lucer/iarida {Aitrelia). a Free-swimming cili- ated embryo; b Hydra-tuba; c Hydra-tuba undergoing transverse fission; d The same with the fission further advanced. can produce extensive colonies in this way ; but it does not obtain the power of generating the essential elements of re- production. Under certain circumstances, however, the Hydra-tuba enlarges, and its body becomes constricted by a REPRODUCTION. 1 I I series of transverse annulations or grooves (fig. 32, c). These grooves go on deepening, and the segments which they mark off become deeply lobed and incised at their margins, till the whole organism assumes the aspect of a pile of saucers arranged one upon another with their concave surfaces up- wards. A new set of tentacles is developed near the base of the organism, and all the segments above this point gradu- ally fall off, and swim away to lead a free life. These libe- rated segments of the little Hydra-tuba (it is about half an inch in height) now lead an independent existence, and were originally described by naturalists as distinct animals (hg. 33). They are provided with a swimming-bell or "urn- Fig. 33. — Hidden-eyed Medusx. Generative zooid of one of the LucemariJa {Chrysaora hysoscelUi). After Gosse. brella," by the contractions of which they are propelled through the water. From the centre of the umbrella is sus- pended a modified polypite with lobed and scalloped lips ; 112 ELEMENTS OF BIOLOGY. • and the margins of the bell carry organs of sense and long tentacles. The central polypite has a mouth and digestive cavity, leading into a complex canal-system. At first of small size, they feed eagerly, and increase largely in bulk, in some cases attaining perfectly colossal dimensions (as much in one species as twenty feet in circumference). After awhile they develop the essential elements of reproduction, and after the fecundation and liberation of their ova, they die. The ova, however, are not developed into the free- swimming and comparatively gigantic organism by which they were immediately produced, but into the minute, fixed, sexless Hydra-tuba. We thus see that a small, sexless zooid, which is capable of multiplying itself by gemmation, produces by fission several independent, locomotive beings, which are capable of nourishing themselves and of performing all the functions of life. In these are produced generative elements, which give rise by their development to the little fixed creature v/ith which the series began. To the group of phenomena of which the above are ex- amples, the name "alternation of generations" was applied by Steenstrup ; but the name is not an appropriate one, since the process is truly an alternation of generation with gemmation or fission. The only generative act takes place in the reproductive zooid, and the production of this from the nutritive zooid is a process of gemmation or fission, and nofa process of generation. The "individual," in fact, in all these cases, must be looked upon as a double being composed of two factors, both of which lead more or less completely independent lives, the one being devoted to nutrition, the other to reproduction. The generative being, however, is in many cases not at first able to mature the sexual elements, and is therefore provided with the means necessary for its growth and nourishment as an ind^Dendent organism. It must also be remembered that the nutritive half of the '• individual " is usually, and the generative half • REPRODUCTION. II3 sometimes, compound ; that is to say, composed of a number of zooids produced by gemmation ; so that the zoological individual in these cases becomes an extremely complex being. These phenomena of so-called "alternation of generations," or " metagenesis," occur in their most striking form amongst the Hydrozoa; but they occur also amongst some of the intestinal worms (Entozoa), and amongst some of the Tunicata (MoUuscoida). d. Parthenogenesis. — " Parthenogenesis " is khe term em- ployed to designate certain singular phenomena, resulting in the production of new individuals by virgin females without the intervention of a male. By Professor Owen, who first employed the term, parthenogenesis is applied also to the processes of gemmation and ^fission, as exhibited in sexless beings or in virgin females ; but it seems best to consider these phenomena separately. Strictly, the term parthenogenesis ought to be confined to the production of new individuals from virgin females by means of ova^ which are enabled to develop themselves without the contact of the male element. The difficulty in this definition is found in framing an exact definition of an ovum, such as will dis- tinguish it from an internal gemma or bud. No body, however, should be called an "ovum" which does not exhibit a germinal vesicle and germinal spot, and which does not exhibit the phenomenon known as segmentation of the yelk. Moreover, ova are almost invariably produced by a special organ, or ovary. As examples of parthenogenesis we may take what occurs in plaht-lice (Aphides) and in the honey-bee ; but it will be seen that in neither of these cases are the phenomena so unequivocal, or so well ascertained, as to justify a positive assertion that they are truly referable to parthenogenesis in the above restricted sense of the term. The Aphides, or plant-lice (fig. 34), which arc so com- monly found parasitic upon plants, are seen towards the / 114 ELEMENTS OF BIOLOGY. * close of autumn to consist of male and female individuals. By the sexual union of these, true ova are produced, which Fig- 34.— Bean Aphis {Aphis fabm)y winged male and wingless female. remain dormant through the winter. At the approach of spring these ova are hatched; but instead of giving birth to a number of males and females, all the young are of one kind, variously regarded as neuters, virgin females, or hermaphrodites. Whatever their true nature may be, these individuals produce viviparoicsly a brood of young which resemble themselves ; and this second generation, in like manner, produces a third, — and so the process may be re- peated, for as many as ten or more generations, throughout the summer. When the autumn comes on, however, the viviparous Aphides produce— in exactly the same manner — a final brood ; but this, instead of being composed entirely of similar individuals, is made up of males and females. Sexual union now takes place, and ova are produced and fecundated in the ordinary manner. The bodies irom which the young of the viviparous Aphides are produced are variously regarded as internal buds, as "pseudova" (z>., as bodies intermediate between buds and ova), and as true ova. Without entering into details, it is obvious that there is only one explanation of these phenomena which will justify us in regarding the case of the viviparous Aphides as one of true parthenogenesis, as above defined. If, namely, the spring broods are true females, and the bodies which they REPRODUCTION. II5 produce in tlicir interior are true ova, then the case is one of genuine parthenogenesis, for there are certainly no males. The case might still be called one of parthenogenesis, even though the bodies from which these broods are produced be regarded as internal buds, or as " pseudova ; " for a true ovum is essentially a bud. If, however, Balbiani be right, and the viviparous Aphides are really hermaphrodite, then, of course, the phenomena are of a much less abnormal character. In the second case "of alleged parthenogenesis which we are about to examine — namely, in the honey-bee — the phenomena which have been described cannot be said to be wholly free from doubt. A hive of bees consists of three classes of individuals: i. A "queen," or fertile female; 2. The "workers," which form the bulk of the community, and are really undeveloped or sterile females ; and 3. The " drones," or males, which are only produced at certain times of the year. We have here three distinct sets ^of beings, all of which proceed from a single fertile individual ; and the question arises, In what manner are the differences between these produced ? At a certain period of the year the queen leaves the hive, accompanied by the drones (or males), and takes what is known as her "nuptial flight " through the air. In this flight she is impregnated by the males, and it is immaterial whether this act occurs once in the life of the queen, or several times, as asserted by some. Be this as it may, the queen, in virtue of this single impregnation, is enabled to produce fresh individuals for a lengthened period, the semen of the males being stored up in a receptacle which communicates by a tube with the oviduct, from which it can be shut off at will. The ova which are to produce workers (undeveloped females) and queens (fertile females) are fertilised on their passage through the oviduct, the semen being allowed to escape into the oviduct for this purpose. The subsequent development of these fecundated ova into workers or queens depends Il6 ELEMENTS OF BIOLOGY. entirely upon the form of the cell into which the ovum is placed, and upon the nature of the food which is supplied to the larva. ■ So far there is no doubt as to the nature of the phenomena which are observed. It is asserted, how- ever, by Dzierzon and Siebold, that the males or drones are produced by the queen from ova which she does not allow- to come into contact with the semen as they pass through the oviduct. This assertion is supported by the fact that if the communication between the receptacle for the semen and Ihe oviduct be cut off, the queen will produce nothing but males. Also, in crosses between the common honey-bee and the Ligurian bee, the queens and workers alone exhibit any intermediate characters between the two forms, the drones presenting the unmixed characters of the queen by whom they were produced. If these observations are to be accepted as established — and, upon the whole, there can be little hesitation in accepting them as in the main correct — then the drones are produced by a true process of parthenogenesis ; but some observers maintain that the development of any given ovum into a drone is really due — as in the case of the queens and workers — to the special circumstances under which the larva is brought up.