ps ii? «T PRIVATE LIBRARY WILSON R. GAY j $ -Yfl casr$.... W . 1)1 $; If I am generous enough to loan you this jj JJJ /;r;c;/', please be thoughtful enough to re- | ^ ///;•// //, witJiout delaying, until invited \ $ to do so. Never take it, or he eh it with- \ w ' « S °u.t my conse*lt<> as such too often engen- « 5g rtfe/vy /^c?^/ fee /ings. This is simply « | " Business." I SEATTLE, WASH. \ THE PRINCIPLES OF BIOLOGY BY HERBERT SPENCER AUTHOR OF SOCIAL STATICS, EDUCATION, STUDY OF SOCIOLOGY ESSAYS : SCIENTIFIC, POLITICAL, AND SPECULATIVE, FACTORS OF ORGANIC EVOLUTION, ETC. IN TWO VOLUMES VOL. II N E W YORK D. APPLETON AND COMPANY 1806 COPYRIGHT, 1867, BY D. zVPPLETON AND COMPANY. PREFACE TO VOL II. THE proof sheets of this volume, like those of the last volume, have heen looked through by Dr. Hooker and Prof. Huxley ; and I have, as before, to thank them for their valuable criticisms, and for the trouble they have taken in checking the numerous statements of fact on which the argu ments proceed. The consciousness that their many duties render time extremely precious to them, makes me feel how heavy is my obligation. Part IV., with which this volume commences, contains numerous figures. Nearly one half of them are repetitions, mostly altered in scale and simplified in execution, of figures, or parts of figures, contained in the works of various Botanists and Zoologists. Among the authors whom I have laid under contribution, I may name Berkeley, Carpenter, Cuvier, Green, Harvey, Hooker, Huxley, Milne-Edwards, Ralfs, Smith. The remaining figures, numbering 150, are from original sketches and diagrams. The successive instalments which compose this volume, were issued to the Subscribers at the following dates : — No. 13 (pp. 1 — 80) in January, 1805 ; No. 14 (pp. 81 — 100) in June, 1865 ; No. 15 (pp. 101—240) in December, 1805 ; No. 10 (pp. 241—320) in June, 1800 ; No. 17 (pp. 321—400) in November, 1800 ; and No. IS (pp. 401—500) in March, 1807. London. March 23r^, 1867. CONTENTS OF VOL. II. PART IV.— MORPHOLOGICAL DEVELOPMENT. CHAP. PAGE I. — THE PROBLEMS OF MORPHOLOGY . 3 II. — THE MORPHOLOGICAL COMPOSITION OF PLANTS . 10 III. THE MORPHOLOGICAL COMPOSITION OF PLANTS, CONTINUED . . . . . . .28 IV. THE MORPHOLOGICAL COMPOSITION OF ANIMALS . 77 V. THE MORPHOLOGICAL COMPOSITION OF ANIMALS, CONTINUED . . . . . . .09 VI. MORPHOLOGICAL DIFFERENTIATION IN PLANTS . 113 VII. THE GENERAL SHAPES OF PLANTS . . .119 VIII. THE SHAPES OF BRANCHES ..... 130 IX. THE SHAPES OF LEAVES 137 X. THE SHAPES OF FLOWERS . . . . .14:6 XL THE SHAPES OF VEGETAL CELLS . . . 159 XII. CHANGES OF SHAPE OTHERWISE CAUSED . . 102 XIII. MORPHOLOGICAL DIFFERENTIATION IN ANIMALS . 106 XIV. THE GENERAL SHAPES OF ANIMALS . . . 109 XV. THE SHAPES OF VERTEBRATE SKELETONS . . 192 XVI. THE SHAPES OF ANIMAL CELLS . . . .210 XVII. SUMMARY OF MORPHOLOGICAL DEVELOPMENT . 213 PART V.— PHYSIOLOGICAL DEVELOPMENT. I. THE PROBLEMS OF PHYSIOLOGY .... 221 II. DIFFERENTIATIONS BETWEEN THE OUTER AND IN NER TISSUES OF PLANTS .... 220 III. DIFFERENTIATIONS AMONG THE OUTER TISSUES OF PLANTS ....... 2-13 IV. DIFFERENTIATIONS AMONG THE INNER TISSUES OF PLANTS . . . . . . .252 V. PHYSIOLOGICAL INTEGRATION IN PLANTS . .275 vii Vlll CONTENTS. CHAP. PAGE VI. — DIFFERENTIATIONS BETWEEN THE OUTER AND IN NER TISSUES OF ANIMALS .... 282 VII. — DIFFERENTIATIONS AMONG THE OUTER TISSUES OF ANIMALS . . . . . 291 VIII. DIFFERENTIATIONS AMONG THE INNER TISSUES OF ANIMALS 310 IX. PHYSIOLOGICAL INTEGRATION IN ANIMALS . . 365 X. SUMMARY OF PHYSIOLOGICAL DEVELOPMENT . . 377 PART VL— LAWS OF MULTIPLICATION. I. THE FACTORS ... ... 391 II. A PRIORI PRINCIPLE . ... 397 III. OBVERSE A PRIORI PRINCIPLE .... 404: IV. DIFFICULTIES OF INDUCTIVE VERIFICATION . . 412 V. — ANTAGONISM BETWEEN GROWTH AND ASEXUAL GENESIS . ... 419 VI. — ANTAGONISM BETWEEN GROWTH AND SEXUAL GENESIS ....... 428 VII. ANTAGONISM BETWEEN DEVELOPMENT AND GEN ESIS, ASEXUAL AND SEXUAL . . . 440 VIII. ANTAGONISM BETWEEN EXPENDITURE AND GENESIS 440 IX. COINCIDENCE BETWEEN HIGH NUTRITION AND GEN ESIS ... . 454 X. SPECIALTIES OF THESE RELATIONS . .463 XL — INTERPRETATION AND QUALIFICATION . . 470 XII. MULTIPLICATION OF THE HUMAN RACE . . 479 XIII. — HUMAN POPULATION IN THE FUTURE . . . 494 APPENDICES. A. — SUBSTITUTION OF AXIAL FOR FOLIAR ORGANS IN PLANTS ....... 511 B. A CRITICISM ON PROF. OWEN^S THEORY OF THE VER TEBRATE SKELETON 517 C. — ON CIRCULATION AND THE FORMATION OF WOOD IN PLANTS . . . .536 PART IV. MORPHOLOGICAL DEVELOPMENT. CHAPTER I. THE PROBLEMS OF MORPHOLOGY. § 175. THE division of Morphology from Physiology, is one which may be tolerably- well preserved, so long as we do not carry our inquiries beyond the empirical generalizations of their respective phenomena ; but it is one which becomes in great measure nominal, when the phenomena are to be rationally interpreted. It would be possible, after analyzing our Solar System, to set down certain general truths respect ing the sizes and distances of its primary and secondary members, omitting all mention of their motions ; and it would be possible to set down certain other general truths respect ing their motions, without specifying their dimensions or positions, further than as greater or less, nearer or more re mote. But on seeking to account for these general truths, arrived at by induction, we find ourselves obliged to con sider simultaneously the relative sizes and places of the masses, and the relative amounts and directions of their motions. Similarly with organisms. Though we may frame sundry comprehensive propositions respecting the arrange ments of their organs, considered as so many inert parts ; and though we may establish several wide conclusions respecting the separate and combined actions of their organs, without knowing anything definite respecting the forms and positions of these organs ; yet we cannot reach such a rationale of the 4 MORI-' IIO LOGICAL DEVELOPMENT. facts as the hypothesis of Evolution aims at, without content plating structures and functions in their mutual relations. Everywhere structures in great measure determine functions ; and everywhere functions are incessantly modify ing structures. In Nature, the two are inseparable co-operators ; and Science can give no true interpretation of Nature, without keeping their co-operation constantly in view. An account of organic evolution, in its more special aspects, must be essentially an account of the inter-actions of structures and functions, as perpetually altered by changes of conditions. Hence, when, treating apart Morphological Development and Physiological Development, all we can do is to direct our attention mainly to the one or to the other, as the case may be. In dealing with the facts of structure, we shall consider the facts of function, only in such general way as is needful to explain the facts of structure ; and conversely when deal ing with the facts of function. § 176. The problems of Morphology fall into two distinct classes, answering respectively to the two leading a;pects of Evolution. In things which evolve there go on two processes — increase of mass and increase of structure. Increase of mass is primary, and in simple evolution takes place almost alone. Increase of structure is secondary, accompanying or following increase of mass with more or less regularity, wherever evolution rises above that form which small inor ganic bodies, such as crystals, present to us. The fundamental antagonism between Dissolution and Evolution consisting in this, that while the one is an integration of motion and dis integration of matter, the other is an integration of matter and disintegration of motion ; and this integration of matter accompanying disintegration of motion, being a necessary antecedent to the differentiation of the matter so inte. grated ; it follows that questions concerning the mode in which the parts are united into a whole, must be dealt with THE PROBLEMS OF MORPHOLOGY. O before questions concerning the mode in which these parta become modified.* This is not obviously a morphological question. But an illustration or two will make it manifest, that fundamental differences maybe produced between aggregates, by differences in the degrees of composition of the increments : the ultimate units of the increments being the same. Thus an accu mulation of things of a given kind may be made by add ing one at a time. Or the things may be tied up into bundles often, and the tens placed together. Or the tens may be united into hundreds, and a pile of hundreds formed. Such unlikenesses in the structures of masses, are habitually seen in our mercantile transactions. Articles which the consumer re cognizes as single, the retailer keeps wrapped up in dozens, the wholesaler sends the gross, and the manufacturer supplies in packages of a hundred gross — that is, they severally increase their stocks by units of simple, of compound, and of doubly- compound kinds. Similarly result those differences of mor phological composition which we have first to consider. An organism consists of units. These units may be aggregated into a mass by the addition of unit to unit. Or they may be united into groups, and the groups joined together. Or these groups of groups may be so combined as to form a doubly- compound aggregate. Hence there arise respecting each organic form, the question — is its composition of the first, second, third, or fourth order ? — does it exhibit units of a singly-compounded kind only ; or are these consolidated into units of a doubly-compounded kind, or a triply-compounded kind ? And if it displays double or triple composition, the * It seems needful here to say, that allusion is made in this paragraph to a pro position respecting- the ultimate natures of Evolution and Dissolution, which ia contained in an essay on The Classification of the Sciences, published in March, ISGi. When the opportunity comes, I hope to make the definition there arrived at, the basis of a re-organization of the second part of First Principles : giving to that work a higher development, and a greater cohesion, than it at present pof>- scssts. MORPHOLOGICAL DEVELOPMENT. komologies of its different parts become problems. Undci the disguises induced by the consolidation of primary, second ary, and tertiary units, it has to be ascertained which answer to which, in their degrees of composition. Such questions are more intricate than they at first appear ; since, besides the obscurities caused by progressive integration, and those due to accompanying modifications of form, further obscurities result from the variable growths of units of the different orders. Just as an army may be augmented by re cruiting in each company, without increasing the number of companies ; or may be augmented by making up the full complement of companies in each regiment, while the number of regiments remains the same ; or may be aug mented by putting more regiments into each division, other things being unchanged ; or may be augmented by adding to the number of its divisions without altering the components of each division ; or may be augmented by two or three of these processes at once ; so, in organisms, increase of mass may be due to growth in units of the first order, or in those of the second order, or in those of still higher orders ; or it may be due to simultaneous growth in units of several orders. And this last mode of integration being the general mode, puts difficulties in the way of analysis. Just as the structure of an army would be made less easy to understand, if com panies often outgrew regiments, or regiments became larger than brigades ; so these questions of morphological composi tion, are complicated by the indeterminate sizes of the units of each kind — relatively-simple units frequently becoming far more bulky than relatively-compound units. § 177. The morphological problems of the second class, are those having for their subject-matter the changes of shape that accompany changes of aggregation. The most general questions respecting the structure of an organism, having been answered when it is ascertained of what units it is composed as a whole, and in its several parts ; there come the more special THE PROBLEMS OF MORPHOLOGY. / questions concerning its form — form in the ordinary sense. After the contrasts caused by variations in the process of integration, we have to consider the contrasts caused by variations in the process of differentiation. To speak speci fically — the shape of the organism as a whole, irrespect ive of its composition, has to be accounted for. Reasons have to be found for the unlikeness between its general out lines and the general outlines of allied organisms. And there have to be answered kindred inquiries respecting the propor tions of its component parts : — Why, among such of these as are homologous with one another, have there arisen the differences that exist ? And how have there been produced the contrasts between them and the homologous parts of or ganisms of the same type ? Very numerous are the heterogeneities of form that present themselves for interpretation under these heads. The ultimate morphological units combined in any group, may be differ entiated individually, or collectively, or both : each of them may undergo changes of shape ; or some of them may be changed and others not ; or the group may be rendered mul tiform by the greater growth of some of its units than of others. Similarly with the compound units, arising by union of these simple units. Aggregates of the second order may be made relatively complex in form, by inequalities in the rates of multiplication of their component units in diverse directions ; and among a number of such aggregates, numer ous unlikenesses may be constituted by differences in their degrees of growth, and by differences in their modes of growth. Manifestly, at each higher stage of composition, the possible sources of divergence are multiplied still further. That facts of this order can be accounted for in detail, is not to be expected — the data are wanting. All that we may hope to do, is to ascertain their general laws. How this is to be attempted we will now consider. § 178. The task before us is to trace throughout thcae O MORPHOLOGICAL DEVELOPMENT. phenomena the process of evolution ; and to show how, ati displayed in them, it conforms to those first principles which evolution in general conforms to. Two sets of factors have to be taken into account. Let us look at them. The factors of the first class are those which tend directly to change an organic aggregate, in common with every other aggregate, from that more simple form which is not in equi librium with incident forces, to that more complex form which is in equilibrium with them. We have to mark how, in corre spondence with the universal law that the uniform lapses into the multiform, and the less multiform into the more multi form, the parts of each organism are ever becoming further differentiated ; and we have to trace the varying relations to incident forces, by which further differentiations are entailed. We have to observe, too, how each primary modification of structure, induced by an altered distribution offerees, becomes a parent of secondary modifications — how, through the neces sary multiplication of effects, change of form in one part brings about changes of form in other parts. And then we have also to note the metamorphoses constantly being induced by the process of segregation — by the gradual union of like parts exposed to like forces, and the gradual separation of like parts exposed to unlike forces. The factors of the second class which we have to keep in view throughout our interpret ations, are the formative tendencies of organisms themselves — the proclivities inherited by them from antecedent organ isms, and which past processes of evolution have bequeathed. We have seen it to be a necessary inference from various orders of facts (§§ 65, 84, 97,) that organisms are built up of certain highly- complex molecules, which we distinguished as physio logical units — each kind of organism being built up of phy siological units peculiar to itself. We found ourselves obliged to recognize in these physiological units, powers of arranging themselves into the forms of the organisms to which they be long ; analogous to the powers which the molecules of inor ganic substances have of aggregating into specific crystalline THE PROBLEMS OF MORPHOLOGY. forms. AVe have consequently to regard this polarity of the physiological units, as producing, during the development of any organism, a combination of internal forces that expend themselves in working out a structure in equilibrium with the forces to which ancestral organisms were exposed ; but not in equilibrium with the forces to \vhich the existing organ ism is exposed, if the environment has been changed. Hence the problem in all cases, is, to ascertain the resultant of inter nal organizing forces, tending to reproduce the ancestral form, and external modifying forces, tending to cause deviations from that form. Moreover, we have to take into account, not only the characters of immediately-preceding ancestors, but also those of their ancestors, and ancestors of all degrees of remoteness. Setting out with rudimentary types, we have to consider how, in each successive stage of evolution, the structures acquired during previous stages, have been ob scured by further integrations and further differentiations ; or, conversely, how the lineaments of primitive organisms have all along continued to manifest themselves under the superposed modifications. § 179. Two ways of carrying on the inquiry suggest themselves. We may go through the several great groups of organisms, with the view of reaching, by comparison of parts, certain general truths respecting the homologies, the forms, and the relations of their parts ; and then, having dealt with the phenomena inductively, may retrace our steps with the view of deductively interpreting the general truths reached. Or, instead of thus separating the two inves tigations, we may carry them on hand in hand — first establishing each general truth empirically, and then pro ceeding to the rationale of it. This last method will, I think, conduce to both brevity and clearness. Let us now thus deal with the first class of morphological problems. 34 CHAPTER II. THE MORPHOLOGICAL COMPOSITION OF PLANTS. § 180. Evolution implies insensible modifications and gradual transitions, which render definition difficult — which make it impossible to separate absolutely the phases of or ganization from one another. And this iridefinitencss of distinction, to be expected a priori, we are compelled to re cognize d posteriori, the moment we begin to group morpho logical phenomena into general propositions. Thus, on in quiring what is the morphological unit, whether of plants or of animals, we find that the facts refuse to be included in any rigid formula. The doctrine that all organisms are built up of cells, or that cells are the elements out of which every tissue is developed, is but approximately true. There are living forms of which cellular structure cannot be asserted ; and in living forms that are for the most part cellular, there are nevertheless certain portions which are not produced by the metamorphosis of cells. Supposing that clay were the only material available for building, the proposition that all houses are built of bricks, would bear about the same relation to the truth, as does the proposition that all organisms are composed of cells. This generalization respecting houses would be open to two criticisms : first, that certain houses of a primi tive kind are formed, not out of bricks, but out of unmoulded clay ; and second, that though other houses consist mainly of bricks, yet their chimney-pots, drain-pipes, and ridge-tiles THE MORPHOLOGICAL COMPOSITION OF PLANTS. 