* There are various other cases in which parthenogenesis is said to occur, but the above will suffice to indicate the general character of the phenomena in question. The theories of parthenogenesis appear to be too complex to be introduced here ; and there is the less to regret in their omission, as naturalists have not yet definitely adopted any * In the case of Polistes Gallica, Von Siebold appears to have proved beyond reasonable doubt that the males are produced by a process of parthenogenesis. Landois, however, asserts that the eggs of insects are of no sex ; that sex is only developed in the larva after its emergence from the egg ; and that in each individual larva the sex is determined ■wholly by the nature of the food upon which it is brought up — abundant nourishment producing females, and scanty diet giving rise to males. RErRODUCTION. II7 one explanation of the phenomena to the exclusion of the rest. e. Law of Quatrcfagcs. — From the phenomena of asexual reproduction in all its forms, M. de Quatrefages has de- duced the following generalisation : — " The formation of new individuals may take place, in some instances, by gemmation from, or division of, the parent being; but this process is an exhaustive one, and cannot be carried out indefinitely: when, therefore, it is necessary to insure the continuance of the species, the sexes must present themselves, and the germ and sperm must be allowed to come in contact with one another." It should be added that the act of sexual reproduction, though it insures the perpetuation of the species, is very destructive to the life of the individual. The formation of the essential elements of reproduction appears to be one of the highest physiological acts of which the organism is cap- able, and it is attended with a corresponding strain upon the vital energies. In no case is this more strikingly exhib- ited than in the majority of insects, which pass the greater portion of their existence in a sexually immature condition, and die almost immediately after they have become sexually perfect, and have consummated the act whereby the per- petuation of the species is secured. Thus, as pointed out by Dr Carpenter, and strongly in- sisted upon by Mr Herbert Spencer, we are to regard sex- ual reproduction as being directly antagonistic to nutrition. This brings us to the further law that the life of an animal whilst sexually immature is generally associated with active growth ; but that when once the generative expenditure has commenced, the nutritive powers can rarely do more than maintain the ^orgajpism in statu quo, whilst they may even fall short of this. If we regard the asexual methods of re- production as being merely forms of growth, we can readily understand how it is that zooidal multiplicatioi^^nerally excludes sexual reproduction for a time. The ^ffe, how- Il8 ELEMENTS OF BIOLOGY. ever, ultimately comes in the life of all organisms when mul- tiplication by gemmation and fission becomes insufficient, when it becomes necessary that the essential elements of reproduction should be produced. The additional tax thus imposed upon the organism is usually borne without injury for a certain length of time ; but the losses thus caused, if slow, are sure, and in some cases they are so great as to end in the immediate extinction of the organism. There are, how- ever, strong grounds for the belief that in this respect man's position differs materially from that of all other animals. • CHAPTER XL REPRODUCTION IN PLANTS. Having treated at some length of the reproductive process in animals, there remains little that need be said as to the reproduction of plants. As amongst animals, plants exhibit both sexual and non-sexual methods of reproduction, though the peculiarities of vegetables render the latter much less con- spicuous than in animals, and, indeed, usually lead to their being completely overlooked. In many of the lower cellular plants reproduction takes place by gemmation or fission, which may be continuous or discontinuous, and the process differs little from what may be observed in many of the lower animals. In the higher plants, however, continuous gemmation is universal, but it is so plainly a mere form of growth that it is never regarded as being of a reproductive nature. Nevertheless, from a philosophical point of view, the gemmation by which a trefe is produced may be in all respects paralleled with that to which the origin of one of the plant-like colonies of the Hydroid Zoophytes is due, if we simply make due allowance for the differences which subsist between animals and plants. Thus the leaves of the tree are truly " nutritive zooids," produced by a process of continuous gemmation from the primitive being which is developed from the ovum ; and they are concerned wJiolly with the nutrition of the organism, 120 ELExMENTS OF BIOLOGY. and take no part in reproduction. That they do not strike us in the same light as do the " polypites " of the Hydroid colony arises merely from the fact that they are devoid of the animal ''functions of relation." In reality, however, they lead a life which is just as independent of the whole, whilst the life of the latter is in no way commensurate with the existence of the leaves. Similarly, the tree ordinarily consists simply of a collec- tion of leaves, or nutritive factors, which have no power of producing the sexual elements. At ceftain times, however, the tree produces special buds — the flowers — in which the generative elements are produced, and by the agency of which the perpetuation of the species is insured. We may, then, regard ordinary plants as colonies consist- ing theoretically of a "trophosome" and "gonosome," each of which is made up of an indefinite number of zooids. The zooids of the trophosome — or leaves — are all like one another, and are devoted to the nutrition of the colony. The zooids of the "gonosome" — or flowers — are also usually all alike, but do not resemble the leaves, though the two can be shown to be morphologically identical. They take no part in the nutrition of the colony, but are simply devoted to the production of new individuals. The inter- esting and important point about this comparison is the clearness with which it brings out the fact that gemmation and fission are merely to be regarded as forms of growth. No one thinks of looking upon the leaves or flowers of a tree as independent or separate beings ; and yet in reality they have just as much claim to this title as have the zooids of the Hydroid colony. On the contrary, every one recog- nises that a tree is the result of a process of growth ; and every one would equally recognise that this is the case with the Hydroids, if the polypites of the latter were endowed with as little power of spontaneous motion and as little sensation as the leaves of a plant. In plants, as in animals, the only genuine form of repro- REPRODUCTION IN PLANTS. 121 (luction consists in the production of two cells having dif- ferent contents — a sperm-cell or spermatozoid, and a germ- cell or ovum. The contact of these gives rise to the direct formation of an embryo, or, in other cases, to the formation of an individual which produces special buds or " spores." In all the higher plants there is a male element or " pollen," and a female element (or ovule), both cellular, and the em- bryo is produced by the coming together of these. In the lower plants considerable modifications occur as to the man- ner in which new individuals are produced ; but in the great majority of cases elements corresponding to the pollen and ovule of the higher forms are produced. It is impossible here to treat of the modifications of the reproductive process of plants at any length ; but we may very briefly describe the method by which new individuals are produced in the ordinary Flowering Plants (Angiosperms) and in Ferns. The male organs of Angiospermous Flowering Plants are called the "stamens" (fig. 35, A), and, like the other parts A B C Fig. 35. — A, Flower of Tulip with the external parts removed, showing the six sta- mens {s) surrounding the pistil (/). B, Single stamen enlarged, showing anther (a) and the filament or stalk (_/). C, Pollen-grains enlarged, one of them dis- charging the fovilla. of the flower-bud, are really to be regarded as modified leaves. Each consists of a folded leaf or " anther" (fig. 35, B), which is generally supported upon a more or less con- I 22 ELEMENTS OF BIOLOGY. spicuous stem or " filament." When mature, the anther is found to be filled with microscopic cellular bodies or "pol- len-grains" (fig. 35, C), which constitute a fine powder, and which are truly the male element of reproduction. The pollen-grains, in turn, are filled with an extremely fine molecular matter which is termed the " fovilla." The par- ticles of the fovilla exhibit more or less active movements, the exact nature of which has not yet been accurately deter- mined ; and it is probable that they are the essential gener- ative elements by which the influence of the male is trans- mitted to the female. The female organs of Angiospermous Flowering Plants constitute the *' pistil " (fig. 36, A) ; and consist in their — a ..... — A.._ Pistil of the Apricot. E,_ Pistil of the Orange. C, Flower of Valerian, cut vertically, a Ovary, containing the ovule or ovules ; b Style ; c Stigma ; d Stamen. simplest and most fundamental form of a folded leaf or " ovary " (^), containing one or more germ-cells or ovules. The summit of the pistil is formed of loose cells, which are uncovered by epidermis, and secrete a viscid fluid, the whole constituting what is known as the " stigma " {c). The REPRODUCTION IN PLANTS. 123 Stigma may be seated directly upon the ovary, or may be separated from it by a longer or shorter stalk, ^vhich is termed the "style" {b). The male and female organs of reproduction are usually present in the same flower, when the plant is "monoecious;" but at other times one individual produces the male flowers and another individual produces female flowers, when the species is " dioecious." Even in bisexual flowers, however, there is reason to believe that there are natural arrange- ments whereby perpetual self-fertilisation is prevented, and the influence of another individual is at intervals secured. In ordinary cases amongst Angiosperms, the process by which the ovule is impregnated may be described as fol- lows : — The anthers, when ripe, burst, and shed their con- tained pollen upon the moist stigmatic surface of the pistil. The viscid secretion of the stigma seems to act in such a manner upon the pollen-grains that their inner lining is protruded in the form of delicate microscopic tubes — the " pollen-tubes." These insinuate their extremities into the loose tissue of the stigma, and, gradually elongating, make their way into the ovary ; the distance traversed in this way varying with the distance between the stigma and ovule, and being enormously great in long-styled plants. During this process, changes have been going on in the ovule, in consequence of which impregnation is possible. The most important of these consists in the enlargement of the so- called " embryo-sac," which truly corresponds with the ovum of animals, and the formation in its interior of from one to three or more vesicular bodies, which are known as the " embryonal vesicles," and which seem to correspond with the germinal vesicle of the ovum of animals. When the pollen-tube reaches the embryo-sac, its further growth seems to be generally arrested, and it is only in rare cases that the pollen-tube perforates the embryo-sac,* if, indeed, * Recent researches demonstrating the possibility of cells making their way through unbroken surfaces, as has been incontestably proved 124 ELEMENTS OF BIOLOGY. this ever really happens. The fluid matter of the pollen- tube, and possibly some of the minutely granular '" fovilla " as well, is now transferred to the embryo-sac ; and as the result of the stimulus thus imparted, one or more of the embryonal vesicles is impregnated, when the pollen-tubes decay. As regards