11 do not result from combination or metamorphosis of bricks, but are made directly out of tlie original clay. And of like natures are the criticisms which must be passed on the generalization, that cells are the morphological units of or ganisms. To continue the simile, the truth turns out to be, that the primitive clay or protoplasm out of which organisms are built, may be moulded either directly, or with various degrees of indirectness, into organic struc tures. The physiological units which we are obliged to as sume as the components of this protoplasm, must, as we have seen, be the possessors of those complex polarities which re sult in the structural arrangements of the organism. The assumption of such structural arrangements may go on, and, in many cases, does go on, by the shortest route ; without the passage through what we call metamorphoses. But where such structural arrangements are reached by a circuitous route, the first stage is the formation of these small aggre gates, which, under the name of cells, are currently regarded as morphological units. The rationale of these truths appears to be furnished by the hypothesis of evolution. We set out with molecules one degree higher in complexity than those molecules of nitro genous colloidal substance into which organic matter iy resolvable ; and we regard these somewhat more complex mo lecules as having the implied greater instability, greater sen sitiveness to surrounding influences, and consequent greater mobility of form. Such being the primitive physiological units, organic evolution must begin with the formation of a minute aggregate of them — an aggregate showing vitality only by a higher degree of that readiness to change its form of aggregation, which colloidal matter in general displays ; and by its ability to unite the nitrogenous molecules it meets with, into complex molecules like those of which it is com posed. Obviously, the earliest forms must have been minute ; since, in the absence of any but diffused organic matter, no torm but a minute one could find nutriment. Obviously, too; L2 MORPHOLOGICAL DEVELOPMENT. it must liave been structurelesss ; since, as differentiations are producible only by the unlike actions of incident forces, there could have been no differentiations before such forces had had time to work. Hence, distinctions of parts like those required to constitute a cell, were necessarily absent at first. And we need not therefore be surprised to find, as we do find, specks of protoplasm manifesting life, and yet showing no signs of organization. A further stage of evolution is reached, when the very imperfectly integrated molecules form ing one of these minute aggregates, become more coherent ; at the same time as they pass into a state of heterogeneity, gradually increasing in its definiteness. That is to say, we may look for the assumption by them, of some distinctions of parts, such as we find in cells, and in what are called uni cellular organisms. They cannot retain their primordial uni formity ; and while in a few cases they may depart from it but slightly, they will, in the great majority of cases, acquire a very decided multiformity — there will result the compara tively integrated and comparatively differentiated Protopfnjta and Protozoa. The production of minute aggregates of physiological units, being the first step ; and the passage of such minute aggregates into more consolidated and more complex forms, being the second step ; it must naturally hap pen that all higher organic types, subsequently arising by further integrations and differentiations, will everywhere bear the impress of this earliest phase of evolution. From the law of heredity, considered as extending to the entire succes sion of living things during the Earth's past history, it folio w&t that since the formation of these small, simple organ isms, must have preceded the formation of larger and more complex organisms, the larger and more complex organisms must inherit their essential characters. We may anticipate that the multiplication and combination of these minute aggregates or cells, will be conspicuous in. the early develop mental stages of plants and animals ; and that through out all subsequent stages, cell-production and cell-differen THE MORPHOLOGICAL COMPOSITION OF PLANTS. It3 tiation will be dominant characteristics. The physiological units peculiar to each higher species, will, speaking generally, pass through this form of aggregation on their way towards the final arrangement they are to assume ; because those primordial physiological units from which they are remotely descended, aggregated into this form. And yet, just as in other cases we found reasons for inferring (§ 131), that the traits of ancestral organization may, under certain conditions, be partially or wholly obliterated, and the ultimate structure assumed without passing through them ; so, here, it is to be inferred that the process of cell-formation may, in some cases, be passed over. Thus the hypothesis of evolution prepares us for those two radical modifications of the cell- doctrine, which the facts oblige us to make. It leads us to expect that as structureless portions of protoplasm must have preceded cells in the process of general evolution ; so, in the special evolution of each higher organism, there will be an habitual production of cells out of structureless blastema. And it leads us to expect that though, generally, the physiolo gical units composing a structureless blastema, will display their inherited proclivities by cell-development and meta morphosis ; there will nevertheless occur cases in which the tissue to be formed, is formed by direct transformation of the blastema. Interpreting the facts in this manner, we may recognize that large amount of truth which the cell-doctrine contains, without committing ourselves to the errors involved by the sweeping assertion of it. We are enabled to understand how it happens that organic structures are usually cellular in their composition, at the same time that they are not universally so. We are shown that while we may properly continue to regard the cell as the morphological unit, we must constantly bear in mind that it is such, only in a greatly-qualified sense.* * Let me here refer those who are interested in this question, to Prof. Hux ley's criticism on the cell-doctrine, published in the Medico -Chirurgical lievw* in 1853. 14 MORPHOLOGICAL DEVELOPMENT. § 181. These aggregates of the lowest order, each formed of physiological units united into a group that is structurely single, and cannot be divided without destruction of its individuality, may, as above implied, exist as independent organisms The assumption to which we are committed by the hypothesis of evolution, that such so-called uni-cellular plants, were at first the only kinds of plants, is in harmony with the fact that habitats not occupied by plants of higher orders, commonly contain these protophytes in great abund ance and great variety. The various species of Protococcus, of Desmidiacece, and Diatomaccee, supply examples of morpho logical units living and propagating separately, under nu merous modifications of form and structure. Figures 1, 2, •and 3, represent a few of the commonest types. Mostly, simple plants are too small to be individually visible without the microscope. But, in some cases, these vegetal aggregates of the first order, grow to appreciable sizes. In the mycelium of some fungi, we have single cells developed into long branched filaments, or ramified tubules, that are of considerable lengths. An analogous structure characterizes certain tribes of Alga, of which Cocliiim adkarens, Fig. 4, may serve as an example. In Hydrogastrum, an other alga, Fig. 5, we have a structure which is described a? THE MORPHOLOGICAL COMPOSITION OF PLANTS. 15 simulating a perfect plant, with root, stem, "bud, and fruit, all produced by the branching of a single cell. And among fungi, the genus Botrytis, Fig. 6, furnishes an illus tration of allied kind. Here, though the size attained is much greater than that of many organisms which are mor phologically compound, we are compelled to consider the morphological composition as simple ; since the whole can no more be separated into minor wholes, than can the branched vascular system of an animal. In these cases, we have con siderable bulk attained, not by a number of aggregates of the first order being united into an aggregate of the second order ; but by the continuous growth of an aggregate of the first order. § 182. The transition to higher forms begins in a very unobtrusive manner. Among these aggregates of the first order, an approach towards that union by which aggregates of the second order are produced, is indicated by mere juxta position. Protophytes multiply rapidly; and their rapid multiplication sometimes causes crowding. When, instead of floating free in the water, they form a thin film on a moist surface, or are imbedded in a common matrix of mucus ; the mechanical obstacles to dispersion result in a kind of feeble integration, vaguely shadowing forth a combined group. Somewhat more definite combination is shown us by such plants as PalmclJa botryoidcs. Here the members of a family of cells, arising by the spontaneous fission of a parent-cell, remain united by slender threads of that jelly-like substance which envelops their surfaces. In some Diatomacece, several individuals, instead of completely separating, hold together by their angles ; and in other DiatomacecB, as the Bacillaria, a variable number of units cohere so slightly, that they are continually moving in relation to one another. This formation of aggregates of the second order, faintly indicated in feeble and variable unions like the above, may he traced through phases of increasing permanence arid de 16 MORPHOLOGICAL DEVELOPMENT. finiteness, as well as increasing extent. In the yeast-plant, Fig. 7, we have cells which may exist singly, or joined into groups of several ; and which have their shapes scarcely at all modified by their connexion. Among the Desmidiacece, it happens in many cases, that the two individuals produced by division of a parent-individual, part as soon as they are fully formed ; but in other cases, instead of parting they compose a group of two. Allied kinds show us how, by subsequent fissions of the adherent individuals and their progeny, there result longer groups ; and in some species, a continuous thread of them is thus produced. Figs. 8, 9, 10, 11, exhibit these several stages. Instead of linear aggregation, some of the Desmidiacece illustrate central aggregation ; as shown in Figs. 12, 13, 14, 15. Other instances of central aggrega tion are furnished by such protophytes as the Goniurn pector- ale, Fig. 16 (a being the front view, and b the edge view), and the Sarcina veutriculi, Fig. 17. Further, we have that spherical mode of aggregation of which the Vohox globator furnishes a familiar instance. Thus far, however, the individuality of the secondary ag gregate is feebly pronounced : not simply in the sense that it is small ; but also in the sense that the individualities of the primary aggregates are very little subordinated. But on seeking further, we find transitions towards forms in which the compound individuality is more dominant, while the sim ple individualities are more obscured. Obscuration of one kind, accompanies mere increase of size in the second- arv Aggregate : in proportion to the greater number of the THE MORPHOLOGICAL COMPOSITION OF PLANTS. 17 morphological units held together in one mass, becomes their relative insignificance as individuals. We see this in the irregularly-spreading lichens that form patches on rocks ; and in such creeping fungi as grow in films or lamina? on decaying wood and the bark of trees. In these cases, how ever, the integration of the component cells is of an almost mechanical kind. The aggregate of them is scarcely more individuated than a lump of inorganic matter : as witness the way in which the lichen extends its curved edges in this or that direction, as the surface favours ; or the way in which the fungus grows round and imbeds the shoots and leaves that lie in its way, just as so much plastic clay might do. Though here, in the augmentation of mass, we see a progress towards the evolution of a higher type ; we have as yet none of that definiteness required to constitute a compound unit, or true aggregate of the second order. Another kind of obscuration of the morphological units, is brought about by their more complete coalescence into the form of some struc ture made by their union. This is well exemplified among the Conferva, and their allies. In Fig, 18, there are re- °Q presented the stages of a growing Mougeotia genuflexa, in which this merging of the simple individualities into the compound individuality, is shown in the history of a single plant ; and in Figs. 19, 20, 21, 22, 23, are represented a series of species from this group, and that of Cladophora, in which we see a progressing integration. While in the lower types, the primitive spheroidal forms of the cells are scarcely 18 MORPHOLOGICAL DEVELOPMENT. altered ; in the higher types, the cells are so fused together as to constitute cylinders divided by septa. Here, how ever, the indefiniteness is still great : there are no specific limits to the length of any thread thus produced ; and none of that differentiation of parts required to give a decided in dividuality to the whole. To constitute something like a true aggregate of the second order, capable of serving as a compound unit, that may be combined with others like itself into still higher aggregates, there must exist both mass and definiteness. § 183. An approach towards plants which unite these cha racters, may be traced in such forms as Bangia ciliaris, Fig. 24. The multiplication of cells here takes place, not in a longitudinal direction only, but also in a transverse direction ; and the transverse multiplication being greater towards the middle of the frond, there results a differ ence between the middle and the two ex tremities — a character which, in a feeble way, unites all the parts into a whole. Even this slight individuatioii is, however, very indefinitely marked ; since, as shown by the figures, the lateral multiplication of cells does not go on in a precise manner. From some such type as this there appear to arise, by slight differences in the modes of growth, two closely-allied groups of plants, having individualities somewhat more pro nounced. If, while the cells multiply lon gitudinally, their lateral multiplication goes on in one direc tion only, there results a flat surface, as in Ulca linza, Fig. 25 ; or where the lateral multiplication is less uniform in its rate, in types like Fig. 26. But where the lateral multipli cation occurs in two directions transverse to one another, a hollow frond may be produced — sometimes irregularly THE MORPHOLOGICAL COMPOSITION OF PLANTS. 11* spheroidal, and sometimes irregularly tubular ; as in Enter o* morpha intestinaJis, Fig. 27. And occasionally, as in Entsro- morpha comprcsw. Fig. 28, this tubular frond becomes branched, Figs. 29 and 30 are magnified portions of such fronds ; show ing the simple cellular aggregation which allies them with the preceding forms. In the common Fuci of our coasts, other and somewhat higher stages of this integration are displayed. We have fronds preserving something like constant breadths ; and dividing dichotomously with approximate regularity. Though the sub-divisions so produced, are not to be regarded at all as separate fronds, but only as extensions of one frond, they foreshadow a higher degree of composition ; and by the com paratively methodic way in which they are united, give to the aggregate a more definite, as well as a more complex, in dividuality. Many of the higher lichens exhibit an analogous advance. While in the lowest lichens, the different parts of the thallus are held together only by being all attached to the supporting surface, in the higher lichens the thallus is so far integrated that it can support itself by attachment to such surface at one point only. And then, in still more developed kinds, we find the thallus assuming a dichotomously -branched form, and so gaining a more specific character as well as greater size. VO MORPHOLOGICAL DEVELOPMENT Where, as in types like these, the morphological units show an inherent tendency to arrange themselves in a man ner that is so far constant as to give characteristic propor tions, we may say that there is a recognizable compound in dividuality. Considering the Thallogens that grow in this way, apart from their kinships, and wholly with reference to their morphological composition, we might not inaptly de scribe them as pseudo-foliar. § 184. Another mode in which aggregation is so carried on as to produce a compound individuality of considerable definiteness, is variously displayed among other families of Alfjcr. When the cells, instead of multiplying longitudin ally alone, and instead of all multiplying laterally as well as longitudinally, multiply laterally only at particular places ; they produce a branched structure. Indications of this mode of aggregation occur among the Conferee and simple plants akin to them, as shown in Figs. 22, 23. Though, in some of the more developed Alyce which exhibit the ramified arrangement in a higher degree, the component cells are, like those of the lower Algcv, united to gether end to end, in such way as but little to obscure their separate forms; as in Cladophora Hutchimice, Fig. 31 ; they nevertheless evince greater subordination to the whole of which they are parts, by arranging themselves more mothod- THE MORPHOLOGICAL COMPOSITION OF PLANTS. 21 ically. Still further pronounced becomes the compound individuality, when, while the component cells of the branches unite completely into jointed cylinders, the com ponent cells of the stem begin to multiply laterally, so as to form an axis distinguished by its relative thickness and com plexity. Such types of structures are indicated by Figs. 32, 33 — figures representing small portions of plants which are quite tree-like in their entire outlines. On examining Figs. 34, 35, 36, which show the structures of the stems in these types, it will be seen, too, that the component cells in becoming more coherent, have undergone changes of form which obscure their individualities more than before : not only are they much elongated, but they are so compressed as to be prismatic rather than cylindrical. This structure, be sides displaying integration of the morphological units car ried on in two directions instead of one; and besides displaying this higher integration in the greater merging of the indi vidualities of the morphological units in the general individu ality ; also displays it in the more pronounced subordination of the branches and branchlets to the main stem. This differ entiation and consolidation of the stem, brings all the second ary growths into more marked dependence ; and so renders the individuality of the aggregate more decided. We might not inappropriately call this type of structure pseud-axial. It simulates that of the higher plants in cer tain leading characters. We see in it a primary axis along which development may continue indefinitely, and from which there bud out, laterally, secondary axes of like na ture, bearing like tertiary axes ; and this is the mode of growth with which Phaeriogams make us familiar. But the resemblance goes no further ; for these pseud-axes are devoid of those lateral appendages — those leaves or foliar organs — which true axes bear, and the bearing of which ordinarily constitutes them true axes. § 185. Some of the larger Algce supply examples of an MORPHOLOGICAL DEVEL DPMEXT. integration stili more advanced: not simply inasmuch as they unite much greater numbers of morphological units into continuous masses; but also inasmuch as they com bine the pseudo-foliar structure with the pseud-axial struc ture. Our own shores furnish an instance of this in the common Laminaria ; and certain gigantic Fuoi of the Antartic seas, supply yet better instances. In some of these, the germ develops a very long slender stem, which eventually expands into a large bladder-like or cylindrical air-vessel ; and from the surface of this there grow out numerous leaf-shaped expansions. Another kind, Lessonia fitscescens, Fig. 37, shows us a massive stem growing up through water many feet deep — a stem which, bifurcating as it approaches the surface, flat tens out the ends of its subdivisions into fronds like ribands. These, however, are not true foliar appendages, since they are merely ex panded continuations of the stem. The whole plant, great as is its size, and made up though it seems to be of many groups of mor phological units, united into a compound group by their marked subordination to a connecting mass, is nevertheless a single thallus. The aggregate is still an aggregate of the second order. But among certain of the highest Algw, we do find some thing more fhan this union of the pseud-axial with the pseudo-foliar structure. In addition to pseud-axes of comparative complexity ; and in addition to pseudo-folia that are like leaves, not only in their general shapes, but in having mid-ribs and even veins ; there are the be ginnings of a higher stage of integration. Figs. 38, 39, and 40, show some of the steps. In Rhodymcma palmata, Fig. 38, the parent-frond is comparatively irregular in shape, and without a mid- rib ; and along with this very imperfect integration, we see that the secondary fronds growing from THE MORPHOLOGICAL COMPOSITION OF PLANTS. 23 the edges are distributed very much at random, and are by no means specific in their shapes. A considerable advance is displayed by Phylloplom rulens, Fig. 39. Here the frond, primary, secondary, or tertiary, betrays some approach to wards regularity in both form and size ; by which, as also by its partially-developed mid-rib, there is established a more marked individuality ; and at the same time, the growth of the secondary fronds no longer occurs anywhere on the edge, in the same plane as the parent frond, but from the surface at specific places. Dclesseria sanguinea, Fig. 40, illustrates a much more definite arrangement of the same kind. The fronds of this plant, quite regularly shaped, have their parts decidedly subordinated to the whole ; and from their mid ribs grow other fronds, which are just like them. Each of these fronds is an organized group of those morphological units which we distinguish as aggregates of the first order. And in this case, two or more such aggregates of the second order, well individuated by their forms and structures, are united together ; and the plant composed of them is thus rendered, in so far, an aggregate of the third order. Just noting that in certain of the most-developed Algce, as 24 MORPHOLOGICAL DEVELOPMENT. the Sargassum, or common gulf- weed, this tertiary degree of composition is far more completely displayed, so as to pro duce among Thallogens a type of structure closely simulating that of the higher plants, let us now pass to the considera tion of these higher plants. § 186. Having the surface of the soil for a support and the air for a medium, terrestrial plants are mechanically circum stanced in a manner widely different from that in which aquatic plants are circumstanced. Instead of being buoyed up by a surrounding fluid of specific gravity equal to their own, they have to erect themselves into a rare fluid which yields no appreciable support. Further, they are dis similarly conditioned in having two sources of nutriment in place of one. Unlike the Alfja, which derive all the mate rials for their tissues from the water bathing their entire- surfaces, and use their roots only for attachment ; most of the plants which cover the Earth's surface, absorb part of their food through their imbedded roots and part through their exposed leaves. These two marked unlikenesscs in the rela tions to surrounding conditions, profoundly affect the respec tive modes of growth. "We must duly bear them in mind while studying the further advance of composition. The class of plants to which we now turn — that of Acrogena - — is nearly related by its lower members to the classes above dealt with : so much so, that some of the inferior liverworts are quite licheniform, and are often mistaken for lichens. Passing over these, let us recommence our analysis with such members of the class, as repeat those indications of progrees towards a higher composition, which we have just observed among the more-developed Algce. The Jungermanniaccce furnish as with a series of types, clearly indicating the transi tion from an aggregate of the second order to an aggregate of the third order. Figs. 41, and 42, indicate the structure among the lowest of this group. Here there is but an incom plete development of the second order of aggregate. The THE MORPHOLOGICAL COMPOSITION OF PLANTS. 25 frond grows as irregularly as the thallus of a. lichen : it is in definite in size and outline, spreading hither or thither as the conditions favour. Moreover, it lacks the differentiations re- quirwd to subordinate its parts to the whole : it is uniformly cellular, having neither mid-rib nor veins ; and it puts out rootlets indifferently from all parts of its under-surface. In Fig 43, Jungermannia epiphylla, we have an advance on this type. There is here, as shown in the transverse section, Fig, 44, a thickening of the frond along its central portion, pro ducing something like an approach towards a mid- rib ; and from this the rootlets are chiefly given off. The outline, too, is much less irregular ; whence results greater distinctness of the individuality. A further step is displayed in Junger mannia furcata, Fig. 45. The frond of this plant, compara tively well integrated by the distribution of its substance around a decided mid-rib, and by its comparatively-definite outlines, produces secondary fronds — there is what is called proliferous growth ; and, occasionally, as shown in Fig. 46, representing an enlarged portion, the growth is doubly-pro liferous. In these cases, however, the tertiary aggregate, so far as it is formed, is but very feebly integrated ; and its in tegration is but temporary. For not only do these younger fronds that bud out from the mid-ribs of older fronds, develop rootlets of their own ; but as soon as they are well grown and adequately rooted, they dissolve their connexions with the parent-fronds, and become quite independent. From these transitional forms we pass, in the higher Jungerman- niacece, to forms composed of many fronds that are perman ently united by a continuous stem. A more-developed ag- 35 26 MORPHOLOGICAL DEVELOPMENT. '/~eVU^ fV^ fv, (dl/0 c-/' *S'| > gregate of the tliird order is thus produced. But though, along with increased defmiteness in the secondary aggregates, there is here an integration of them so extensive and so re gular, that they are visibly subordinated to the whole they form ; yet the subordination is really very incomplete. In some instances, as in ./. complanata, Fig. 47, the leaflets de velop roots from their under surfaces, just as the primitive frond does ; and in the majority of the group, as in J". capitata, Fig. 48, roots are given off all along the connecting stem, at the spots where the leaflets or frondlets join it : the result being, that though the connected frondlets form a physical whole, they do not form, in any decided manner, a physiological whole ; since successive portions of the united series, carry on their functions independently of the rest. Finally, the most developed members of the group, present us with tertiary aggregates that are physio logically as well as physically integrated. Not lying prone like the kinds thus far described, but growing erect, the stem and attached leaflets become dependent upon a single root or group of roots ; and being so prevented from carrying on their functions separately, are made members of a compound indi vidual — there arises a definitely-established aggregate of the third degree of composition. The facts as arranged in the above order, are suggestive. Minute aggregates, or cells, the grouping of which we traced in § 182, showed us analogous phases of indefinite union, which appeared to lead the way towards definite union. We THE MORPHOLOGICAL COMPOSITION OF PLANTS. 27 see here among compound aggregates, as we saw there among simple aggregates, the establishment of a specific form, and a size that falls within moderate limits of varia tion. This passage from less definite extension to more de finite extension, seems in the one case, as the other, to be ac companied by the result, that growth exceeding a certain rate, ends in the formation of a new aggregate, rather than an enlargement of the old. And on the higher stage, as oa the lower, this process, irregularly carried out in the simpler types, produces in them unions that are but temporary ; while in the more-developed types, it proceeds in a systematic way, and ends in the production of a permanent aggregate that is doubly compound. Must we then conclude, that as cells, or morphological units, are integrated into a unit of a higher order, which we call a thalhis or frond ; so, by the integration of fronds, there is evolved a structure such as the above-delineated species possess ? Whether this is the interpretation to be given of these plants, we shall best see when considering whether it is the interpretation to be given of plants that rank above them. Thus far we have dealt only with the Cryptogamia. We have now to deal with the Phanerogaiuia or Phaenogaonia. CHAPTER III. THE MORPHOLOGICAL COMPOSITION OF PLANTS, CONTINUED. § 187. THAT advanced composition arrived at in the Acrogens, is carried still further in the Endogens and Exo- gens. In these most- elevated vegetal forms, aggregation of the third order is always distinctly displayed ; and aggre gates of the fourth, fifth, sixth, &c., orders are very common. Our inquiry into the morphology of these flowering plants, may be advantageously commenced by studying the development of simple leaves into compound leaves. It is easy to trace the transition, as well as the conditions under which it occurs ; and tracing it will prepare us for under standing how, and when, metamorphoses still greater in de gree, take place. § 188. If we examine a branch of the common bramble, when in flower or afterwards, we shall not uiifrequently find a simple or undivided leaf, at the insertion of one of tho lateral flower-bearing axes, composing the terminal cluster of flowers. Sometimes this leaf is partially lobed ; sometimes cleft into three small leaflets. Lower down on the shoot, if it be a lateral one, occur larger leaves, composed of three leaflets ; and in some of these, two of the leaflets may be lobed more or less deeply. On the main stem, the leaves, usually still larger, will be found to have five leaflets. Sup- THE MORPHOLOGICAL COMPOSITION OF PLANTS. 29 posing tLe plant to be a well- grown one, it will furnish all gradations between the simple, very small leaf, and the large composite leaf, containing sometimes even seven leaflets. Figs. 50 to 64, represent leading stages of the transition. 54: 5 WTiat determines this transition ? Observation shows that the quintuple leaves occur where the materials for growth are supplied in greatest abundance ; that the leaves become less 30 MORPHOLOGICAL DEVELOPMENT. and less compound, in proportion to their remoteness from the main currents of sap ; and that where an entire absence of divisions or lobes is observed, it is on leaves within the flower-bunch : at the place, that is, where the forces that cause growth are nearly equilibrated by the forces that oppose growth ; and where, as a consequence, gamogenesis is about to set in (§ 78) . Additional evidence that the degree of nutrition determines the degree of composition of the leaf, is furnishod by the relative sizes of the leaves. Not only, on the average, is the quintuple leaf much larger in its total area than the triple leaf; but the component leaflets of the one, are usually much larger than those of the other. The like con trasts are still more marked between triple leaves and simple leaves. This connexion of decreasing size with decreasing composition,, is conspicuous in the series of figures : the differ ences shown, being not nearly so great as may be frequently observed. Confirmation may be drawn from the fact, that when the leading shoot is broken or arrested in its growth, the shoots it gives off* (provided they are given off after the injury), and into which its checked currents of sap are thrownf produce leaves of five leaflets, where ordinarily leaves of three leaflets occur. Of course incidental circumstances, as varia tions in the amounts of sunshine, or of rain, or of matter sup plied to the roots, are ever producing changes in the state of the plant as a whole ; and by thus affecting the nutrition of its leaf-buds at the times of their formation, cause irregularities in the relations of size and composition above described. But taking these causes into account, it is abundantly manifest that a leaf-bud of the bramble, will develop into a simple leaf or into a leaf compounded in different degrees, according to the quantity of assimilable matter brought to it at the time when the rudiments of its structure are being fixed. And on studying the habits of other plants — on observing how annuals that have compound leaves, usually bear simple leaves at the outset, when the assimilating surface is but small ; and how, when compound-leaved plants in full growth THE MORPHOLOGICAL COMPOSITION OF PLANTS. 31 oear simple leaves in the midst of compound ones, the rela tive smallness of such simple leaves shows that the buds from which they arose were ill-supplied with sap ; it will cease to be doubted that a foliar organ may be metamorphosed into a group of foliar organs, if furnished, at the right time, with a quantity of matter greater than can be readily organized round a single centre of growth. An examination of the transitions through which a compound leaf passes into a doubly -compound leaf, as seen in the various intermediate forms of leaflets in Fig. 65, will further enforce this conclusion. d €5 Tlcre we may advantageously note, too, how in such cases, the leaf-stalk undergoes concomitant changes of structure. In the bramble-leaves above described, it becomes compound simultaneously with the leaf — the veins become mid-ribs while the mid-ribs become petioles. Moreover, the secondary stalks, and still more the main stalks, bear thorns similar in their shapes, and approaching in their sizes, to those on the stem; 32 MORPHOLOGICAL DEVELOPMENT. besides simulating the stem in colour and texture. In the petioles of large compound leaves, like those of the com mon Heracleum, we still more distinctly see both internal and external approximations in character to axes. Nor are there wanting plants whose large, though simple, leaves, are held out far from the stems, by foot- stalks that are, near the ends, sometimes so like axes, that the transverse sections of the two are indistinguishable; as instance the Calla Ethiopica. One other fact respecting the modifications which leaves undergo, should be set down. Not only may leaf-stalks as sume to a great degree the characters of stems, when they have to discharge the functions of stems, by supporting many leaves or very large leaves ; but they may assume the cha racters of leaves, when they have to undertake the functions of leaves. The Australian Acacias furnish a remarkable illustration of this. Acacias elsewhere found, bear pinnate leaves ; but the majority of those found in Australia, bear what appear to be simple leaves. It turns out, however, that these are merely leaf- stalks flattened out into foliar shapes : the lamina) of the leaves being undeveloped. And the proof is, that in young plants, showing their kinships by their em bryonic characters, these leaf- like petioles bear true leaflets at their ends. A metamorphosis of like kind occurs in Oxalis bupleurifolia, Fig. 66. The fact most deserving of notice, however, is, that these leaf stalks, in usurping the gene ral aspects and functions of leaves, have also usurped their structures : though their ve nation is not like that of the leaves they replace, yet they have veins, and in some cases mid-ribs. Reduced to their most general expression, the truths above shadowed forth are these : — That group of morphologi cal units, or cells, which we see integrated into the compound unit called a leaf, has, in each higher plant, a typical form; due to the special arrangement of these cells around a mid-rib and THE MORPHOLOGICAL COMPOSITION OF PLANTS. 33 veins. If the multiplication of morphological units, at the time when the leaf-bud is taking on its main outlines, exceeds a certain limit, these units begin to arrange themselves round secondary centres, or lines of growth, in such ways as to re peat, in part or wholly, the typical form : the larger veins become transformed into imperfect mid-ribs of partially inde pendent leaves ; or into complete mid-ribs of quite separate leaves. And as there goes on this transition from a single aggregate of cells to a group of such aggregates, there simul taneously arises, by similarly-insensible steps, a distinct structure which supports the several aggregates thus pro duced, and unites them into a compound aggregate. These phenomena should be carefully studied ; since they give us a key to more involved phenomena. § 189. Thus far we have dealt with leaves ordinarily so called : briefly indicating the homologies between the parts of the simple and the compound. Let us now turn to the homo logies among foliar organs in general. These have been made familiar to readers of natural history, by popularized outlines of " The Metamorphosis of Plants " — a title, by the way, which is far too extensive ; since the phenomena treated of under it, form but a small portion of those it properly in cludes. Passing over certain vague anticipations that have been quoted from ancient writers, and noting only that some clearer recognitions were reached by Joachim Jung, a Ham burg professor, In the middle of the 17th century ; we como to the Thooria Generationis, which Wolff published in 1759, and in which he gives a definite form to the conceptions that have since become current. Specifying the views of Wolff, Dr Masters writes, — " After speaking of the homologous nature of the leaves, the sepals and petals, an homology consequent on their similarity of structure and identity of origin, he goes on to state that the ' pericarp is manifestly composed of several leavea as in the calyx, with this differ- 34 MORPHOLOGICAL DEVELOPMENT. erice only, that the leaves which are merely placed in close contact in the calyx, are here united together ; ' a view which he corroborates hy referring to the manner in which many capsules open and separate ' into their leaves/ The seeds, too, he looks upon as consisting of leaves in close combination. His reasons for considering the petals and stamens as homologous witli leaves, are based upon the same facts as those which led Linnaeus, and, many years afterwards, Goethe, to the same conclusion. ' In a word,' says Wolff, ' we see nothing in the whole plant, whose parts at first sight differ so remark ably from each other, but leaves and stem, to which latter the root is referrible.' ' It appears that Wolff, too, enunci ated the now-accepted interpretation of compound fruits: basing it on the same evidence as that since assigned. In the essay of Goethe, published thirty years after, these rela tions among the parts of flowering plants were traced out in greater detail, but not in so radical a way ; for Goethe did not, as did Wolff, verify his hypothesis by dissecting buds in their early stages of development. Goethe appears to have arrived at his conclusions independently. But that they were original with him, and that he gave a more variously-illus trated exposition of them than had been given by Wolff, does not entitle him to anything beyond a secondary place, among those who have established this important generaliza tion. Were it not that these pages may be read by some to whom Biology, in all its divisions, is a new subject of study, it would be needless to name the evidence on which this now- familiar generalization rests. For the information of such it will suffice to say, that the fundamental kinship existing among all the foliar organs of a flowering plant, is shown by the transitional forms which may be traced between them, and by the occasional assumption of one another's forms. " Floral leaves, or bracts, are frequently only to be distin guished from ordinary leaves by their position at the base oi the flower ; at ether times the bracts gradually assume more THE MORPHOLOGICAL COMPOSITION OF PLANTS. 