the reproduction of the Flowerless Plants (Cryptogams)^ the process varies much in different cases; but in the higher forms the essential element of the process consists in the production of sperm-cells or spermatozoa, and a germ-cell or ovum. There are, however, some very singu- lar complexities in the manner in which these essential generative elements are produced, and we may notice the phenomena which have been observed in Ferns : — The ordinary Ferns are well known to produce at certain seasons what are commonly spoken of as the "organs of fructification. '^ In the commoner species these take the form of little rounded masses, which are generally placed upon the back of the adult frond (fig. 37, A). When examined microscopically, each of these spots of fructifica- tion is found to consist of an aggregation of minute recep- tacles or '' spore-cases," containing in their interior still more minute cellular bodies or " spores." If one of these spores be liberated from the spore-case, and placed under favourable conditions, it germinates, giving off roots on the one hand, and producing on the other hand a little cellular expansion or leaf, which is termed the "prothallus" (fig. 37, D). This prothallus, however, is not itself developed into a new fern, but it is a mere temporary or provisional body, upon which are produced male and female organs of reproduction. The male organs are produced upon the under side of the prothallus, and they have the form of minute cellular eminences, containing reproductive cells. in the case of the white corpuscles of the blood, render it by no means unlikely that the fovilla itself reaches the embryo-sac without any neces- sary rupture of the walls of this cavity or of the pollen-tube. REPRODUCTION IN PLANTS. 125 These cells are liberated, when they burst, and give exit to true spermatozoa in the form of ciliated spiral filaments. The female organs are also placed upon the under surface of the prothallus, and also have the form of cellular pro- rig- 37- — A, Portion of tlie frond of Polypodlinn vidga7-e, showing the organs of fructification B, Spore-cases of the same, magnified. C, Spore of a Fern, greatly enlarged. D, Cellular prothallus of a Fern, produced by a spore {s), and giving off a root (r). minences. The cells of these prominences are so arranged that they form a canal, leading down to a large central cell or ovule. The spermatozoa liberated from the male organs pass down the central canal, and gain access to the ovule. As the result of this, segmentation of the ovule is set up, and an embryo is produced from which the frond of the ordinary fern is developed, the prothallus perishing when this is accomplished. The sequence of phenomena here indicated may in some respects be fairly compared with those formerly alluded to under the head of " alternation of generations." The " spores " produced in the spore-cases of the ordinary fern are to be regarded simply as buds, since they are not pro- duced by any generative act, whilst they have the power of developing themselves without contact with a second dis- 126 ELEMENTS OF BIOLOGY. similar element. These spores give rise to a temporary organism, the sole function of which is to develop the special organs in which the essential elements of reproduc- tion may be produced. The contact of these elements gives rise to an embryo, which is developed into the original " sporangiferous " frond by which the spores were pro- duced, and not into the temporary cellular expansion on which the generative elements were carried. There is thus an alternation of gemmation with generation, the generative process being carried on by a minute provisional organism, developed by budding from a conspicuous leafy frond, which latter is produced in a true sexual manner. CHAPTER XII. SPONTANEOUS GENERATION. " Spontaneous Generation," or " Abiogenesis," is the term applied to the alleged production of living beings without the pre-existence of germs of Siny kind, and there- fore without the pre-existence of parent organisms. - The question as to the possibility of spontaneous generation is one which has been long and closely disputed, and which cannot be said to be yet definitely settled. It will be suffi- cient, therefore, to indicate some of the facts upon which the belief in Abiogenesis is founded, and to point out some general considerations upon the same. If an animal or vegetable substance be soaked in hot or cold water, we obtain what is called an " organic infusion " — that is to say, a fluid holding organic matter in solution. If such an infusion be boiled, any adult living beings which might be present in it are destroyed, and the fluid certainly becomes temporarily deprived of all active life. If, how- ever, such an infusion be exposed for a certain length of time to the air, a series of changes is inaugurated which end in its becoming tenanted by numerous living organisms. The first phenomenon observable is usually the forma- tion upon the surface of the infusion of a delicate film or scum. If a fragment of this film or pellicle be examined microscopically, it is found to consist of numberless moving 128 ELEMENTS OF BIOLOGY. points, particles, or molecules (fig. 38, A). The largest of these may not be more than one ten-thousandth of an inch in diameter; the smallest may not exceed one forty-thou- sandth of an inch. Every increase in the magnifying ::oV,>.'-<<:»;.-„,»rj:T7«uv",r,'i ••.-0- ..ai'O A.-^.,o ?:^?~;^?(J?^:^ »■ u — O fl " o .' o'•-»v%^S'C-'*--<^■*i^'^• A B Fig. 38. — A, Living particles or molecules developed in organic infusions. B, Bacteria developed in organic infusions. (After Beale.) power of the microscope has simply served to bring to light myriads of smaller and smaller particles ; and the highest powers of the microscope known to us — enormous as they are — only leave us in the certainty that if we could obtain still higher powers, we should almost infallibly dis- cover particles still more minute. All the particles of the scum are seen to be in active and incessant movement, and there is no question as to their being truly living organisms, though it is uncertain whether they are of an animal or vegetable nature, or whether they may not be partly the one and partly the other. If the fluid be examined at a later period, in addition to the minuter moving particles, there will be found many little moving filaments of a larger size. Some of these are short and staff-shaped, and are known as "bacteria" (fig. 38, B). Others are long and worm-like, and move about actively, twisting from side to side. These are known as " vibrios." Both the bacteria and vibrios are unquestionably alive, though in this case, also, it is a matter of some doubt whether we have to deal with animal or vegetable organ- isms. Upon the whole, however, it seems tolerably certain SPONTANEOUS GENERATION. 1 29 that the bacteria and vibriones are to be regarded as be- longing to the vegetable kingdom. Lastly, at a still later period, the fluid may be found to ccmtain forms of the so-called " Infusorian Animalcules." These are undoubted animals, and though not standing very high in the zoological scale, they are by no means the humblest or most lowly organised members of the animal kingdom. The phenomena just recounted are altogether beyond doubt, and may be observed by any one for himself with a little trouble and a tolerably good microscope. Their ex- planation, however, has been the subject of one of the most vigorous controversies which has ever divided the scientific world into two opposing camps ; and it cannot be regarded as by any means near its final settlement. The point to be settled is this : — How does a fluid which, to begin with, is wholly without living beings, become the home of unquestionable living organisms ? Two answers have been given to this question. The oldest theory, and one which was in vogue long anterior to the discovery of the facts just mentioned, was, that these living beings formed themselves spontaneously and de novo out of the dead materials of the fluid. Very ancient is this belief, that living beings could be produced by the spontaneous action of a genial and prolific nature upon dead matter ; and many animals, both real and imaginary, have been asserted to have been generated in this fashion. Nowadays, how- ever, the theory of spontaneous generation has been wholly given up as regards all the cases in which the ancients placed credence ; and it has become entirely restricted to a group of minute organisms, the very existence of which has only been known since the microscope has reached something like its present perfection. Stated briefly, then, as far as concerns the facts above described, it is held by one school that the microscopic organisms which make their appear- ance in organic infusions, after exposure to the air, have 130 ELEMENTS OF BIOLOGY. been produced spontaneously by the action of physical and chemical forces upon the organic, but dead, materials held in solution in the fluid. By another school, on the other hand, it is held that the facts of the case may be explained upon the supposition that the air, all fluids exposed to the atmosphere, and many solid bodies, are crowded with the microscopic germs of minute living beings, animal or vegetable; that these germs may remain dormant for indefinite periods, having the power of withstanding temperatures which would be fatal to adult organisms ; but that they spring into active life the moment the conditions which surround them are favourable for their development. Such conditions are presented by any fluid holding organic matter in solution ; and it is believed that the living organisms which appear in an organic infusion are merely developed from inconceiv- ably minute germs, which fall into the infusion from the air, or are contained in the fluid to begin with. It must be admitted that the above is to a certain extent an hypothesis ; but it is not only supported by various ab- stract considerations of great weight, but also rests upon a firm if somewhat narrow basis of fact. Thus it has been shown, beyond a question, that such germs are present in the atmosphere, and in many other localities as well. We may therefore safely assume as proved, that the air, most fluids, and many organic and inorganic substances, contain the germs of organisms which are capable of being developed in active life, when once they are placed under suitable con- ditions. We may regard this as proved wholly irrespective of the belief that certain low organisms can be produced spontaneously, without the presence of pre-existent germs. Even if spontaneous generation were proved to be part of the order of nature, the importance or validity of the fact just stated would be in no way affected thereby. Even if we were to admit the possible formation of living beings out of dead matter, it would still remain certain that all nature SPONTANEOUS GENERATION. 