35 und more of the appearance of the sepals." The sepals, or divisions of the calyx, are not unlike undeveloped leaves : sometimes assuming quite the structures of leaves. In other cases, they acquire partially or wholly the colours of the petals — as, indeed, the bracts and uppermost stem -leaves occasionally do. Similarly, the petals show their alliances to the foliar organs lower down on the axis, and to those higher up on the axis : on the one hand, they may develop into or dinary leaves that are green and veined ; and, on the other hand, as so commonly seen in double flowers, they may bear anthers on their edges. All varieties of gradation into neighbouring foliar organs, may be witnessed in stamens. Flattened and tinted in various degrees, they pass insensibly into petals, and through them prove their homology with leaves ; into which, indeed, they are transformed in flowers that become wholly foliaceous. The style, too, is occasionally changed into petals or into green leaflets ; and even the ovules are now and then seen to take on leaf- like forms. Thus we have clear evidence that in Phsenogams, all the ap pendages of the axis are homologues : they are all modified leaves. Wolff established, and Goethe further illustrated, another general law of structure in flowering plants. Each leaf commonly contains in its axil, a bud, similar in structure to the terminal bud This axillary bud may remain unde veloped ; or it may develop into a lateral shoot like the main shoot ; or it may develop into a flower. If a shoot bearing lateral flowers be examined, it will be found that the internode, or space which separates each leaf with its axillary flower from the leaf and axillary flower above it, becomes gradually less towards the upper end of the shoot. In some plants, as in the fox- glove, the internodes constitute a regularly-diminishing series. In other plants, the series they form suddenly begins to diminish so rapidly, as to bring the flowers into a short spike — instance the common orchis. And again, by a still more sudden dwarfing of the intcrnodes, the 36 MORPHOLOGICAL DEVELOPMENT. flower? are brought into a cluster ; as they are in the cows lip. On contemplating a clover-flower, in which this clustering has been carried so far as to produce a com pact head ; and on considering what must happen if, by a further arrest of axial development, the foot-stalks of the florets disappear ; it will be seen that there must result a crowd of flowers, seated close together on the end of the axis. And if, at the same time, the internodes of the upper stem- leaves also remain undeveloped, these stem-leaves will be grouped into a common calyx or involucre : we shall have a composite flower, such as the thistle. Hence, to modifications in the developments of foliar organs, have to be added modi fications in the developments of axial organs. Comparisons disclose the gradations through which axes, like their append ages, pass into all varieties of size, proportion, and structure. And we learn that the occurrence of these two kinds of metamorphosis, in all conceivable degrees and combinations, furnishes us with a proximate interpretation of morpho logical composition in Phcenogams. I say a proximate interpretation, because there remain to be solved certain deeper problems ; one of which at once presents itself to be dealt with under the present head. Leaves, petals, stamens, &c., being shown to be homologous foliar organs ; and the port to which they are attached, proving to be an indefinitely- extended axis of growth, or axial organ ; we are met by the questions, — What is a foliar organ ? and What is an axial organ ? The morphological com position of a Phsenogam is undetermined, so long as we can not say to what lower structures leaves and shoots are homo logous ; and how this integration of them originates. To these questions let us now address ourselves. § 190-1. Already, in § 78, reference has been made to the Dccasional development of foliar organs into axial organs: che special case there described, being that of a fox- glove, in *rhich some of the sepals were replaced by flower-buda THE MORPHOLOGICAL COMPOSITION OF PLANTS. 37 43 The observation of these and some analogous monstrosities, raising the suspicion that the distinction between foliar organs and axial organs is not absolute, led me to examine into the matter ; and the result has been the deepening oi' this suspicion into a conviction. Part of the evidence is given in Appendix A Some time after having reached this conviction, I found on looking into the literature of the subject, that analogous ir regularities have suggested to other observers, beliefs similarly at variance with the current morphological creed. Diffi culties in satisfactorily defining these two elements, have served to shake this creed in some minds. To others, the strange leaf-like developments which axes undergo in certain plants, have afforded reasons for doubting the constancy of this distinction which vegetal morphologists usually draw. And those not otherwise rendered sceptical, have been made to hesitate by such cases as that of the Nepaul-baiiey ; in which the glume, a foliar organ, becomes developed into an axis, and bears flowers. In his essay — " Vegetable Morphology : its History and Present Condi tion," * whence I have already quoted, Dr Masters indicates sundry of the grounds for thinking, that there is no impassable demarcation between leaf and stem. Among other difficult ies which meet us if we assume that the distinction is abso lute, he asks — " What shall we say to cases such as those afforded by the leaves of G-uarea and Trichilia, where the leaves after a time assume the condition of branches and de velop young leaflets from their free extremities, a process less perfectly seen in some of the pinnate-leaved kinds of Berberis or Mahonia, to be found in almost every shrubbery ? " A class of facts on which it will be desirable for us nere to dwell a moment, before proceeding to deal with the matter deductively, is presented by the Cactacece. In this remark able group of plants, deviating in such varied ways from the ordinary phaenogamic type, we find many highly instructive * Sec British and Foreign Medico-Chirurgical Review for January, 186$. 44 MORPHOLOGICAL DEVELOPMENT. modifications of form and structure. By contemplating the changes here displayed within the limits of a single order, we shall greatly widen our conception of the possibilities of metamorphosis in the vegetal kingdom, taken as a whole. Two different, but similarly-significant, truths are illustrated. First, we are shown how, of these two components of a flowering plant, commonly regarded as primordially distin guished, one may assume, throughout numerous species, the functions, and to a great degree the appearance, of the other. Second, we are shown how, in the same individual, there may occur a re-metamorphosis — the usurped function and appearance being maintained in one part of the plant, while in another part, there is a return to the ordinary appearance and function. We will consider these two truths separ ately. Some of the Euphorbiacece, w^hich simulate Cactuses, show us the stages through which such abnormal structures are arrived at. In Euphorbia splendens, the lateral axes are considerably swollen at their distal ends, so as often to be club-shaped : still, however, being covered with bark of the ordinary colour, and still bearing leaves. But in kindred plants, as Euphorbia ncriifolia, this swelling of the lateral axes is carried to a far greater extent ; and, at the same time, a green colour and a fleshy consistence have been acquired : the typical relations nevertheless being still shown, by the few leaves that grow out of these soft and swollen axes. In the Cactacece, which are thus resembled by plants not otherwise allied to them, we have indications of a parallel transformation. Some kinds, not commonly brought to England, bear leaves ; but in the species most familiar to us, the leaves are undeveloped and the axes assume their functions. Passing over the many varieties of form and combination which these green succulent growths display, we have to note that in some genera, as in Pliyllocactns, they become flattened out into foliaceous shapes, having mid-riba arid something approaching to veins. So that here, and ill the genus Epiphyllum, which has this character still more THE MORPHOLOGICAL COMPOSITION OF PLANTS. 45 marked, the plant appears to be composed of fleshy leaves growing one upon another. And then, in Rhip salts, the same parts are so leaf-like that an uncritical observer would regard them as leaves. These which are axial organs in their homologies, have become foliar organs in their analogies. When, instead of comparing these strangely-modified axes in different genera of Cactuses, wo compare them in the same individual, we meet with transform ations no less striking. Where a tree-like form is pro duced by the growth of these foliaceous shoots, one on another ; and where, as a consequence, the first-formed of them become the main stem that acts as support to secondary and tertiary stems; they lose their green, succulent character, acquire bark, and become woody — in resuming the functions of axes they resume the structures of axes, from which they had de viated. In Fig. 71 are shown some of the leaf-like axes of Rliipsalw rhombea in their young state ; while Fig. 72 repre sents the oldest portion of the same plant, in which the foli aceous characters are quite obliterated, and there has re sulted an ordinary stem-struc ture. One further fact is to be noted. At the' same time that their leaf-like appearances are lost, the ixes also lose their separate individualities. As they become stem-like, they also become integrated ; and they do this so effectually, that their original points of junction, at first so strongly marked, are effaced, and a consolidated trunk is produced. Joined with the facts previously specified, these facts help us to conceive how, in the evolution of flowering plants in general, the morphological components that were once distinct, may become extremely disguised. We may ration ally expect that during so long a course of modification, much greater changes of form, and much more decided fusions 4:6 MORPHOLOGICAL DEVELOPMENT. of parts, have taken place. Seeing how, in an individual plant, the single leaves pass into compound leaves, by the devel opment of their veins into mid- ribs while their mid-ribs begin to simulate axes ; and seeing that leaves ordinarily exhibit ing definitely-limited developments, occasionally produce other leaves from their edges ; we are led to suspect the pos sibility of still greater changes in foliar organs. When, fur ther, we find that within the limits of one natural order, petioles usurp the functions and appearances of leaves, at the same time that in other orders, as in Ruscus, lateral axes so completely simulate leaves that their axial nature would never have been supposed, did they not bear flowers on their mid ribs or edges ; and when, among Cactuses, we perceive that such metamorphoses and re-metamorphoses take place with great facility ; our suspicion that the morphological elements of Phosnogams admit of profound transformations, is deepened. And then, on discovering how frequent are the monstrosities that do not seem satisfactorily explicable without admitting the development of foliar organs into axial organs ; we become ready to entertain the hypothesis, that during the ^.volution of the phaenogamic type, the distinction between leaves and axes has arisen by degrees. With our pre-conceptions loosened by such facts, and carrying with us the general idea which such facts suggest, let us now consider in what way the typical structure of a flowering plant may be interpreted. § 192. To proceed methodically, we must seek a clue to the structures of Endogens and Exogens, in the structures cf those inferior plants that approach to them — Acrogens. The various divisions of this class present, along with sundry characters which ally them with Thallogens, other charac ters by which the phaenogamic structure is shadowed forth. While some of the inferior Hepaticce or Liverworts, severally consist of little more than a thallus-like frond ; among the higher members of this group, and still more among the Tire MORPHOLOGICAL COMPOSITION OF PLANTS. 47 \fosses and Ferns, wo find a distinctly marked stem.* Some Acrogeiis have foliar expansions that are indefinite in their forms ; and some have quite definitely-shaped leaves. Eoota are possessed by all the more developed genera of the class ; but there are other genera, as Sphagnum, which have no roots. Here the fronds are thallus-like, in being formed of only a single layer of cells ; and there a double layer gives them a more leaf-like character — a difference exhibited between closely-allied genera of one order, the Mosses. Equally varied are the developments of the foliar-organs in their detailed structures : now being without mid-ribs or veins ; now having mid-ribs but no veins ; now having both mid-ribs and veins. Where stem and leaves exist, their imperfect differentiation is shown by the fact, that in many cases the stem is covered by an epidermis containing stomata. Nor must we omit the similarly-significant circumstance, that whereas in the lower Aero gens, the reproductive elements are immersed here and there in the thallus-like frond ; they are, in the higher orders, seated in well- specialized and quite distinct fructifying organs, having analogies with the flowers of Phocnogams. Thus, many facts imply that if the pha3nogamic type is to bo analyzed at all, we must look among the Acrogens for its mor phological components, and the manner of their integration. Already we have seen among the lower Cryptogamia, how * Schleiden, who chooses to regard as an axis, that which Mr Berkeley, with more obvious truth, calls a mid-rib, says :— " The flat stem of the Liverworts pre sents many varieties, consisting frequently of one simple layer of thin-walled cells, or it exhibits in its axis the elements /v the ordinary stem." This passage exemplifies the wholly gratuitous hypotheses which men will sometimes espouse, to escape hypotheses they dislike. Schleiden, with the positiveness characteristic of him, asserts the primordial distinction between axial organs and foliar organs. In the higher Acrogens, he sees an undeniable stem. In the lower Acrogens, clearly allied to them by their fructification, there is no structure having the remotest resemblance to a stem. Hut to save his hypothesis, Schleiden calls that "a flat stem," which is very obviously a structure in which stem and leaf are not differ entiated. He is the more to be blamed for this unphilosophical assumption, since he is merciless in his strictures on the unphilosophical assumptions of other 'jotanists. 48 MORPHOLOGICAL DEVELOPMENT. as tney become integrated and definitely limited, aggregated acquire the habit of budding out other aggregates, on reach ing certain stages of growth. Cells produce other cells endogenously or exogenously ; and fronds give origin to other fronds from their edges or surfaces. We have seen, too, that the new aggregates so produced, whether of the first order or the second order, may either separate or remain connected. Fissiparously-multiplying cells in some cases fly asunder, while in other cases they unite into threads or laminae or masses ; and fronds originating proliferously from other fronds, sometimes when mature disconnect themselves from their parents, and sometimes continue attached to them. Whether they do or do not part, is clearly determined by their nutrition. If the conditions are such that they can severally thrive better by separating after a certain develop ment is reached, it will become their habit then to separate ; since natural selection will favour the propagation of those which separate most nearly at that time. If, conversely, it profits the species for the cells or fronds to continue longer attached, which it can only do if their growth and subse quent powers of multiplication are thereby increased ; it must happen, through the continual survival of the fittest, that longer attachment will become an established characteristic ; and by persistence in this process, permanent attachment will result, when permanent attachment is advantageous. That disunion is really a consequence of relative innu trition, and union a consequence of relative nutrition, is clear, a posteriori. On the one hand, the separation of the new individuals, whether in germs or as developed aggregates, is a decaying awTay of the connecting tissue ; and this implies that the connecting tissue has ceased to perform its function as a channel of nutriment. On the other hand, where, as we see among Phoenogams, there is about to take place a separation of new individuals in the shape of germs, at the point where the nutrition is the lowest, a sudden increase of nutrition will cause the impend- THE MORPHOLOGICAL COMPOSITION v)F PLANTS. 49 ing separation to be arrested ; and the fructifying elements will revert towards the ordinary form, and develop in con nexion with the parent. Turning to the Acrogens, we tind among them, many indications of this transition from dis continuous development to continuous development. Thus the Liverworts give origin to new plants by cells which they throw off from their surfaces ; as, indeed, we have seen that much higher plants do. " According to Bischoff," says Schleiden, "both the cells of the stem (Jungermannia biden- tata) and those of the leaves (/. cxsccta) separate themselves as propagative cells from the plant, and isolated cells shoot out and develop while still connected with the parent plant into small cellular bodies (J. violacea), which separate from the plant, and grow into new plants, as in Mnium androgyimm among the Mosses." Now in the way above explained, these propagative cells and proliferous buds, may continue de veloping in connexion with the parent, to various degrees before separating ; or the buds \vhich are about to become fructifying organs, may similarly, under increased nutrition, develop into young fronds. As Sir AY. Hooker says of the male fructification in Jungermannia fur 'cata, — " It has the ap pearance of being a young shoot or innovation (for in colour and texture I can perceive no difference) rolled up into a spherical figure." On finding in this same plant, that some times the proliferously-produced frond, buds out from itself another frond before separating from the parent, as shown in Fig. 46 ; it becomes clear that this long- continued connexion, may readily pass into permanent connexion. And when we see how, even among Pha3nogams, buds may either detach themselves as bulbils, or remain attached and become shoots ; we can scarcely doubt that among inferior plants, less de finite in their modes of organization, such transitions must continually occur. Let us suppose, then, that Fig. 73 is the frond of some primitive Acrogen, similar in general characters to Junger- "iiannia epiphylla, Fig. 43 ; bearing, like it, the fructifying buds 50 MORPHOLOGICAL DEVELOPMENT. on its upper surface, and having a slightly- marked mid-rib and rootlets. And sup pose that, as shown, a secondary frond is proliferousty produced from the mid-rib, ^ and continues attached to it. Evidently, the ordinary discontinuous development, can thus become a continuous development, only on condition that there is an adequate supply, to the secondary frond, of such materials as are furnished by the rootlets : the remaining materials being obtainable by itself from the air. Hence, that portion of the mid-rib lying between the secondary frond and the chief rootlets, having its function increased, will increase in bulk. An additional consequence will be, a greater concentration of the rootlets — there will be extra growth of those which are most serviceably placed. Observe, next, that the structure so arising, is likely to be maintained. Such a variation implying, as it does, circumstances especially favour able to the growth of the plant, will give to the plant extra chances of leaving de scendants ; since the area of frond sup- ' ,; •. ported by a given area of the soil, being greater than in other individuals, there may be a greater production of spores. And then, among the more numerous descendants thus secured by it, the varia tion will give advantages to those in which it recurs. Such a mode of growth having, in this manner, become established, let us ask what is next likely to result. If it becomes the habit of the primary frond to bear a secondary frond from its mid-rib, this secondary frond, composed of physiological units of the same kind, will inherit the habit ; and supposing THE MORPHOLOGICAL COMPOSITION OF PLANTS. 51 that the supply of mineral matters obtained by the rootlets suffices for the full development of the secondary frond, there is a likelihood that the growth from it of a tertiary frond, will become an habitual characteristic of the variety. Along with the establishment of such a tertiary frond, as shown in Fig. 74, there must arise a further development of mid-rib in the primary frond, as well as in the secondary frond — a develop ment which must bring with it a greater integration of the two ; while, simultaneously, extra growth will take place in such of the rootlets as are most directly connected with this main channel of circulation. Without further explanation it will be seen, on inspecting Figs. 75 and 76, that there may in this manner result an integrated series of fronds, placed alternately on opposite sides of a connecting vascular struc ture. That this connecting vascular structure will, as shown in the figures, become more distinct from the foliar surfaces as these multiply, is no unwarranted assumption ; for we have seen in compound-leaved plants, how, under analogous con ditions, mid-ribs become developed into separate supporting parts, which acquire some of the characters of axes while as suming their functions. And now mark how clearly the structure thus built up by integration of proliferously- growing fronds, corresponds with the structure of the more- developed Jungermanniacece. Each of the fronds successively produced, repeating the characters of its parent, will bear roots ; and will bear them in homologous places, as shown. Further, the united mid-ribs having but very little rigidity, will be unable to maintain an erect position. Hence there will result the recumbent, continuously-rooted stem, whiclt these types exhibit. Nay, the parallelism is more complete than the figures show. To avoid confusion, the fronds thus supposed to be progressively integrated, have been repre sented as simple. But, as shown in Fig. 45, these lower types ordinarily have fronds which divide dichotomously, in such way that one division is larger than the other ; and this 52 MORPHOLOGICAL DEVELOPMENT is just the character of the successive leaves in the higher types. As shown in Fig. 47, each leaf is usually composed of two unequal lobes. A natural concomitant of the mode of growth here de scribed, is, that the stem, while it increases longitudinally, increases scarcely at all transversely : hence the name Acrogens. Clearly the transverse development of a stem, is the correlative, partly of its function as a channel of circula tion, and partly of its function as a mechanical support. That an axis may lift its attached leaves into the air, implies thickness and solidity proportionate to the mass of such leaves ; and an increase of its sap- vessels, also proportionate to the mass of such leaves, is necessitated when the roots are all at one end and the leaves at the other. But in the generality of Acrogens, these conditions, under which arises the necessity for transverse growth of the axis, are absent, wholly or in great part. The stem habitually creeps belor/ the surface, or lies prone upon the surface ; and where it grows in a vertical or inclined direction, does this by at taching itself to a vertical or inclined object. Moreover, throwing out rootlets, as it mostly does, at intervals through out its length, it is not called upon in any considerable de gree, to transfer nutritive materials from one of its ends to the other. Hence this peculiarity which gives their name to the Acrogens, is a natural accompaniment of the low degree of specialization reached in them. And that it is an incidental and not a necessary peculiarity, is demonstrated by two converse facts. On the one hand, in those higher Acrogens which, like the tree-ferns, lift large masses of foliage into the air, there is just as decided a transverse ex pansion of the axis as in Exogens. On the other hand, in those Exogens which, like the common Dodder, gain sup port and nutriment from the surfaces over which they creep, there is no more lateral expansion of the axis than is habit ual among Acrogens. Concluding, as we are thus fully justi fied in doing, that the lateral expansion accompanying longi- THE MORPHOLOGICAL COMPOSITION OF PLANTS. 53 tiulinal extension, which is a general characteristic of Endogens and Exogens as distinguished from Acrogens, is nothing more than a concomitant of their usually-vertical growth;* let us now go on to consider how vertical growth originates, and what are the structural changes it involves. § 193. Plants depend for their prosperity mainly on air and light : they dwindle where they are smothered, and thrive where they can expand their leaves into free space and sunshine. Those kinds which assume prone positions, consequently labour under disadvantages in being habitually interfered with by one another — they are mutually shaded and mutually injured. Such of them, however, as happen, by variations in mode of growth, to get at all above the rest, are more likely to flourish and leave offspring than the rest. That is to say, natural selection will favour the more upright- growing forms : individuals with structures that lift them above the rest, are the fittest for the conditions ; and by the continual survival of the fittest, such structures must become established. There are two essentially-different ways in which the integrated series of fronds above described, may be modified so as to acquire the stiffness needful for main taining perpendicularity. We will consider them separately. A thin layer of substance gains greatly in power of re sisting a transverse strain, if it is bent round so as to form a tube — witness the difference between the pliability of a sheet of paper when outspread, and the rigidity of the same sheet of paper when rolled up. Engineers constantly recognize * I am indebted to Dr Hooker for pointing out farther facts supporting this view. In his Flora Antarctica, he describes the genus Lcssonia (see Fig. 37} and especially L. ovata, as having a mode of growth simulating that of the Exogens. The tall vertical stem thickens as it grows, by the periodical addition of layers to its periphery. Among lichens, too, it seems that there is an analogous case. That even Thallogens should thus, under certain conditions, present a transversely- increasing axis, shows that there is nothing absolute in the character which gives the names to the two highest classes of plants, in contradistinction to the class ueareut to them. 54 MO11PH9 LOG ICAL DE VELOPM K N T. 77 "%_ this truth, in devising appliances by which the greatest strength shall be obtained at the smallest cost of material ; and among organisms, we see that natural selection habit ually establishes structures conforming to the same principle, wherever lightness and stiffness are to be combined. The cylindrical bones of mammals and birds, and the hollow shafts of feathers, are examples. The lower plants, too, furnish cases where the strength needful for maintaining an upright position, is acquired by this rolling up of a flat thallus or frond. In Fig. 77, we have an Alga which ap proaches towards a tubular distribution of substance ; and which has a consequent rigid ity. Sundry common forms of lichen, having the thallus folded into a branched tube, still more decidedly display ing the connexion between this structural arrangement and this mechanical advantage. And from the particular class of plants we are here dealing with — the Acrogens — a type is shown in Fig. 78, Riclla helicopkylla, similarly cha racterized by a thin frond that is made stiff enough to stand, by an incurving which, though it does not produce a hollow cylinder, produces a kindred form. If, then, as we have seen, natural selection or survival of the fittest, will favour such among these recumbent Acrogens, as are enabled, by variations of their structures, to maintain raised postures ; it will favour the formation of fronds that curve round upon themselves, and curve round upon the fronds growing out of them. What, now, will be the result should such a modification take place in the group of proliferous fronds represented in Fig. 76 ? Clearly, the result will be a structure like that shown in Fig. 79. And if this inrolling becomes more complete, a form like Junqe^mannia cordifolia. THE MORPHOLOGICAL COMPOSITION OF PLANTS. 55 represented in Fig. 80, will be produced. When the successive fronds are thus folded round so com pletely that their opposite edges meet, these opposite edges will be apt to unite : not that they will grow together after being formed, but that they will develop in connexion; or, in botanical language, will become " adnate." That foliar surfaces which, in their embryonic state, are in close contact, often join into one, is a familiar fact. It is habitually so with sepals or divisions of the calyx. In all campanulato flowers it is so with petals. And in some tribes of plants it is so with stamens. We are therefore well-warranted in inferring, that under the conditions above described, the suc cessive fronds or leaflets will, by union of their remote edges, first at their points of origin, and afterwards higher up, form sheaths inserted one within another, and including the axis. This incurving of the successive fronds, ending in the formation of sheaths, may be accompanied by different sets of modifications. Supposing Fig. 81 to be a transverse section of such a type (a being the mid-rib, and b the expansion of an older frond ; while c is a younger frond proliferously developed within it), there may begin two di vergent kinds of changes, leading to two contrasted struc tures. If, while frond continues to grow out of frond, the series of united mid-ribs continues to be the channel of circu lation between the uppermost fronds and the roots — if, as a consequence, the compound mid-rib, or rudimentary axis, con tinues to increase in size laterally ; there will arise the series of transitional forms represented by the transverse sections 82, 83, 84, 85 ; ending in the production of a solid axis, everywhere wrapped round by the foliar surface of the frond, as an outer layer or sheath. But if, on the other 56 MORPHOLOGICAL DEVELOPMENT. hand, circumstances favour a form of plant which maintains its uprightness at the smallest cost of substance — if the S3 vascular bundles of each succeeding mid-rib, instead of re maining concentrated, become distributed all round the tube formed by the infolded frond ; then the structure eventually reached, through the transitional forms 86, 87, 88, 89, will be a hollow cylinder. And now observe how the two structures thus produced, correspond with two kinds of Endogens. Fig. 90 represents a species of Dendrobium, in which we see clearly how each leaf is but a continuation of the external layer of a solid axis — a sheath such as would result from the infolded edges of a frond becoming adnate ; and on examining how the sheath of each leaf includes the one above it, and how the successive sheaths include the axis, it will be manifest that the relations of parts are just such as exist in the united series of fronds shown in Fig. 79 — the successive nodes answering to the successive points of origin of the fronds. Conversely, the stem of a grass, Fig. 91, dis plays just such relations of parts, as would result from the de velopment of the type shown in Fig. 79, if instead of the mid ribs thickening into a solid axis, the matter composing them became evenly distributed round the foliar surfaces, at the THE MORPHOLOGICAL COMPOSITION OF PLANTS. 57 tame time that the incurved edges of the foliar surfaces united. The arrangements of the tubular axis and its ap pendages, thus resulting, are still more instructive than those of the solid axis. For while, even more clearly than in the Deiidrobium, we see at the point b, a continuity of structure between the substance of the axis below the node, and the substance of the sheath above the node ; we see that this rfieath, instead of having its edges united as in Dendrobium, has them simply overlapping, so as to form an incomplete hollow cylinder which may be taken off and unrolled; and we see that were the overlapping edges of this sheath, united all the way from the node a to the node fe, it would constitute a tubular axis, like that which precedes it or like that which it includes. And then, giving an unexpected conclusiveness to the argument, it turns out that in one family of grasses, the overlapping edges of the sheaths do unite : thus furnishing us with a demonstration that tubular structures are produced by the incurving and joining or foliar surfaces ; and that so, hollow axes may be interpreted as above, without making any assumption unwarranted by fact. One further correspondence between the type thus ideally constructed, and the endogenous type, must oe noted. If, as already pointed out, the transverse growth of 58 MORPHOLOGICAL DEVELOPMENT. an axis arises, when the axis comes to be a channel of circu lation between all the roots at one of its extremities and all the leaves at the other ; and if this lateral bulging must in crease, as fast as the quantity of foliage to be brought in communication with the roots increases — especially if such foliage has at the same time to be raised high above the earth's surface ; what must happen to a plant constructed in the manner just described ? The elder fronds or foliar or gans, ensheathing those within them, as well as the incipient axis serving as a bond of union, are at first of such circum ference only as suffices to inclose these undeveloped parts. What, then, will take place when the inclosed parts grow — - when the axis thickens while it elongates ? Evidently the earliest-formed sheaths, not being large enough for the swelling axis, must burst ; and evidently each of the later- formed sheaths must, in its turn, do the. like. There must result a gradual exfoliation of the successive sheaths, like that indicated as beginning in the above figure of Denclro- bium; which, at a, shows the bud of the undeveloped parts just visible above the enwrapping sheaths, while at b, and c, it shows the older sheaths in process of being split open. That is to say, there must result the mode of growth which helps to give the name Endogens to this class. The other way in which an integrated series of fronds may acquire the rigidity needful for maintaining an erect position, has next to be considered. If the successive fronds do not acquire such habit of curling as may be taken ad- Vantage of by natural selection, so as to produce the requisite stiffness ; then, the only way in which the requisite stiffness appears producible, is by the thickening and hardening of the fused series of mid-ribs. The incipient axis will not, in this case, be inclosed by the rolled-up fronds ; but will con tinue exposed. Survival of the fittest will favour the genesis of a type, in which those portions of the successive mid- ribs that enter into the continuous bond, become more bulky than the disengaged portions of the mid-ribs : the individuals THE MORPHOLOGICAL COMPOSITION OF PLANTS. 59 which thrive and have the best chances of leaving offspring, being, by the hypothesis, individuals having axes stiff enough to raise their foliage above that of their fellows At the same time, under the same influences, there will tend to result an elongation of those portions of the mid-ribs, which become parts of the incipient axis ; seeing that it will profit the plant to have its leaves so far removed from one another, as to prevent mutual interferences. Hence, from the recumbent type, there will evolve, by indirect equilibration, (§ 167) such modifications as are shown in Figs. 92, 93, 94 : ff'T the first of which is a slight advance on the ideal type represented in Fig. 76, arising in the way described ; and the others of which are actual plants — Jungermannia Hooker I, and J". decipiens. Thus the higher Acrogens show us how, along with an assumption of the upright attitude, there docs go on, as we see there must go on, a separation of the leaf- producing parts from the root-producing parts ; a greater development of that connecting portion of the successive fronds, by which they are kept in communication with the roots, and raised above the ground ; and a consequent in creased differentiation of such connecting portion from the parts attached to it. And this lateral bulging of the axis, directly or indirectly consequent on its functions as a suupo.rl 60 MORPHOLOGICAL DEVELOPMENT. and -a channel, being here unrestrained by the early-forra«xl fronds folded round it, goes on without the bursting of these. Hence arises a leading character of what is called exogenous growth — a growth which is, however, still habitually accom panied by exfoliation, in flakes, of the outermost layer, con tinually being cracked and split by the accumulation of layers within it. And now if we examine plants of the exogenous type, we find among them many displaying the stages of this metamorphosis. In Fig. 95, is shown a form in which the continuity of the axis with the mid-rib of the leaf, is manifest — a continuity that is conspicuous in the common thistle. Here the foliar expansion, running some distance down the axis, makes the included portion of the axis a part of its mid-rib ; just as in the ideal types above drawn. By the greater growth of the intemodes, which are very variable, not only in different plants but in the same plant, there results a modification like that delineated in Fig. 96. And then, in such forms as Fig. 97, there is shown the arrangement that arises when, by more rapid develop ment of the proximal portion of the mid- rib, the distal part of the foliar surface is separated from the part which em braces the axis : the wings of the mid-rib still serving, how ever, to connect the two portions of the foliar surface. Such a separation is, as pointed out in § 188, an habitual occur rence ; and in some compound leaves, an actual tearing of the inter- veinous tissue, is caused by extra growth of the mid-rib. Modifications like this, and the further one in Fig. 98, we may expect to be established by survival of the fittest, among THE MORPHOLOGICAL COMPOSITION OF PLANTS. 61 those plants which, produce considerable masses of leaves, Kiice the development of mid- ribs into footstalks, by throw ing the leaves further away from the axes, will diminish the shading of the leaves, one by another. And then, among plants of bushy growth, in which the assimilating surfaces become still more liable to intercept one another's light, natural selection will continue to give an advantage to those which carry their assimilating surfaces at the ends of the petioles, and do not develop assimilating surfaces close to the axis, where they are most shaded. Whence will result a disappearance of the stipules and the foliar fringes of the mid- ribs ; ending in the production of the ordinary stalked leaf, Fig. 99, which is characteristic of trees. Meanwhile, the axis thickens in proportion to the number of leaves it has to carry, and to put in communication with the roots ; and so there conies to be a more marked contrast between it and the uetioles, severally carrying a leaf each.