131 teems with a life invisible except to the higher powers of the microscope — a life which reproduces itself by the ordinary and natural methods, and which is ever on the alert to catch the first opportunity of springing into active instead of po- tential vitality. It is not necessary here to enter upon the experimental evidence upon this subject. Upon no single question, pro- bably, in the whole range of Biology, have greater pains been expended, and more elaborate experiments carried out ; and upon no single question have the actual results of the inquiry been so singularly contradictory and unsatisfactory. All the experiments which have been set on foot with a view of settling this question have been directed to one of two ends — ^viz., to prove that no life would appear in organic infu- sions from which germs were rigidly excluded, or to. show that living organisms appeared in fluids in which it was im- possible that any germs could be present. Neither end has as yet been satisfactorily attained ; and from the nature of the case it is difficult to believe that the experimental evi- dence could, under any circumstances, ever amount to actual demonstration. For our present purpose it will be sufficient, very briefly, to consider some recent experiments carried out by Dr Charlton Bastian with a view of proving the occurrence of Abiogenesis. The most important experiments carried out by this ob- server consisted in taking an organic infusion — such as an infusion of turnip — boiling it, to expel the air as far as pos- sible, as well as to kill any germs which might be present in the fluid, and then hermetically sealing the neck of the flask in the flame of a spirit-lamp. By this procedure it will be at once evident that the experimenter had an infusion con- taining dead organic matter, but ostensibly containing no living germs, enclosed in a flask from which all, or nearly all, the atmospheric air had. been expelled by boiling. The flask thus prepared was submitted for hours to a tempera- ture considerably over the boiling-point of water, and then 132 ELEMENTS OF BIOLOGY. allowed to remain unopened for a varying period. It only remains to add, that in some of the experiments the rigour of the conditions was still further increased by the substitu- tion for the organic infusion of mere solutions of certain salts, such as tartar emetic, phosphate of ammonia, or phos- phate of soda. With regard to the alleged results of these apparently crucial experiments, Dr Bastian asserts that in almost every instance the fluid in the flask, in the course of a certain time, was found under the microscope to exhibit numerous living organisms, chiefly, though not exclusively, of a vege- table nature. With regard to the value of these results, it should be remarked, in the first place, that the conditions of the ex- periment were such as we should, upon a priori grounds, have believed to be utterly fatal to the possibility of the development of life even in its humblest forms. The fluid experimented on was subjected to a temperature exceeding that of boiling water, and the flasks were hermetically sealed at a time when they were filled with steam, so that the atmospheric air was thereby excluded from them. It is true that the experiments of Pasteur have shown that some of the organisms of infusions — e.g.^ the bacteria — can exist without free oxygen ; but there is certainly no reason to believe that any living beings can thrive in a complete and perfect vacuum. In the second place, it is to be noticed that in spite of the fearfully deterrent conditions under which the fluid in the flasks was placed, living beings are alleged to have made their appearance therein nearly or quite as abundantly as would have been the case if an ordinary organic infusion had been taken, subjected to ordinary con- ditions, and allowed an unrestrained access of air. In the third place, there is absolutely no proof that the heat to which the fluids experimented on were subjected is sufficient to kill any or all living germs. It is quite true that, so far as we know, no adult organism can withstand SPONTANEOUS GENERATION. I 33 for any length of time exposure to a temperature equal to that of boiling water. But it by no means follows from this that the same temperature would necessarily suffice to de- stroy all the indescribably minute germs from which some of the lower animals and plants are produced. In point of fact, many instances are known in which the eggs of various animals, and the seeds of many plants, can withstand in- jurious conditions to an extent of which the adults are wholly incapable. And Mr Grace - Calvert has recently shown that vibrios can endure a temperature in some cases exceeding 300° Fahr. without being killed thereby. Lastly, many of the vegetable organisms present in the infusions of Dr Bastian were seen fructifying and producing spores in the ordinary manner. Had they been produced spontaneously, and had this mode of production been the natural one, it would, to say the least of it, be a very remark- able fact if they should straightway proceed to reproduce their kind in the manner which is believed to be the normal and regular mode. This argument is a still stronger one when applied to the Infusorian Animalcules, which are so commonly found in organic infusions, but which do not appear to have made their appearance in any of the fluids experimented on by Dr Bastian. In this case, not only is the organisation of the animals of a comparatively high type, but we are perfectly familiar with their modes of re- production ; and it would appear to be most unnecessary that they should be produced spontaneously in the manner alleged, since their fecundity by the ordinary methods of reproduction is very great. Upon the whole, then, we can hardly avoid the conclu- sion that some fallacy lurks under the experiments carried out by Dr Bastian. Probably the living germs of the low- est animals and plants are 7iot destroyed by a temperature equal to that of boiling water; whilst some of the lower forms of life may be able to endure conditions which might at first sight be regarded as inevitably destructive of vitality. 7 CHAPTER XIII. ORIGIN OF SPECIES. We have already seen reasons to conclude that the term " species'' must be regarded as being merdy a convenient abstraction by which we denote assemblages of individuals having certain characters in common. We are thus led to the belief that what naturalists ordinarily call " species" are not unvar}dng and immutable quantities. We cannot, there- fore, retain, in the sense in which he used it, the dictum of Forbes, that " every true species presents in its individuals certain features, specific characters, which distinguish it from every other species ; as if the Creator had set an exclu- sive mark or seal on each type." On the contrary, the researches of Darwin, Wallace, and others, have compelled the admission that all so-called species vary more or less, and that these variations are sometimes so extensive that the limits of specific distinctness are overstepped. Still, it has not yet been demonstrated that these variations are indefinite, either in direction or amount ; and it remains, therefore, possible that the process of specific variation is bounded by fixed, if widely extended, limits, however pro- bable the contrary may appear. It is impossible here to do more than merely indicate, in the briefest manner, the two fundamental ideas which are at the bottom of the leading theories which are entertained as ORIGIN OF SPECIES. 135 to the origin of species. The opinions of scientific men are still divided upon this subject ; and it will be sufficient to give an outline of the two more important hypotheses, with- out adducing any of the reasoning upon which they are based. "^ I. Doctrine of Special Creation. — Upon this doctrine of the origin of species, it is believed that species are to all practical intents and purposes immutable productions, each of which has been specially created at some point within the area in which we now find it, subsequently spreading from this spot as far as the conditions of life were suitable for it. Each species upon this view has a " specific centre,"^ll^ere it was primitively created, and from which it extended itself over a larger or smaller area, until its progress was stopped by unsuitable conditions. Upon this theory, therefore, if a species is found occupying two widely remote areas, this can only be in consequence of some geological change by which the original area became divided, or in conse- quence of the species having been carried in some acci- dental manner to a considerable distance from its original home. II. Doctrine of Evolution. — On the other hand, it is believed that species are not permanent and immutable, but that they " undergo modification, and that the existing forms of life are the descendants by true generation of pre-existing forms " (Darwin). Upon this view the resemblances which we express by the terms species, genus, family, order, and the like, indicate really the existence of a true blood-relation- ship between the organisms thus grouped together, each group denoting a less and less close degree of relationship as we recede from the ''species" in the direction of the "sub-kingdom." Whilst most naturalists are inclined to admit the truth of the general doctrine of Evolution, as * The author would ask his readers to remember that the mere state- ment of the leading propositions of two opposing theories in no way commits the writer to the support or rejection of either. 136 ELEMENTS OF BIOLOGY. expressed in the above proposition, considerable difference of opinion obtains as to the viethod in which evolution has been brought about. On Lamarck's theory of the evolution of species, the means of modification were ascribed to the action of exter- nal physical agencies, the interbreeding of already existing forms, and the effects of habit, or the use and disuse of cer- tain organs. The doctrine of the evolution of species by variation and *' Natural Selection" — propounded by Mr Darwin, and com- monly knoAvn as the Darwinian theory — is based upon the folldii^ng fundamental propositions : — 1. The progeny of all species of animals and plants exhibit variations amongst themselves in all parts of their organisation, no tvvo individuals being exactly and in all respects alike. In other words, in every species the indi- viduals, whilst inheriting a general likeness to their progeni- tors, tend by variation to diverge from the parent type in some particular or other. 2. Variations arising in any part of the organism, how- ever minute, may be transmitted to future generations, under certain definite and discoverable laws of inheritance. 3. By "artificial selection," or by breeding from indi- viduals possessing any particular variation, man, in succes- sive generations, can produce a breed in which the variation will be permanent, the divergence from the parent type being usually intensified by the process of interbreeding. The races thus artificially produced by men are often as widely different as are distinct species of wild animals. 4. The world in which all living beings are placed is one not absolutely unchanging, but is liable, on the contrary, to subject them to very varying conditions^ 5. All animals and plants give rise to more numerous young than can by any possibility be preserved, each spe- cies tending to increase in numbers in a geometrical progression. ORIGIN OF SPECIES. 