* § 194. When, in the course of the process above sketched out, there has arisen such community of nutrition among the fronds thus integrated into a series, that the younger ones are aided by materials which the older ones have elaborated ; the younger fronds will begin to show, at earlier and earlier periods of development, the structures about to originate from them. Abundant nutrition will abbreviate the intervals between the successive prolifications ; so that eventually, while each frond is yet imperfectly formed, the rudiment of the next will begin to show itself. All embryology justifies this inference. The analogies it furnishes lead us to expect that when this serial arrangement becomes organic, the growing part of the series will show the general relations of * Since tbi* paragraph, was put in type, I have observed that in some varieties of Cineraria, as probably in other plants, a single individual furnishes all these forms of leaves— all gradations between unstipulntod leaves on long petioles, and leaves that embrace the axis. It may be added that the distribution of theoc va~ riouy forms, is quite in harmony with the rationale above given. 62 MORPHOLOGICAL DEVELOPMENT. the forthcoming parts, while they are very small and im- specialized. What will in such case be the appearances they assumed ? We shall have no difficulty in perceiving what it will be, if we take a form like that shown in Fig. 92, and dwarf its several parts at the same time that we generalize them. Figs. 100, 101, 102, and 103, will show the result; and in Fig. 104, which is the bud of an exogen, we see how clear is the morphological correspondence: a being the rudiment of a foluir organ beginning to take shape ; b being the almost formless rudiment of the next foliar organ ; and c being the quitc-undifferentiated part whence the rudiments of subsequent foliar organs are to arise. And now we are prepared for entering on a still-remaining question respecting the structure of Phoonogams — what is the origin of axillary buds ? As the synthesis at present stands, it does not account for these ; but on looking a little more closely into the matter, we shall find that the axillary buds are interpretable in the same manner as the terminal buds. So to interpret them, however,, we must return to that pro cess of proliferous growth with which we set out, for the pur pose of observing some facts not before named. Ddesseria hypoglossum, Fig. 105, represents a seaweed of the same genus as one outlined in Fig. 40 ; but of a species in which pro liferous growth is carried much further. Here, not only does the primary frond bud out many secondary fronds from its mid-rib ; but most of the secondary fronds similarly bud out several tertiary fronds ; and even by some of the tertiary fronds, this prolification is repeated. Besides being shown that the budding out of several fronds from one frond, may become habitual ; TTC are also shown that it may become a habit inherited by the fronds so produced, and also by tlm TIFE MORPHOLOGICAL COMPOSITION OF PLANTS. 63 fronds they produce : the manifestation of the tendency, being probably limited only by failure of nutrition. That under fit conditions, an analogous mode of growth will occur in fronds of the acrogenic type, like those we set out with, ia shown by the case of Jungermannia furcata, Figs. 45, 46, in which such compound prolification is partially displayed. Let us suppose then, that the frond a, Fig. 106, produces not only a single secondary frond 1), but also another such secondary frond, I'. Let us suppose, further, that the frond b is in like manner doubly proliferous : producing both c and cf. Lastly, let us suppose that in the second frond // which a produces, as well as in the second frond c1 which // produces, the doubly-proliferous habit is manifested. If, now, this habit grows organic — if it becomes, as it natur ally will become, the characteristic of a plant of luxuriant growth, the unfolding parts of which can be fed by the un folded parts ; it will happen with each lateral series, as with the main series, that its successive components will begin to fihcw themselves at earlier and earlier stages of development. &nd in the same way that, by dwarfing and generalizing 37 B MORPHOLOGICAL DEVELOPMENT. the original series, we arrive at a structure like that of the terminal bud; by dwarfing and generalizing a lateraj series, as shown in Figs. 107 — 110, we arrive at a structure an swering in nature and position to the axillary bud. JO'/ Facts confirming these interpretations, are afforded by the structure and distribution of buds. The phsenogamic axis in its primordial form, being an integrated scries of folia ; and the development of that part by which these folia are held together at considerable distances from one another, taking place afterwards ; it is inferable from the general principles of embryology, that in its rudimentary stages, the phoonogamic axis will have its foliar parts much more clearly marked out than its axial parts. This we see in every bud. Every bud consists of the rudiments of leaves packed to gether without any appreciable internodal spaces ; and the internodal spaces begin to increase with rapidity, only when the foliar organs have been considerably developed. More over, where nutrition is defective, and arrest of development takes place — that is, where a flower is formed — the inter- nodes remain undeveloped : the process of unfolding ceases before the later-acquired characters of the phacnogamic axis are assumed. Lastly, as the hypothesis leads us to expect, axillary buds make their appearances later than the foliar organs which they accompany ; and where, as at the ends of axes, these foliar organs show failure of chlorophyll, the axillary buds are not produced at all. That these are in ferable traits of structure, will be manifest on contemplating Figs. 106 — 110; and on observing, first, that the doubly- proliferous tendency of which the axillary bud is a result, im plies abundant nutrition ; and on observing, next, that the original place of secondary prolification, is such that the foliar THE MORPHOLOGICAL COMPOSITION OF PLANTS, purface on which it occurs, must grow to some extent before the bud appears. On thus looking at the matter — on contemplating afresli the ideal type shown in Fig. 106, and noting how, by the conditions of the case, the secondary prolifications must cease before that primary prolification which produces the main axis ; we are enabled to reconcile all the phenomena of axil lary gemmation. We see harmony among the several facts — first, that the axillary bud becomes a lateral, leaf-bearing axis if there is abundant material for growth ; second, that its development is arrested, or it becomes a flower-bearing axis, if the supply of sap is but moderate ; third, that it is absent when the nutrition is failing. We are no longer o O committed to the gratuitous assumption, that in the phsono- gamic type, there must exist an axillary bud to each foliar organ ; but we are led to conclude, a priori, that which we find, a posteriori, that axillary buds are as normally absent in flowers as they are normally present lower down the axis. And then, to complete the argument, we are prepared for the corollary that axillary prolification may naturally arise even at the ends of axes, provided the failing nutrition which causes the dwarfing of the foliar organs to form a flower, be suddenly changed into such high nutrition as to transform the components of the flower into appendages that are green, if not otherwise leaf- like — a condition under which only, this phenomenon is proved to occur. § 195. One more question presents itself, when we con trast the early stages of development in the two classes of Phoenogams ; and a further answer supplied by the hypothe sis, gives to the hypothesis a further probability. It is cha racteristic of an endogen, to have a single seed-leaf or coty ledon ; and it is characteristic of an exogen, to have at least two cotyledons, if not more than two. That is to say, the monocotyledonous mode of germination everywhere co exists with the endogenous mode of growth ; and along with 66 MORPHOLOGICAL DEVELOPMENT. the exogenous mode of growth, there always goes either a dicotyledonous or polycotyledonous germination. Why is this ? Such correlations cannot be accidental — cannot be meaningless. A true theory of the phaenogamic types, in their origin and divergence, should account for the connex ion of these traits. Let us see whether the foregoing theory does this. The higher plants, like the higher animals, bequeath to their offspring more or less of nutriment and structure. Superior organisms of either kingdom do not, as do all in ferior organisms, cast off their progeny in the shape of minute portions of protoplasm, unorganized and without stocks of material fit for them to organize ; but they either deposit along with the germs they cast off, certain quantities of albumenoid substance, fit for them to appropriate while they develop themselves, or else they continue to supply such substance while the germs partially-develop themselves before their detachment. Among plants, this constitutes the dis tinction between seeds and spores. Every seed contains a store of food to serve the young plant during the first stages of its independent life ; and usually, too, before the seed is detached, the young plant is so far advanced in structure, that it bears to the attached stock of nutriment much the same relation that the young fish bears to the appended yelk- bag at the time of leaving the egg. Sometimes, indeed, the development of chlorophyll gives the seed-leaves a bright green, while the seed is still contained in the parent - pod. This early organization of the phamo- gam, must be supposed rudely to indicate the type out of which the phaBnogamic type arose. On the foregoing hypo thesis, the seed-leaves therefore represent the primordial fronds — which, indeed, they simulate in their simple, cellular, unveined structures. And the question here to be asked is — do the different relations of the parts in young endogens and exogens correspond with the different relations of the primor dial fronds, severally implied by the endogenous and tha THE MORPHOLOGICAL COMPOSITION OF PLANTS. t>< exogenous modes of growth ? We shall find that they do. Starting, as before, with the proliferous form shown in Fig. Ill, it is clear that if the strength required for main taining the vertical attitude, is obtained by the rolling up of the fronds, the primary frond will more and more conceal the secondary frond within it. At the same time, the secondary frond must continue to be dependent on the first for its nutri tion ; and being produced within the first, must be prevented by defective supply of light and air, from ever becoming syn chronous in its development with the first. Hence, this infolding which leads to the endogenous mode of growth, implies that there must always continue such pre-eminence of tho ilrst-formed frond or its representative, as to make the germination monocotylcdonous. Figs. Ill to 115, show the transitional forms that would result from the infolding of the fronds. In Fig. 116, a vertical section of the form repre sented in Fig. 115, are exhibited the relations of the succes- 68 MORPHOLOGICAL DEVELOPMENT. sive fronds to each other. The modified relations that would result, if the nutrition of the embryo admitted of anticipatory development of the successive fronds, is shown in Fig. 117. And how readily the structure' may pass into that of the nionocotyledonous germ, will be seen 011 inspecting Fig. 118 ; which is a vertical section of an actual monocotyledon at an early stage — the incomplete lines at the left of its root, indi cating its connexion with the seed.* Contrariwise, where the strength required for maintaining an upright atti tude is not obtained by the rolling up of the fronds, but by the strengthening of the continuous mid-rib, the second frond, so far from being less favourably circumstanced than the first, becomes in some respects even more favourably circumstanced : being above the other, it gets a greater share of light, and it is less restricted by surrounding obstacles. There is nothing, therefore, to prevent it from rapidly gaining an equality with the first. And if we assume, as the truths ot embryology entitle us to do, an increasing tendency towards anticipation in the development of subsequent fronds — if we assume that here, as in other cases, structures which were originally produced in succession, will, if the nutrition allows and no mechanical dependence hinders, come to be pro duced simultaneously ; there is nothing to prevent the pas sage of the type represented in Fig. Ill, into that represented * Since these figures were put on the block, it has occurred to me that the relations would be still clearer, were the primary frond represented as not taking part in these processes of modification, which have been described as giving rise to the erect form ; as, indeed, the rooting of its under surface will prevent it from doing in any considerable degree. In such case, each of the Figs. Ill to 117, should have a horizontal rooted frond at its base, homologous with the pro-em- fcryo among Acrogens. This primary frond would then more manifestly stand in the same relation to the rest, as the cotyledon does to the plumule— both by position, and as a supplier of nutriment. Fig. 117 «, which I am enabled to add, shows that this would complete the interpretation. Of the dicotyledonous series, it is needful to add no further explanation than that the difference in habit of growth, will permit the second frond to root itself as well as the first ; and so to become an additional source of nutrition, similarly circumstanced to the firs* and equal with it. THE MORPHOLOGICAL COMPOSITION OF PLANTS. C9 in Fig. 122. Or rather, there is everything to facilitate it; seeing that natural selection will continually favour the pro duction of a form in which the second frond grows in such way as not to shade the first, and in such way as allows the axis readily to assume a vertical position. Thus, then, is interpretable the universal connexion between monocotyledon ous germination and endogenous growth ; as well as the similarly-universal connexion between exogenous growth and the development of two or more cotyledons. That it explains these fundamental relations, adds very greatly to the probability of the hypothesis. § 196. While we are in this manner enabled to discern the kinship that exists between the higher vegetal types themselves, as well as between them and the lower types ; wo are at the same time supplied with a rationale of those truths which vegetal morphologists have established. Those homo- logies which Wolrf indicated in their chief outlines and Goethe followed out in detail, have a new meaning given to them when we regard the phocnogamic axis as having been evolved in the way described. Forming the modiiied con ception which we are here led to do, respecting the units of which a flowering plant is composed, we a,re no longer left without an answer to the question — What is an axis ? And we are helped to understand the naturalness of those cor respondences which, the successive members of each shoot display. Let us glance at the facts from our present stand point. Tho unit of composition of a Pliocnogam, is such portion of a shoot as answers to one of the primordial fronds. This portion is neither one of the foliar appendages nor one of the internodes ; but it consists of a foliar appendage together with the preceding intcrnode, including the axillary bud where this is developed. The parts intercepted by the dotted lines in Fig. 123, constitute such a segment ; and the true homology is between this and any other foliar organ with the 70 MORPHOLOGICAL DEVELOPMENT. portion of the axis below it. And now observe how, when wo take this for the unit of composition, the metamorphoses which the phrcnogamic axis displays, are inferable from known laws of development. Embryology teaches us that arrest of development shows itself first in the absence of those parts that have arisen latest in the course of evolution ; that if defect of nutrition causes an earlier arrest, parts that are of more ancient origin abort ; and that the part alone produced when the supply of materials fails near the outset, is the prim ordial part. We must infer, therefore, that in each seg ment of a Pheenogam, the foliar organ, which answers to the primordial frond, will be the most constant element ; and that the internode and the axillary bud, will be successively less constant. This we find. Along with a smaller size of foliar surface implying lower nutrition, it is usual to see a much-diminished internode and a less-pronounced axillary bud, as in Fig. 124. On approaching the flower, the J2Z J2Q axillary bud disappears ; and the segment is reduced to a small foliar surface, with an internode which is in most cases very short if not absent, as in 125 and 126. In the flower itself, axillary buds and internodes are both want ing: there remains only a foliar surface (127), which, though often larger than the immediately preceding foliar surface, shows failing nutrition by absence of chlorophyll. And then, in the quite terminal organs of fructification (129), we have the foliar part itself reduced to a mere rudiment. Though these progressive degenerations are by no means regular, being in many cases varied by adaptation to par ticular requirements, yet it cannot, I think, be questioned, THE MORPHOLOGICAL COMPOSITION OF PLANTS. 