1 37 6. As these young are none of them exactly aHke in all respects, a process of " Natural Selection " will ensue, where- by those individuals which possess any variation, however slight, favourable to the peculiarities of the species, will tend to be preserved. Those individuals, on the other hand, which do not possess any such favourable variation, will be placed at a disadvantage in the *' struggle for existence," and will tend to be gradually exterminated. The individuals, therefore, composing any species, are thus subjected to a rigid process of sifting, by which those least adapted to their environment are being perpetually weeded out, whilst " the survival of the fittest " is secured. 7. Other conditions remaining the same, the individuals which survive in the struggle for existence will transmit the variations, to which they owe their preservation, t|| future generations. 8. By a repetition of this process, " varieties " are first established ; these become permanent, and " races " are produced ; finally, in the lapse of time, the differences thus caused become sufficiently marked to constitute distinct *' species," 9. If we grant that past time has been practically infinite, it is conceivable that all the different animals and plants which we see at present upon the globe, may have been produced by the action of Natural Selection upon the off- spring of a few primordial forms, or, it may be, of a single primitive being. Originally, Mr Darwin appears to have believed that *' Natural Selection " would alone be found to be a suffi- cient cause to have given rise to all existing species by a process of Evolution from pre-existing forms. In view, however, of certain objections which had been brought for- ward, Mr Darwin seems to have abandoned this position ; and a cause supplementary to "Natural Selection" was sought for in what Mr Darwin terms " Sexual Selection." The action of Sexual Selection in a supposed process of 138 ELEMENTS OF BIOLOGY. Evolution, according to Mr Darwin's views, may be stated in the following two propositions : — a. The males of many species of animals are known to engage in very severe contests for the possession of the females, these latter yielding themselves to the victor. In such contests certain males will inevitably have certain ad- vantages over the others, either in point of strength or ac- tivity, or in consequence of the possession of more efficient offensive weapons. There will therefore always be a pro- bability that certain males will get possession of the females in preference to others ; and thus there will be a tendency in the individuals of many species of animals to secure a preponderance of offspring from the strongest males. The peculiarities which enable certain males to succeed in these contests will, C(zteris paribus, be transmitted to their male offspring, and in this way variations may be perpetuated, initiated, or intensified. b. In the preceding cases, the females are believed to be perfectly passive, and the selection is a " natural " one, the final result depending solely upon the natural advantages which certain males possess over others in actual combat. It is alleged, ho\wever, that there are other cases in which the selection is truly " sexual," since its result is determined by spontaneous preference, and not by brute force alone. It is asserted, namely, that amongst certain species of ani- mals, the females exercise a free choice as to the particular male with which they will pair; the males being passive agents in the matter, except in so far as each uses, or may use, his utmost exertions to secure that the choice of the female may fall upon him. The circumstances supposed to influence, and ultimately determine, the choice of the female, are of course, in the main, the pergonal attractions of some particular male, the female being captivated by some " beauty of form, colour, odour, or voice," which such a male may possess. If it be admitted that the females of some of the lower ORIGIN OF SPECIES. 1 39 animals have the power of expressing and exercising a pre- ference in the manner above indicated, then it is easy to understand how variations might be transmitted or intensi- fied in this wa)'-. The male who is most attractive to the female, will, other things being equal, have the best chance of propagating his species, and is likely to leave the largest number of descendants. His male offspring will inherit the peculiarities by which their sire was rendered pre-eminently attractive in the eyes of their mother, and thus a well-marked breed might be produced, by the preservation or intensifi- cation of characters of this nature. Mr Darwin is disposed to believe that colour and song in most, if not in all, animals are thus to be ascribed to the action of Sexual Selection, through numerous successive generations ; but other com- petent authorities are unable to concur in this view. Numerous objections have been brought forward to prove the insufficiency of the view that the Evolution of species has been effected by Natural Selection. The student de- sirous of making himself acquainted with this subject should consult Mr Mivart's ' Genesis of Species ; ' but the follow- ing are the chief difficulties which the advocate of Natural Selection has to meet : — I. Natural Selection, whilst doubtless capable of preserv- ing favourable variations, cannot initiate changes of any kind. The origin^ therefore, of variations is not elucidated in any way by the doctrine of Natural Selection, and we are compelled to believe that the variability of the individuals of a species depends upon some internal law with which we are not as yet acquainted. It thus remains open for us to believe that the law which gives rise to variations is in every way a more important one than that under which they are simply preserved. Unfavourable variations must be at least as common as those which are advantageous, and whilst Natural Selection can produce neither, it can at best hvX preserve the latter. It seems clear also that many variations which, when fully developed, are very useful to 140 ELEMENTS OF BIOLOGY. m the species, would, to begin with, be so minute as to be use- less, if not injurious, in which case their preservation and ultimate intensification must have been caused by something else than Natural Selection alone. 2. Whilst Natural Selection cannot 'initiate even the smallest variations, the behef in its being a constant and universal agent in modifying all living beings, requires that variations should be continually occurring, and that they should not be extensive in amount. The probability, how- ever, that all variations depend upon some internal law far below the surface, and unconnected with outside conditions, is greatly increased by the undoubted occurrence of sudden and striking variations, for which no cause can be shown, and for which Natural Selection is unable to account. 3. It has been shown that it is not sufficient for the pro- duction of a new breed or variety simply that a favourable variation should occur, unless the change should occur simultaneously in a greater or less number of individuals of the species. However favourable a variation might be, there would be little or no chance of its being perpetuated, unless it presented itself in more than one individual at the same time. But the probabilities are enormously against the simultaneous appearance of the same variation in numer- ous individuals of a species. We are thus led to doubt if even highly favourable variations would necessarily, or even probably, end in the establishment through natural selection of a permanent new breed or variety. 4. The same parents may give rise to several groups of individuals which differ very widely from one another, and from their parents in their characters, but which are sexless, and are therefore unable to transmit their peculiarities to future generations. Thus the workers and 'the soldiers amongst the Termites differ greatly both from one another and from the fertile individuals, both in their actual struc- ture and in their instincts ; and yet both are neuter and have no power of transmitting their peculiarities by the ORIGIN OF SPECIES. 141 way of inheritance. Yet it is only by the medium of her- edity that Natural Selection can possibly act. 5. Whilst it is undeniable that the individuals composing any species vary more or less amongst themselves, there is no proof that the variability of any species is indefinite. On the contrary, there are reasons to believe that each species is bounded by an uncertain but definite range of variability. The extreme terms of this range may lie very far apart, but between these runs somewhere a normal line or '* line of safety," which is occupied by those individuals which may be regarded as the type of the species. The doctrine how- ever, of the evolution of species by natural selection de- mands our assent to the belief that the variability of a species is indefinite. 6. The theory of the evolution of species by natural selec- tion implies of necessity that one species can only be con- verted into another through the medium of a great number of successive forms, graduating into one another, each member of the series differing from its immediate neighbours in but minute characters. If, therefore, any existing species has descended from any pre-existing species, there must at one time have existed between the two species a graduated series of intermediate forms. When we consider the enor- mous number of living animals and plants, and the still more enormous number of extinct forms which we know, or may infer, to have existed in past time, it becomes clear — if evolution be true — that the number of minutely inter- mediate forms must have been incalculably great. We have therefore the clear right to expect that Palaeontology should reveal to us such intermediate forms, amongst the vast series of fossil remains with which we are acquainted. We cannot, however, in any case point to such forms. It is quite true that there are many instances in which fossil animals may be regarded as intermediate forms between great groups of living forms, as missing links in the zoological chain. Such intermediate forms, however, are 142 ELEMENTS OF BIOLOGY. invariably sharply separated from the forms which they connect ; and no case is yet known to us, even taking the Tertiary period alone, in which we can point to a graduated series of intermediate forms, by which one well-marked species can be shown to pass into another equally well- marked species. 