71 that the general relations are as described, and that they are such as the hypothesis leads us to expect. Nor are we without a kindred explanation of certain remaining traits of foliar organs in their least-developed, forms. Petals, stamens, pistils, &c., besides reminding us of the primordial fronds by their diminished sizes, and by the want of those several supplementary parts which, the preceding segments possess, also remind us of them by their histological charac ters : they consist of simple cellular tissue, scarcely at all differentiated. The fructifying cells, too, which here make their appearance, are borne in ways like those in which the lower Acrogens bear them — at the edge of the frond, or at the end of a peduncle, or immersed in the general substance ; as in Figs. 128 and 129. JNTay, it might even be said that the colours assumed by these terminal folia, call to mind the plants out of which we conclude that Phsenogams have been evolved ; for it is said of the fronds of the Jungermanniacece, that "though under certain circumstances of a pure green, they are inclined to be shaded with red, purple, chocolate, or other tints." As thus understood, then, the hornologies among the parts of the phsenogamic axis are interpretable, not as due to a needless adhesion to some typical form or fulfilment of a pre determined plan ; but as the inevitable consequences of the mode in which the phsenogamic axis originates. § 197. And now it remains only to observe, in confirmation of the foregoing synthesis, that it at once explains for us various irregularities. When we see leaves sometimes pro ducing leaflets from their edges or extremities, we recogiii/e in the anomaly, a resumption of an original mode of growth : fronds frequently do this. When we learn that a flowering plant, as the Droscra intermedia, has been known to develop a young plant from the surface of one of its leaves, we are at once reminded of the proliferous growths and fructifying organs in the Liverworts. The occasional production of bul- 72 MORPHOLOGICAL DEVELOPMENT. bils by Phaenogams, ceases to be so surprising when \ve find it to be habitual among the inferior Acrogens ; and when W9 see that it is but a repetition, on a higher stage, of that self- detachment which is common among proliferously-produced fronds. Nor are we any longer without a solution of that transformation of foliar organs into axial organs, which not uncommonly takes place. How this last irregularity of development is to be accounted for, we will here pause a moment to consider. Let us first glance at our data. The form of every organism, we have seen, must depend jii the structures of its physiological units. Any group of such physiological units will tend to arrange itself into the complete organism, if it is uncontrolled and placed in fit conditions. Hence the development of fertilized germs ; and hence the development of those self-detached cells which characterize some plants. Conversely, physiological units which form a small group involved in a larger group, and are subject to all the forces of the larger group, will become sub ordinate in their structural arrangements to the larger group — will be co-ordinated into a part of the major whole, in stead of co-ordinating themselves into a minor whole. This antithesis will be clearly understood on remembering how, oil the one hand, a small detached part of a hydra soon moulds itself into the shape of an entire hydra ; and how, on the other hand, the cellular mass that buds out in place of a lobster's lost claw, gradually assumes the form of a claw — has its parts so moulded as to complete the structure of the organism : a result which we cannot but ascribe to the forces which the rest of the organism exerts upon it. Con sequently, among plants, we may expect that whether any portion of protoplasm moulds itself into the typical form around an axis of its own, or is moulded into a part subor dinate to another axis, will depend on the relative mass of its physiological units — the accumulation of them that has taken place before the assumption of any structural arrange ment. A few illustrations will make clear the validity of THE MORPHOLOGICAL COMPOSITION OF PLANTS. /•! this inference. In the compound leaf, Fig. 65, the several lateral growths a, b, c, d, are manifestly homologous ; and on comparing a number of such leaves together, it will be seen that one of these lateral growths may assume any de gree of complexity, according to the degree of its nutrition. Every fern leaf exemplifies the same general truth still bet ter. Whether each sub-frond remains an undeveloped wing of the main frond, or whether it organizes itself into a group of frondlets borne by a secondary rib, or whether, going further, as it often does, it gives rise to tertiary ribs, is clearly determined by the supply of materials for growth ; since such higher developments are habitually most marked at points where the nutrition is greatest ; namely, next the stein. But the clearest evidence is afforded among the Al common support, and establish among them a more decided community of nutrition. Among the Ascidians, another order of the Molluscoida, this general law of morphological composition is once more dis played. Each of these creatures subsists on the nutritive particles contained in the water which it draws in through one orifice and sends out through another ; and it may thus subsist either alone, or in connexion with others that are in some cases loosely aggregated and in other cases closely aggregated. Fig. 156, Phallusia mcntula, is one of the soli tary forms. A type in which the individuals are united by a Btolon that gives origin to them by successive buds, is shown in PerrspTiora-y Fig. 157. Among the BotryllicUe, of which one THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 87 kind is drawn on a small scale in Fig. 159, and a portion ol the same on a larger scale in Fig. 158, there is a combination of the individuals into annular clusters, which are themselves imbedded in a common gelatinous matrix. And in this group there are integrations even a stage higher, in which several such clusters of clusters grow from a single base. Here the compounding and re-compounding, appears to be carried further than anywhere else in the animal kingdom. Thus far, however, among these aggregates of the third order, we see what we before saw among the simpler aggre gates of the second order — we see that the component indi vidualities are but to a very small extent subordinated to the individuality made up of them. In nearly all the forms in dicated, the mutual dependence of the united animals is so slight, that they are more fitly comparable to societies, of which the members co-operate in securing certain common benefits. There is scarcely any specialization of functions among them. Only in the last type described do we see a number of individuals so completely combined as to simulate a single individual. And even here, though there appears to be an intimate community of nutrition, there is no physio logical integration beyond that implied in several mouths and stomachs having a common vent. § 204. We come now to an extremely interesting ques tion. Does there exist in other sub-kingdoms composition ol the third degree, analogous to that which we have found so prevalent among the Cwknterata and the Molhiscoida ? The question is not whether elsewhere there are tertiary aggregates produced by the branching or clustering of secondary aggre gates, in ways like those above traced ; but whether elsewhere there are aggregates which, though otherwise unlike in the arrangement of their parts, nevertheless consist of parts so similar to one another that we may suspect them to be united secondary aggregates. The various compound type*. 88 MORPHOLOGICAL DEVELOPMENT. above described, in which the united animals maintain their individualities so distinctly that the individuality of the aggregate remains vague, are constructed in such ways that the united animals carry on their several activities with scarcely any mutual hindrance. The members of a branched Hydrozoon such as is shown in Fig. 149 or Fig. 150, are so placed that they can all spread their tentacles and catch their prey as well as though separately attached to stones or weeds. Packed side by side on a flat surface or forming a tree like assemblage, the associated individuals among the Polyzoa are not unequally conditioned ; or if one has some advantage over another in a particular case, the mode of growth and the relations to surrounding objects are so irregular as to prevent this advantage re-appearing with constancy in suc cessive generations. Similarly with the Ascidians growing from a stolon or those forming an annular cluster : each of them is as well placed as every other for drawing in the currents of sea-water from which it selects its food. Li these cases the mode of aggregation does not expose the united individuals to multiform circumstances ; and there fore is not calculated to produce among them any structural multiformity. For the same reason no marked physiologi cal division of labour arises among them ; and consequently no combination close enough to disguise their several indi vidualities. But under converse conditions we may expect converse results. If there is a mode of integration which necessarily subjects the united individuals to unlike sets of incident forces, and does this with complete uniformity from generation to generation, it is to be inferred that the united individuals will become unlike. They will severally assume such different functions as their different positions enable them respectively to carry on with the greatest advantage to the assemblage. This heterogeneity of function arising umong them, will be followed by heterogeneity of structure ; as also by that closer combination which the better enables them to utilize one another's functions. And hence, while THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 89 the oiiginally-like individuals are rendered unlike, they will have their homologies further obscured by their progressing fusion into an aggregate individual of a higher order.. These converse conditions are in nearly all cases fulfilled where the successive individuals arising by continuous devel opment are so budded-off as to form a linear series. I say in nearly all cases, because there are some types in which the associated individuals, though joined in single file, are not thereby rendered very unlike in their relations to the environment ; and therefore do not become differentiated and integrated to any considerable extent. I refer to such Asci- dians as the Salpidce. These creatures float passively in the sea, attached together in strings. Being placed side by side and having mouths and vents that open laterally, each of them is as well circumstanced as its neighbours for absorb ing and emitting the surrounding water; nor have the in dividuals at the two extremities any marked advantages over the rest in these respects. Hence in this type, and in the allied type Pyrosoma, which has its component indivi duals built into a hollow cylinder, linear aggregation may exist without the minor individualities becoming obscured and the major individuality marked : the conditions under which a differentiation and integration of the component individuals may be expected, are not fulfilled. But where the chain of individuals produced by gemmation, is either habitually fixed to some solid body by one of its extremities or moves actively through the water or over submerged 8tones and weeds, the several members of the chain become differently conditioned in the way aoove described ; and may therefore be expected to become unlike while they become united. A clear idea of the contrast between these tw) linear arrangements and their two diverse results, will be obtained by considering what happens to a row of soldiers, when changed from the ordinary position of a single rank to the position of Indian file. So long as the men stand shoulder to shoulder, they are severally able to use their •90 MORPHOLOGICAL DEVELOPMENT. weapons in like ways with like efficiency ; aud could, if called on, similarly perform various manual processes directly or indirectly conducive to their welfare. But when on the word of command "right face," they so place themselves that each has one of his neighbours before him and another behind him, nearly all of them become incapacitated for fighting and for many other actions. They can walk or run one after another, so as to produce movement of the file in the direction of its length ; but if the file has to oppose an enemy or remove an obstacle lying in the line of its march, the front man is the only one able to use his weapons or hands to much purpose. And manifestly such an arrange ment could become advantageous only if the front man pos sessed powers peculiarly adapted to his position, while those behind him facilitated his actions by carrying supplies, &c. This simile, grotesque as it seems, serves to convey better perhaps than any other could do, a clear idea of the relations that must arise in a chain of individuals arising by gemma tion, and continuing permanently united end to end. Such a chain can arise by natural selection, only on condition that combination is more advantageous than separation ; and for it to be more advantageous, the anterior members of the series must become adapted to functions facilitated by their posi tions, while the posterior members become adapted to func tions which their positions permit. Hence, survival of the fittest must tend continually to establish types in which the connected individuals are more unlike one another, at the same time that their several individualities are more dis guised by the integration consequent on their mutual dependence. Such being the anticipations warranted by the general laws of evolution, we have now to inquire whether the~e are any animals which fulfil them. Very little search suffices ; for structures of the kind to be expected are abund ant. In that great division of the animal kingdom called Annnlosa, especially if the Annuloida be regarded as part of THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 91 It, we find a variety of types having the looked- for charac ters. Let us contemplate some of them. § 205. An adult Annelid is composed of segments which repeat one another in their details as well as in their general shapes. Dissecting one of the lower orders, such as is shown in Fig. 160, proves that the successive segments, be- having like locomotive appendages, like branchiae, and sometimes even like pairs of eyes, also have like internal organs. Each has its enlargement of the alimentary canal ; each its contractile dilatation of the great blood-vessel ; each its portion of the double nervous cord, with ganglia when these exist ; each its branches from the nervous and vascular trunks answering to those of its neighbours ; each its simi larly answering set of muscles ; each its pair of openings through the body- wall ; and so on throughout, even to the organs of reproduction. That is to say, every segment is in great measure a physiological whole — every segment con tains most of the organs essential to individual life and mul tiplication : such essential organs as it does not contain, being those which its position as one in the midst of a chain, prevents it from having or needing. If we ask what is the meaning of these homologies, no adequate answer is supplied by any current hypothesis. That this " vegetative repetition " is carried out to fulfil a prede termined plan, was shown to be quite an untenable notion (§§ 133, 134). On the one hand, we found nothing satis factory in the conception of a Creator who prescribed to him 92 MORPHOLOGICAL DEVELOPMENT. self a certain unit of composition for all creatures of a par ticular class, and then displayed his ingenuity in building up a great variety of forms without departing from the " arche typal idea." On the other hand, examination made it mani fest that even were such a conception worthy of being enter tained, it would have to be relinquished ; since in each class there are numerous deviations from the supposed " archetypal idea." Still less can these traits of structure be accounted for teleologically. That certain organs of nutrition and re spiration and locomotion are repeated in each segment of a dorsibranchiate annelid, may be regarded as functionally ad vantageous for a creature following its mode of life. But why should there be a hundred or even two hundred pairs of ovaries ? This is an arrangement at variance with that physiological division of labour which every organism pro fits by — is a less advantageous arrangement than might have been adopted. That is to say, the hypothesis of a designed adaptation fails to explain the facts. Contrariwise, these structural traits are just such as might naturally be looked for, if these annulose forms have arisen by the in tegration of simpler forms. Among the various compound animals already glanced at, it is very general for the united individuals to repeat one another in all their parts — repro ductive organs included. Hence if, instead of a clustered or branched integration, such as the Ccvlenterata and Molluscoida exhibit, there occurs a longitudinal integration ; we may ex pect that the united individuals will habitually indicate their original independence by severally bearing germ-producing or sperm-producing organs. The reasons for believing one of these creatures to be an aggregate of the third order, are greatly strengthened when we turn from the adult structure to the mode of develop ment. Among the Dorsibranchiata and Tubicolce, the em bryo leaves the egg in the shape of a ciliated gemmule, not much more differentiated than that of a polype. As shown in Fig. 162, it is a nearly globular mass ; and its interior THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 93 consists of imtransformed cells. The first appreciable change is an elongation and a simultaneous commencement of seg mentation. The segments multiply by a modified gemma tion, which takes place from the hinder end of the penultimate segment. And considerable progress in marking out these divisions is made before the internal organization begins. Figs. 163, 164, 165, represent some of these early stages. In S65 Annelids of other orders, the embryo assumes the segmented form while still in the egg. But it does this in just the same manner as before. Indeed, the essential identity of the two modes of development is shown by the fact that the seg mentation within the egg is only partially carried out : in all these types the segments continue to increase in number for some time after birth. Now this process is as like that by which compound animals in general are formed, as the different conditions of the case permit. When new individuals are budded-out laterally, their unfolding is not hindered — there is nothing to disguise either the process or the product. But gemmoc produced one from another in the same straight line, and remaining connected, restrict one another's developments ; and that the resulting segments are so many gemmiparously-produced individuals, is necessarily less obvious. § 206. Evidence remains which adds very greatly to the weight of that already assigned. Thus far we have studied only the individual annulose animal ; considering what may be inferred from its mode of evolution and final organization. 94 MORPHOLOGICAL DEVELOPMENT. "We have now to study annulose animals in general. Com parison of them will disclose various phases of progiessive integration of the kind to be anticipated. Among the simpler Annuloida, as in the Nemertido& and in some kinds of Planaria, transverse fission occurs. A por tion of a Planaria separated by spontaneous constriction, becomes an independent individual. Sir J. Gr. Dalyell found that in some cases numerous fragments artificially separated, grew into perfect animals. In these annuloids which thus remind us of the lowest Hydrozoa in their powers of agamo- genetic multiplication, the individuals produced one from another, do not continue connected. As the young ones laterally budded-off by the Hydra separate when complete, so do the young ones longitudinally budded-off by the Pla naria. Fig. 16G indicates this. But there are allied types which show us a more or less persistent union of homologous parts, or individuals, similarly arising by longitudinal gem ination. The cestoid Entozoa furnish illustrations. Without dwelling on the fact that each segment of a Tcenia, like each separate Planaria, is an independent hermaphrodite, or on the fact that both develop their ova by the peculiar method of forming germinal vesicles in one canal and surrounding them with yelk that is secreted in another canal ; and without specifying the sundry common structural traits which add probability to the suspicion that there is some kinship be tween the individuals of the one order and the segments of the other ; it will suffice to point out that the two types are so far allied as to demand their union under the same sub class title. And recognizing this kinship, we see significance in the fact that in the one case the longitudinally-produced gemmae separate as complete individuals, and in the other continue united as segments in smaller or larger numbers and for shorter or longer periods. In Tcenia cchinococcus, represented in Fig. 167, we have a species in which the number of segments thus united does not exceed four. In Echinolothrium typus there are eight or ten ; and in cestoids THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 95 generally they are numerous.