7. The changes in the life of the globe revealed to us by geology are so vast and so numerous that the imagination is utterly powerless to grasp the inconceivable lapse of time required for the bringing about of these changes by the tardy action of natural selection alone. Physical geology teaches us that geological time is something as inconceiv- ably vast as astronomical space ; but it may fairly be doubted if the utmost lapse of time required by the phe- nomena of physical geology can be regarded as more than a mere drop in the ocean, as compared with the time re- quired for the zoological revolutions indicated by the study of Palaeontology — if these revolutions have been brought about by the action of natural selection. It can hardly be reasonably asserted that the time necessary for such biolo- gical changes is fixed by physical geology alone ; and that if this latter informs us that the geological changes of the earth have taken place within a given limited period, then we must simply change our beliefs as to the time required for the conversion of one species into another by natural selection. This certainly appears to be a species of reason- ing in a circle. The very essence of the theory of." Evolu- tion by Natural Selection " is the almost entire impossibility of one species being converted into another otherwise than by an extremely slow process, during which a vast number of generations lived and died. AVe have also, upon the doctrine of " the adequacy of existing causes," certain definite data as to the duration of species. For we know that many existing species have lived without change during what may justly be considered a very vast period of time. It is therefore for Evolution to say how long a period is ORIGIN OF SPECIES. 143 required for the biological revolutions which we know to have occurred since the Laurentian period ; and if physical geology or astronomy can show that the period demanded is too great, Evolution will hardly evade the difficulty by shortening the time required for the conversion of one species into another. At present, however, it can only be said that whilst physical geology does not absolutely need the time demanded by the theory of Evolution, there is nothing in the facts of the former which would forbid our yielding to the requirements of the latter. There are, on the other hand, good grounds to be drawn from other de- partments of physical science, as shown by Sir William Thomson, for the belief that the period which has elapsed since the introduction of life upon the earth is much below that which is required by the theory of evolution by natural selection. CHAPTER XIV. DISTRIBUTION IN SPACE. Under the general term of "Distribution" come all^the facts concerning the external or objective relations of ani- mals — that is to say, their relations to the external conditions by which they are surrounded. The geographical distribution of animals is concerned with the determination of the areas within which every species of animal is at the present day confined. Some species are found almost everywhere, when they are said to be " cos- mopolitan ; " but, as a rule, each species is confined to a limited and definite area. Not only are species limited in their distribution, but it is possible to divide the earth's sur- face into a certain number of geographical regions or " zoo- logical provinces," each of which is characterised by the occurrence in it of certain associated forms of animal life. The number of these provinces has not yet been universally agreed upon, and it is unnecessary here to enter into this subject in detail. There are, however, some general con- siderations which may be briefly alluded to. The geographical distribution of land animals is condi- tioned partly by the existence of suitable surroundings, and partly by the presence of barriers preventing migrations. Thus, certain contiguous regions might be equally suitable for the existence of the same animals, but they might belong DISTRIBUTION IN SPACE. I45 to different zoological provinces, if separated by any impass- able barrier, such as a lofty chain of mountains. Owing to their power of flight, the geographical distribution of birds is much less limited than that of mammals; and many migra- tory birds may be said to belong to two zoological provinces. In spite of their powers of locomotion, however, birds are limited by the necessities of their life to definite areas, and a zoological province may be marked by its birds just as well as by its quadrupeds. The geographical distribution of an animal at the present day by no means necessarily coincides with its former ex- tension in space. Many species are known which now occupy a much more restricted area than they did formerly, owing to changes in climate, the agency of man, or other causes. Similarly, there are species whose present area is much wider'than it was originally. Zoological provinces must always have existed ; but those of the present day by no means correspond with those of former periods of the earth's history, but are, on the con- trary, of comparatively recent origin. As regards the Mammals, the sztciq fonns are found occu- pying the same regions in the later Tertiary period as they do at present ; but the species are difi"erent. The distribu- tion, therefore, of certain groups, dates back to a period an- terior to the appearance of the now existing species of the same groups. Thus, to take a single example. South Ame- rica at the present day has amongst its many peculiar animals none more characteristic than the Sloths and Armadillos {Edentata). In late Tertiary time, however, Edentate ani- mals were equally characteristic of the South American fauna, though none of the living species then existed. Thus, the modern Sloths are represented by the gigantic Megathe- rium^ Afylodofi, and Megalonyx, and the little armour-plated Armadillos find their ancient representative in the colossal Glyptodon. It is to be remembered, however, that the law thus indicated holds good for the later Tertiary period only, 146 ELEMENTS OF BIOLOGY. and does not apply in any manner that admits of being traced to early geological epochs. The general result of this law is, that existing zoological provinces are in some cases older than the species by which they are now char- acterised. The vertical 01 /^^z/^)''^'^^^^''^^'^ distribution of animals relates to the limits of depth within which each marine species is confined. In many cases it is found that marine animals occupy definite bathymetrical zones, existence being impos- sible, or at any rate difficult, at depths greater or less than those comprised within the limits of the zone which each inhabits. In accordance with the facts at that time known, naturalists formerly accepted the following four bathymetrical zones, as being characterised each by its peculiar fauna : — T. The Littoral zone, or the tract between tide-marks. 2. The Laminarian zone, from low water to 15 fathoms. 3. The Coralline zone, from 15 to 50 fathoms. 4. The Deep-sea coral zone, from 50 to 100 fathoms or more. Beyond a depth of something between 100 and 200 fa- thoms it was formerly believed that marine life did not ex- tend. Recent researches, however, especially those by Drs Carpenter and Wyville Thomson and Mr Gwyn Jeffreys, have greatly modified the above generalisation, and have led to the establishment of conclusions of the greatest import- ance and interest. The value of the Littoral zone, or the tract between tide-marks, as a marine province, has not been aftected by these discoveries, but the importance of the others has been greatly reduced ; and we might well adopt the views of Mr Gwyn Jeffreys, and consider that there are but two chief bathymetrical zones, the littoral and the sub- marifte. The next important point which has been brought to light is, that life extends to all depths in the ocean, marine animals having been dredged in abundance from a depth of 2300 fathoms, or not far short of three miles. If, there- DISTRIBUTION IN SPACE. I47 fore, we are to retain the four zones above mentioned, we must now add to these a fifth or Abyssal zone, extending from 100 fathoms up to at least 2500 fathoms, and doubt- less really extending to all depths in the ocean. The most important result, however, of these inquiries is the discovery of the fact that, beyond a very limited depth, the distribution of marine animals is conditioned, not by the depth of the water, but by its temperature. Thus the bat/iy- metrical distribution is truly a thermometrical one. Similar forms, namely, are found inhabiting areas in which the bottom-temperature is the same, wholly irrespective of the depth of water. It may happen, therefore, that two dis- tinct faunae may inhabit contiguous areas of the sea-bottom, and may be even sharply marked off from one another, as when on.e area is swept by a warm current, whilst a neigh- bouring area has its temperature lowered by a cold current. The conditions under which the animals of the deep sea live, are so different to those to which the inhabitants of shallow waters are subjected, that a few remarks upon this subject may advantageously be added here. It was formerly beHeved that the pressure of the water at great depths would be so enormous as to preclude the pos- sibility of life being present. This, however, is a fallacy ; since the internal pressure of any body immersed in a fluid, and admitting fluid into its interior, is in all cases the exact equivalent of the external pressure. In other words, marine animals are in this respect- in the same position as an un- corked bottle sunk at the bottom of the sea. AVhatever the depth may be, there is no pressure upon the sides of the bottle, because the pressure of the water outside the bottle is exactly neutralised by the pressure of the water in its interior. In the second place, it is a well-known generalisation that animals are, mediately or immediately, dependent upon plants for their subsistence. Plants, however, can- not exist unless supplied with solar light, and there is 148 ELEMENTS OF BIOLOGY. reason to conclude that the sun's rays can at most but penetrate to a depth of a few hundred feet below the sur- face of the sea. In the absence, therefore, of any positive knowledge, it was a justifiable conclusion that animal life could not extend to very great depths in the ocean, since vegetable life would of necessity be absent. In the deep sea, however, we find an assemblage of animals, not essen- tially different from those of shallow seas, living without, or . almost without, vegetable life of any kind. A few micro- scopical plants there may possibly be ; but unquestionably there is nothing that could for one moment be regarded as supplying vegetable food to any considerable number of animals. The question then arises, How do these animals support existence ? Some, of course, feed upon the others ; but, equally of course, this must have a limit ; and there must be some which have the means of obtaining food in some other manner. Two explanations have been put forward to account for this singular fact. On the one hand, it has been thought that some of the deep-sea animals might perhaps have the power possessed by plants, of taking inorganic substances from the surrounding medium, and building up these into the matter of life. This theory, if provable, would be all that is needed ; because then, in point of fact, some of the animals of the deep sea, as regards their mode of feeding, would be really plants, and thus the balance of organic nature would be maintained in equilibrium. This, however, is a mere hypothesis ; and it has been shown, on the other hand, that the sea-water at great depths holds in solution a very much larger propor- tion of organic matter than is normally present ; so that it may practically be regarded as a very dilute soup. It has therefore been suggested, with great probability, that the lower forms of life in these abysses can support life solely upon this dissolved and soluble organic matter. Thirdly, it might have been reasonably anticipated that the water at great depths in the ocean would have been DISTRIBUTION IN SPACE. I49 devoid of the oxygen necessary for the support of animal life. The sea-water is mainly oxygenated by the agitation of the waves, and this extends but to a very limited depth below the surface. It is now known, however, that the depths of the ocean, though tranquil and undisturbed by storms, are nevertheless renovated by a vast and com- plex system of oceanic currents. In this way the oxy- genated life- supporting surface-water is being constantly transferred from the face of the deep to take the place of the airless strata of the abysses, from which the oxygen ' has been removed by the agency of living beings. Lastly, we have to consider how the animals of the deep sea manage to exist in the absence of light. For plant-life light is absolutely essential ; and though it is possible for animals to exist in total darkness, the cases in which this occurs are few, and the absence of light is generally ac- companied by the loss of organs of vision. As a general rule, however, light is all -important to animal life in its higher developments, if only for the reason that without light the predaceous animals could not see to capture their prey. Without dogmatising as to the depth below the surface to which light may penetrate, it seems certain that Egyptian darkness must prevail at all depths below a few hundred fathoms. This would at once account for the absence in the deep sea of all vegetable life, with the exception of such microscopic plants as most probably live at or near the surface, and only fall to the bottom when dead. Nevertheless, in the face of this, we find animals living at a depth of more than two thousand fathoms with perfect and well-developed eyes, as perfect as the organs of vision possessed by animals living in illuminated regions. It has been suggested by Sir Charles Lyell, as an explanation of this fact, that the deep-sea animals are enabled to see by their own phosphorescence. It is certain that many of them phosphoresce brilliantly, and in the absence of any other source of light it seems almost certain that they must owe 150 ELEMENTS OF BIOLOGY. their means of vision to this property. If this be the case, v/e have here one of the most wonderful adaptations in the whole range of animated nature, by which life is rendered possible amidst the most apparently hostile conditions. Good authorities, however, are indisposed to accept this view, and some other explanation of the facts may yet be found. CHAPTER XV. DISTRIBUTION IN TIME. All the facts which concern the existence of living beings in past periods of the earth's history come under the general head of " Distribution in Time." The laws of distribution in time are, however, from the nature of the case, less perfectly known than are the laws of lateral or vertical distribution, since these latter concern beings which we are able to examine directly. The follow- ing are the chief facts which it is necessary for the student to bear in mind : — 1. The rocks which compose the crust of the earth have been formed at successive periods, and may be roughly divided into aqueous or sedimentary rocks, and igneous rocks. 2. The igneous rocks are produced by the agency of heat, are mostly tmstnatijied (/. ^., are not deposited in distinct layers or strata), and, with itw exceptions, are destitute of any traces of past life. 3. The sedimentary or aqueous rocks owe their origin to the action of water, are stratified (i. e., consist of separate layers or strata), and mostly exhibit " fossils " — that is to say, the remains or traces of animals or plants which were in existence at the time when the rocks were deposited. 4. The series of aqueous rocks is capable of being divided 152 ELEMENTS OF BIOLOGY. into a number of definite groups of strata, which are technically called ^' formations." 5. Each of these definite rock-groups, or " formations,"' is characterised by the occurrence of an assemblage of fossil remains more or less peculiar and confined to itself. 6. The majority of these fossil forms are " extinct ; " that is to say, they do not admit of being referred to any species at present existing. 7. No fossil, however, is known, which cannot be referred to one or other of the primary subdivisions of the Animal Kingdom, which are represented at the present day. 8. When a species has once died out it never reappears. 9. The older the formation, the greater is the divergence between its fossils and the animals and plants now existing on the globe. 10. All the known formations are divided into three great groups, termed respectively Palaeozoic or Primary, Mesozoic or Secondary, and Kainozoic or Tertiary. The Palaeozoic or Ancient-life period is the oldest, and is characterised by the marked divergence of the life of the period from all existing forms. In the Mesozoic or Middle-life period, the general fades of the fossils approaches more nearly to that of our existing fauna and flora; but — with very few exceptions — the characteristic fossils are all specifically distinct from all exist- ing forms. In the Kainozoic or New-life period, the approximation of the fossil remains to existing living beings is still closer, and some of the forms are now specifically identical with recent species ; the number of these increasing rapidly as we ascend from the lowest Kainozoic deposit to the Recent period. DISTRIBUTION IN TIME. 153 Ideal Section of the Crust of the Earth. o o < )4 u o o CO o S) O < PL. fig- 39- I'ost-Tcrliary and Recent. Pliocene. '^^yzr-'.'zsl )p Miocene. Eocene. .'_'L — j'^ .•!_ 1 L_ :'--_■' — [ Crjlaceous. ■=• Oolitic or Juiassi assic II n 11 I' . I' II II ti II li li li il il II II u M JO COocQOC'fOU OQ 0° o O » " , 11, , II , II ,11, :l I! II li II II 11 ll II II II I I I! il I' II ' I I '11 It" I I ' Silurian. Triassic. y? Permian. Carboniferous. Devonian or Old Red Sandstone. nan. Iluronian. Lauren tian. 154 ELEMENTS OF BIOLOGY. Subjoined is a table giving the more important subdivi- sions of the three great geological periods, commencing with the oldest rocks and ascending to the present day. (See fig. 39.) I. Palaeozoic or Primary Rocks. 1. Laurentian. (Lower and Upper.) 2. Cambrian. (Lower and Upper, with Huronian Rocks?) 3. Silurian. (Lower and Upper.) 4. Devonian, or Old Red Sandstone. (Lower, Middle, and Upper.) 5. Carboniferous. (Mountain-limestone, Millstone Grit, and Coal-measures.) 6. Permian. ( = The lower portion of the New Red Sandstone.) IL Mesozoic or Secondary Rocks. ' 7. Triassic Rocks. (Bunter Sandstein, or Lower Trias ; Muschelkalk, or Middle Trias; Keuper, or Upper Trias.) 8. Jurassic Rocks. (Lias, Inferior Oolite, Great Oolite, Oxford Clay, Coral Rag, Kimmeridge Clay, Portland Stone, Purbeck beds.) 9. Cretaceous Rooks. (Wealden, Lower Greensand, Gault, Upper Greensand, White Chalk, Maestricht beds.) IIL Kainozoic or Tertiary Rocks. 10. Eocene. (Lower, Middle, and Upper.) 11. Miocene. (Lower and Upper.) 12. Pliocene. (Older Pliocene and Newer Pliocene.) 13. Post-tertiary. (Post-pliocene and Recent.) « Contemporaneity of Strata. — When groups of beds in different regions contain the same fossils, or rather an assemblage of fossils in which many identical forms occur, they are ordinarily said to be "contemporaneous;" that is to say, they are ordinarily supposed to have been formed m DISTRIBUTION IN TIME. 155 at the same period in the history of the earth, and belong to the same geological epoch. This statement, however, can only be received with some important qualifications. Beds containing the same specific forms are often so widely removed from one another in point of distance, and occur at so many different points of the earth's surface, that it becomes inconceivable that they are "contemporaneous" in the Hteral sense of this term. Such a supposition would imply an ocean not only more widely extended but presenting more uniform conditions than any with which we are at the present day acquainted. Besides, we know that strictly contemporaneous beds would rarely contain exactly the same species of fossils. Thus, if we could examine the bed of the Atlantic, we should un- doubtedly find it occupied by a series of deposits which would be "contemporaneous" in the strictest sense of the term, but they would neither have the same mineral char- acters, nor contain the same or even nearly-related fossils. Some of the deposits, for example, would consist of chalky beds, crowded with Foraminifera, Siliceous Sponges, Crinoids, and Sea-urchins. Others would be composed of sand and mud, and would contain the remains of Arctic shells. Others, again, would have the characters of shore-deposits, and would yield the remains of littoral animals. If this be the case with a single ocean, such as the Atlantic, still more is it the case when we consider all the oceans of the globe, the deposits of which are nevertheless contemporaneous, in the sense that they have been formed at the same time. Contemporaneous beds, then, if separated from one another in point of distance, are by no means likely to contain the same species of fossils. We are thus driven to seek for another explanation of the fact that specifically identical fossils are often found in formations very widely removed from one another. The true explanation of this fact is to be sought in the phenomenon of " migration." If we imagine a given assemblage of animals to be inhabit- 156 ELEMENTS OF BIOLOGY. ing a given area of the sea-bottom, and we suppose the con- ditions of that area to be changed for the worse, either by an elevation of the sea-bottom or from any other cause, a migratio?i of the fauna will be set on foot. The locomotive animals will shift their quarters in search of some other area in which the conditions are more favourable to their exist- ence. As sedentary animals have almost universally loco- motive young, we may from this point of view regard all the animals of such an area as capable of migrating. A general migration of the fauna of the area will commence, and in this way some of the species of the area will be transferred to another area. By a repetition of this process the same species may ultimately come to inhabit an area removed by a hemisphere from its original habitat ; and in this way the same species may present itself in beds at the most distant parts of the earth's surface. It is quite clear, however, from the above, that the iden- tity of fossils in widely distant strata, is, upon the whole, a proof that the beds in question are jaot strictly contempo- raneous. A migration is a work of time, and one of the two sets of beds must obviously and necessarily be younger than the other by the period consumed in the migration. Still the interval between two such sets of beds would not be long, geologically speaking, and both groups of strata would belong approximately to the same geological horizon. If, therefore, we still apply the name of " contemporaneous'' to beds which contain the same fossils but are widely separated from one another in point of distance, we must do so on the clear understanding that the term must be taken in a wider and looser sense than that in which it is ordinarily employed. Geological Continuity. — The entire series of Stratified or Fossiliferous rocks, as before remarked, admits of a natu- ral division into a certain number of definite "rock-groups" or " formations," each of which is characterised by a peculiar and distinctive assemblage of fossils, constituting the "life" DISTRIBUTION IN TIME. I 57 of the period in which the formation was deposited. It is a matter of importance to understand clearly how far these subdivisions are natural, and what value we may attach to them. The older and very natural view held that the close of each formation was signalised by a general destruction of all the forms of life characteristic of the period, and that the commencement of each new formation was accompanied by the creation of a number of new forms. On the more modern view, it is held that the great formations, and many of the minor subdivisions, are separated by longer or shorter lapses of time not represented by any deposition of rock in the area where the formations in question are in contact. Upon this view we have to admit that what we call the great " formations " are purely artificial divisions rendered possible by the gaps in our knowledge only ; and that if we had a complete series of rock-groups, we could have no such lines of demarcation. It is unnecessary to consider here why it is that we can never hope to find a complete series of intermediate rock- groups by which any two great formations might be linked together. It is sufllicient to say that we may well have the strong conviction that such intermediate deposits have at one time existed, or must still exist, whilst there are per- fectly valid reasons for the belief that we can never know more than fragments of them. Most modern geologists, then, would hold that there is a geological " continuity," such as we see in other departments of nature. There can have been in reality no break in the great series of stratified deposits ; but there must have been a complete "continuity" of life and deposition from the Laurentian period to the present day. There was, and could have been, no such continuity in any one area; but it is inconceivable that the chain should have been snapped at one point and again taken up at another wholly different one. The links of the chain may, indeed must, have been forged in different places, but its continuity must neverthe- «8 • 158 ELEMENTS OF BIOLOGY. less have remained unbroken. From this point of view there would be little impropriety in saying that we are still living in the Silurian period ; but we could only say so in a very limited sense. Most geologists would freely admit that there must in nature have been an actual continuity of the great geological periods. Nevertheless it remains cer- tain that we can never dispense with the division of the stratified series into definite rock-groups and life-periods. We can never hope to discover all the lost links of the geological chain ; and the great formations are likely ever to remain separated by more or less pronounced physical or palasontological breaks, or both combined. The utmost we can at present do is to arrive at the conviction that the lines of demarcation between the great formations only mark gaps in our knowledge, and that there can be truly no hiatus in the long series of fossiliferous deposits. Imperfection of the Pal^ontological Record. — As has been 'just pointed out, the series of the stratified formations is to be regarded as an imperfect one, in which many links are missing. The causes of this " imperfection of the geological record," as it has been termed by Darwin, are various ; but the most important ones are our as yet limited knowledge of vast areas of the earth's surface, the process of denudation, and the fact that many of the miss- ing groups are buried beneath other deposits, whilst more than half of the superficies of the globe is hidden from us by the waters of the sea. The imperfection of the geological record necessarily implies an equal imperfection of the " palseontological record ; " but, in truth, the record of life is far more imperfect than the mere physical series of de- posits. The following are the chief causes of the imperfec- tion of the palaeontological record : — I. In the first place, even if the series of the stratified deposits had been preserved to us in its entirety, and we could point to sedimentary accumulations belonging to every period in the earth's history, there would still have been DISTRIBUTION IN TIME. ' 1 59 enormous gaps in the palseontological record, owing to the different faciHties with which different animals may be pre- served as fossils. It is impossible here to enter at length into this subject ; but there are obvious reasons why certain groups of animals should never be found as fossils, or should at best be but sparingly and imperfectly represented. Thus, many animals are entirely soft-bodied, destitute of hard parts capable of being preserved in a fossil condition, and we can therefore never obtain evidence of the past existence of such forms, though this affords no presump- tion that they were non-existent at any given period. Again, most sedimentary deposits have been laid down in the sea, and contain, therefore, the remains of marine animals, if not exclusively, at any rate in preponderating numbers. Marine groups of animals are therefore much more likely to be preserved than the inhabitants of lakes or rivers. Lastly, almost all sedimentary accumulations have been deposited in water, whether salt or fresh; and it follows from this that the preservation of terrestrial or aerial animals must always have been of an accidental nature, so to speak, depending upon the chance falling of such an animal into water where sediment was being accumulated. It is only in the rare cases in which an old land-surface has been pre- served to us that we meet with the remains of such animals as fossils properly belonging to the deposit in which they occur. 2. In the second place, as shown by the imperfection of the geological record, there are vast periods in the earth's history which are not known to us to be represented by any deposits. This of necessity leads to our being totally ignorant of the life of these same periods. As already remarked, we can never expect wholly to fill up these periods of "unrepresented time" by the discovery of new deposits, and our palaeontological knowledge will therefore ever remain more or less interrupted and incomplete. 3. In the third place, we can seldom or never point to l6o ELEMENTS OF BIOLOGY. « more than one or two classes of the deposits which must have been formed in every great period. We may have the deep-sea deposits of the period, or the littoral accumu- lations, or the sediments which were laid down in its rivers and lakes ; but we very seldom, if ever, obtain all of these. AVe can therefore rarely expect to acquire a complete knowledge of even the aquatic animals alone of any period. 4. Lastly, we have every reason to believe that the life of vast periods of the earth's history will ever remain to us wholly, or almost wholly, unknown, in consequence of the fact that the deposits of these periods have been subjected to such change that all traces of their contained fossils have been destroyed INDEX. Abiogenesis, 127; experiments of Bas- tian on, 131-133. Abyssal zone, 147. Acrojence, 42. Actinozoa, 37. Air, as a condition of life, 14. Alliumen, 67, 69. Altemation of generations, 104, 112, 113, 125. Amoeba, 9, 17, 2S, 29, 30. Amphibia, 41. Analogy, 44. Anarthropoda, 3S. Angiosperms, 43 ; reproduction of, 124- 126. Animal functions, 27, 77. Animals, form of, 20; internal structure of, 21 ; chemical composition of, 21 ; motor power of, 22; food of, 23; re- spiration of, 24. Animals and plants, differences between, 19-25. Annelida, 3S. Annuloida, 34; definition of, 37. Annulosa, 3i; definition of, 33. Anophyta, 42. Aphides, parthenogenesis of, 114, 115. Arachnida, 39. Armadillos, 145, Arthropoda, 39. Ascidians, cellulose in, 21. As.similation, 2, So. Atrophy, 88. Bacteria, 14, 128, 132. Bathybius, 24. Bathyraetrical distribution, 146. Bees, parthenogenesis of, 115, 116. Biology, definition of, 1. Bioplasm, 7, 70; nature of, 71; move- ments of, 71. Brachiopoda, 30. Cacti, 51. Campanularia, 107. Caseine, 67, 69. Cells, 72; wall of, 73; contents of, 74; nucleus of, 74 ; multiplication of, 75 ; life of, 79. Cellulose, 21, 69. Cephalopoda, 40. Chcetognatha, 39. Chemistry, of living beings, 64, 65; of animals, 65, 68; of vegetables, 68, 69. Chlorophyll, 22. Class, definition of, 62. Classification, 66; linear, 62. Clytia, 107. Coelenterata, 22, 34 ; definition of, 36. Conditions of life in the deep sea, 147-150. Contemporaneity of strata, 154-156. Continuity, geological, 156-158. Correlation, functions of (see Relation). Correlation of growth, 54, 55. Crustacea, 39. Cryptogams, 42; reproduction of, 124. Cytogenesis, 75, 76. Darwintan theory, 136, 137; objec- tions to, 139-143. Dead bodies, chemical composition of, 4; form of, 5; arrangement of parts of, 5, Dead and living bodies, differences be- tween, 2. Death, 15, 88. Deep sea, condition of life in, 147-150. Desmids, 22. Development, 89, 91 ; Von Baer's law of, 93 ; retrograde, 95. Diatoms, 22. Dicotyledons, 43. Dictyogence, 42. T)ifferences between different orgauism^, 26. Diraorphio plants, 60. Distribution, 144; in. space, 144; geo- graphical, 144-146 ; b'athjTnetrioal,14tt- 150; in time, 151-160. EcMnodermata, 37. Edentata, distribution of, 146. 1 62 INDEX. Embryology, 26. Undogence, 42. Endogenous cell-multiplication, 75. Epizoa, 95. EuphorbUe, 51. Evolution, theory of, 94, 135; views of Lamarck on, 136; views of Darwin on, 136. Family, definition of, 62. Ferns, reproduction of, 124-126. Fibrine, 67, 69. Fission, 98. 101; of Param§2.50. "This long-expected work will be cordially welcomed by all students and teachers of Co nparaiive Anatomy as a compendious, reliable, and, notwithstand- ing its small dimensions, most comprehensive guide on the subject of which it treats. To praiic or to criticise the work of so accomplished a master of Ids favorite science would be equally out of place. It is enough to say that it realizes in a remarkable degree the anticipations which have been formed of it ; and th.it it presents an extraordinary combination of wide, general views, with the clear, accurate, and succinct statement of a prodigious number of individual facte.'" — Nature. THE WOELD BEFOEE THE DELUGE. By Louis FiGUiER. The Geolog;ical portion newly revised by II. W. Bristow, F. R. S., of the Geological Survey of Great Britain, Hon. 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