* A considerable hiatus occurs between this phase of integration and the next higher phase which we meet with ; but it is not greater than the hiatus between the types of the Annuloida and the Annelida, which present the two phases. Though it is doubtful whether separation of single segments occurs among the Annelida, yet very often we find strings of segments, arising by repeated longitudinal budding, which after reach ing certain lengths undergo spontaneous fission : in some cases doing this so as to form two or more similar strings of segments constituting independent individuals ; and in other cases doing it so that the segments spontaneously separated are but a small part of the string. Thus a SyUix, Fig. 168, after reaching a certain length, begins to trans- form itself into two individuals . one of the posterior seg ments develops into a head, and simultaneously narrows its connexion with the preceding segments, from which it * I find that the reasons for regarding the segment of a Tcenia as answering to an individual of the second order of aggregation, are much stronger than I sup posed when writing the above. Van Beneden says: — " Le Proglottis (segment) ayaut acquis tout son developpement, se detache ordinairement de la colonie et continue encore a croitre dans 1'intestin du merae animal ; il change memo sou- vent de forme et semble doue d'une nouvelle vie ; ses angles s'effacent, tout le corpe s'arrondit, et ii nage comme une Planuire au milieu des -nuscosites iiitestinalee ' 39 96 MORPHOLOGICAL DEVELOPMENT. eventually separates. Still more remarkable is the extent to which this process is carried in certain kindred types ; which exhibit to us several individuals thus being simultaneously formed out of groups of segments. Fig. 169, copied (omit ting the appendages) from one contained in a memoir i>y M. Milne-Edwards, represents six worms of different ages in course of development : the terminal one being the eldest, the one having the greatest number of segments,, and the one that will first detach itself ; and the success ively anterior ones, with their successively smaller numbers of segments, being successively less advanced towards fitness for separation and independence. Here among groups of segments we see repeated what in the previous cases occurs with single segments. And then in other Annelids we find that the string of segments arising by gemmation from a single germ becomes a permanently united whole : the tendency to any more complete fission than that which marks out the seg ments, being lost ; or, in other words, the integration having become relatively complete. Leaving out of sight the question of alliance among the types above grouped together, that which it here concerns us to notice is, that longitudinal gemmation does go on ; that it is displayed in that primitive form in which the gemmsc separate as soon as produced ; that we have types in which such gemma? hang together in groups of four, or in groups of eight and ten, from which however the gemmae successively separate as individuals ; that among higher. types we have long strings of similarly- formed gernmse which do not become individually independ ent, but separate into organized groups ; and that from these we advance to forms in which all the gemmas remain parts of a single individual. One other significant class of facts must be added. A few cases have been pointed out, one of them quite recently, in which Annelids mul tiply by lateral gemmation. M. Pagenstecher alleges this of the Exogone gcmmifera : describing a certain number of the segments of the body as severally bearing on their dorsal THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 97 surfaces a bud on each side. And M. L. Yaillant, after citing this observation of M. Pagenstecher, gives an account of a species of Syllis in which a great number of buds were borne by a single segment. That the longitudinally-produced gemma) which compose an Annelid, should thus have, one of them or several of them, the power of laterally budding- off gemmae, from which no doubt other annelids arise, gives fur ther support to the hypothesis that, primordially, the seg ments were independent individuals. And it suggests this be lief the more strongly because, in certain types of Caelenterata, we see that longitudinal and lateral gemmati n do occur to gether, where the longitudinally-united gemin-T) are dcmon- strably independent individuals. § 207. It would add to the probability of this conclusion coidd we identify the type out of which the annulose type may have arisen by the process of integration. I believe there may be pointed out such a type — a type which, by a slight modification carrying somewhat further an habitual mode of development, would produce not only a unit of com position for the annulose type, but also as a bond uniting it with the other types, and these with one another. It is un desirable, however, here to enter upon the numerous explan ations involved by opening the question of these i elation- ships ; both because it would necessitate a long digression, encumbering too much the general argument, and because, being highly speculative, it woidd be impolitic to let the general argument be even apparently implicated by it. But even supposing it impossible now to identify the unit of composition of the annulose type, the foregoing evidence still goes far towards showing that an annulose animal is an aggre gate of the third order. This repetition of segments, some times numbering several hundreds, like one another in all their organs even down to those of reproduction, while it is otherwise unaccountable, is fully accounted for if these seg ments are homologous with the separate individuals of some 98 MORPHOLOGICAL DEVELOPMENT. lower type. The gemmation by which these segments are pro duced, is as similar as the conditions allow, to the gemmation by which compound animals in general are produced. Aa among plants and as among demonstrably-compound animals, we see that the only thing required for the formation of a per manent chain of gemmiparously-produced individuals, is that by remaining associated, such individuals will have advantages greater than are to be gained by separation. Further, by comparison of the annuloid and lower annulose forms, we discover a number of those transitional phases of integration which the hypothesis leads us to expect. And, lastly, the differences among these united individuals or successive segments, are not greater than the differences in their posi tions and functions explain — not greater than such differences are known to produce among other united individuals : wit ness sundry compound Hydrozoa. Indirect evidence of much weight has still to be given. Thus far we have considered only the less-developed Annu- losa. The more integrated and more differentiated types of the class remain. If in them we find a carrying further of the processes by which the lower types are here supposed to have been evolved, we shall have additional reason for be lieving them to have been so evolved. If we find that in these superior orders the individualities of the united seg ments are much less pronounced than in the inferior, we fchall have grounds for suspecting that in the inferior the indivi dualities of the segments are less pronounced than in those lost forms which initiated the annulose sub-kingdom CHAPTER V. THE MORPHOLOGICAL COMPOSITION OF AXIMALS, CONTINUED. § 208. INSECTS, Arachnids, Crustaceans, and Myriapods, are all members of that higher division of the Annulosa called Articulata or Arthropoda. Though in these creatures the formation of segments may be interpreted as a disguised gemmation ; and though in some of them the number of seg ments increases by this modified budding after leaving the egg, as among the higher Annelids ; yet the process is not nearly so dominant : the segments are usually much less numerous than we find them in the types last considered. In most cases, too, the segments are in a greater degree dif ferentiated one from another, at the same time that they are severally more differentiated within themselves. Nor is there any instance of spontaneous fission taking place in the series of segments composing an articulate animal. On the contrary, the integration, always great enough permanently to unite the segments, is frequently carried so far as to hide very completely the individualities of some or many of them ; and occasionally, as among the Acari, the consolidation, or the arrest of segmentation, is so decided as to leave scarcely a trace of the articulate structure : the type being in these cases indicated chiefly by the presence of those character istically-formed limbs, which give the alternative name Arthropoda to all the higher Annulosa. Omitting the para sitic orders, which, as in other cases, are aberrant members of 100 MORPHOLOGICAL DEVELOPMENT. their sub-kingdom, comparisons between the different orders prove that the higher are strongly distinguished from the lower, by the much greater degree in which the individual ity of the tertiary aggregate dominates over the individual ities of those secondary aggregates called segments or " somites," of which it is composed. The successive Figs. 170 — 176, representing (without their limbs) a Julus, a Scolopendra, an isopodous Crustacean, and four kinds of decapodous Crustaceans, ending with a Crab, will convey at a glance an idea of the way in which that greater size and heterogeneity reached by the higher types, is accompanied by an integration which, in the extreme cases, almost obliter ates all traces of composite structure. In the Crab the posterior segments, usually folded underneath the shell, alone preserve their primitive distinctness : so completely confluent are the rest, that it seems absurd to say that a Crab's carapace is composed of as many segments as there are pairs of limbs, foot-jaws, and antennae attached to it ; and were it not that during early stages of the Crab's develop ment the segmentation is faintly marked, the assertion might be considered illegitimate. That all articulate animals are thus composed from end to end of homologous segments, is, however, an accepted doc trine among naturalists. It is a doctrine that rests on care- THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 101 ful observation of three classes of facts — the correspondences of parts in the successive (< somites " of an adult articulate animal ; the still more marked correspondences of such parts as they exist in the embryonic or larval articulate ani mal ; and the maintenance of such correspondences in some types, which are absent in types otherwise near akin to them. The nature of the conclusion which these evidences unite in supporting, will best be shown by the annexed copies from the lecture -diagrams of Prof. Huxley; exhibiting the typical structures of a Myriapod, an Insect, a Spider, and a Crustacean, with their relations to a common plan, as in terpreted by him. Insert r\//7 fffS Treating of these homologies, Prof. Huxley says " that a striking uniformity of composition is to be found in the heads of, at any rate, the more highly organized members of these four classes, and that, typically, the head of a Crustacean, an Arachnid, a Myriapod, or an Insect, is composed of six somites (or segments corresponding with those of the body) and their appendages, the latter being modified so as to serve the purpose of sensory and manducatory organs." And omitting the Myriapods, he also finds among these groups the further unity that in most of them the entire animal contains twenty of these homologous segments. 102 MORPHOLOGICAL DEVELOPMENT. Thus even in the higher Annulosa, the much greater con so. lidation and much greater heterogeneity do not obliterate the evidence of the fact, that the organism is an aggregate of the third order. Beyond all question it is divisible into a number of proximate units, each of which has essentially the same structure as its neighbours, and each of which is an aggregate of the second order, in so far as it is an organized combination of those aggregates of the first order which we call morphological units or cells. And that these segments or somites, which make up an annulose animal, were origin ally aggregates of the second order having independent in dividualities, is an hypothesis which gathers further support from the contrast between the higher and the lower articu late types, as well as from the contrast between the Articu- lata in general and the inferior Annulosa. For if that masking of the individualities of the segments which we find distinguishes the higher forms from the lower, has been going on from the beginning, as we may fairly assume ; it is to be inferred that the individualities of the segments in the lower forms, were originally more marked than they now are. Reversing those processes of change by which the most developed Annulosa have arisen from the least developed ; and applying in thought this reversed process to the least developed, as they were described in the last Chapter ; we are brought to the conception of attached segments that are completely alike, and have their individualities in no ap preciable degree subordinated to that of the chain they com pose. From which there is but a step to the conception of gemmiparously-produced individuals which severally part one from another as soon as they are formed. § 209. We must now return to a point whence we di verged some time ago. As before explained under the head of Classification, organisms do not admit of uni-serial ar rangement, either in general or in detail ; but everywhere form groups within groups. Hence, having traced tho THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 103 phases of morphological composition up to the highest forms in any sub-kingdom, we find ourselves at the extremity of a great branch, from which there is no access to another great branch, except by going back to some place of bifurcation low down in the tree. The nearest relatives of the Mollusca are those molluscoid forms treated of early in the last Chapter. A Brachiopod or a solitary Ascidian, though widely unlike a Mussel or a Snail or a Cuttle-fish, is nearer akin to them than is any coelenterate animal or annulose animal or vertebrate animal. One of the leading distinctions, however, botween the Mol- luscoida and the Mollusca, considered as groups, is that whereas the Molluscoida are very frequently, or indeed generally, compound, the Mollusca are invariably single. No true Mollusk multiplies by gemmation, either continuous or discontinuous ; but the product of every fertilized germ is a single individual. It is a significant fact that here, where for the first time we have homogenesis holding throughout an entire sub- kingdom, we have also throughout an entire sub-king dom no case in which the organism is divisible into two, three, or more, like parts. There is neither any such clustering or branching as a coclentcrate or molluscoid ani mal usually displays ; nor is there any trace of that seg mentation which characterizes the Annulosa. Among these animals in which no single egg produces several individuals, no individual is separable into several homologous divisions. This connexion will be seen to have a probable meaning, on remembering that it is the converse of the connexion which obtains among the Annulosa, considered as a group. A Mollusk, then, is an aggregate of the second order. Not only in the adult animal is there no sign of a multiplicity of like parts that have become obscured by integration ; but there is no sign of such multiplicity in the embryo. And this unity is just as conspicuous in the lowest Lamelli- branch as in the highest Cephnlopod. 104 MORPHOLOGICAL DEVELOPMENT. It may be well to note, however, more especially because it illustrates a danger of misinterpretation presently to be guarded against, that there are certain Mollusks which si mulate the segmented structure. Externally a Chiton, Fig. 188, appears to be made up of divisions substantially like those of the creature Fig 189 ; and one who judged only by externals, would say that the creature Fig. 190 differs as much from the creature Fig, 189, as this does from the preceding one. But the truth is, that while 190 and 189 are closely- allied types, 189 differs from 188 much more widely than a man does from a fish. And the radical distinction between them is this ; that whereas in the Crustacean the segmentation is carried transversely through the whole mass of the body, so as to render the body more or less clearly divisible into a series of parts that are similarly composed ; in the Mollusk the segmentation is limited to the shell which it carries on its upper surface, and leaves its body as completely undivided as is that of a common slug. Were the body cut through at each of the divisions, the sec- t'on of it attached to each portion of the shell would be unlike all the other sections. Here the segmentation has a purely functional derivation — is adaptive instead of genetic. The similarly-formed and similarly-placed parts, are not homolo gous in the same sense as are the appendages of a phoenoga- mic axis or the limbs of an insect. § 210. In studying the remaining and highest sub-king dom of animals, it is important to recognize this radical dif ference in meaning between that likeness of parts which is produced by likeness of modifying forces, and that likeness of parts which is due to primordial identity of origin. On our recognition of this difference depends the view we take THE MORPHOLOGICAL COMPOSITION OF AXIMALS. 105 of certain doctrines that have long been dominant, and have still a wide currency. Among tho Vsrtebrata, as among the Mollusca, homogenesis is universal. The two sub-kingdoms are like one another a IK! unlike the remaining sub-kingdoms in this, that in nil the types they severally include, a single fertilized ovum produces only a single individual. It is true that as the ef unsymmetry are not in action ; and here, though there is a general leaning to the unsymmetrical form, a large proportion of the leaves become quite symmetrical THE SHAPES OF LEAVES. 145 differentiated in proportion as their relations to incident forces become different. And here, as before, we see that in each unit, considered by itself, the differences of dimension are greatest in those directions in which the parts are most differently conditioned ; while there are no differences be tween the dimensions of the parts that are not differently conditioned.* * It was by an observation on the forms of leaves, that I was first led to the views set forth in the preceding and succeeding chapters on the morphological differentiation of plants and animals. In the year 1851, during a country ramble in which the structures of plants had been a topic of conversation with a friend— Mr G. II. Lewes — I happened to pick up the leaf of a buttercup, and drawing it by its foot-stalk through my fingers so as to thrust together its deeply- cleft divisions, observed that its palmate and almost radial form was changed into a bilateral one ; and that were the divisions to grow together in this new position, an ordinary bilateral leaf would result. Joining this observation with the familiar fact that leaves, in common with the larger members of plants, habitually turn themselves to the light, it occurred to me that a natural change in the circumstances of the leaf might readily cause such a modification of form as that which I had produced artificially. If, as they often do with plants, soil and climate were greatly to change the habit of the buttercup, making it branched and shrub-like ; and if these palmate leaves were thus much over shadowed by each other; would not the inner segments of the leaves grow towards the periphery of the plant where the light was greatest, and so change the palmate form into a more decidedly bilateral form ? Immediately I began to look round for evidence of the relation between the forms of leaves and the general characters of the plants they belonged to ; and soon found some signs of con nexion. Certain anomalies, or seeming anomalies, however, prevented me from then pursuing the inquiry much further. But consideration cleared up these difficulties; and the idea afterwards widened into the general doctrine here elaborated. Occupation with other things prevented me from giving expression to this general doctrine until Jan. 1859 ; when I published an outline of it in the Medico -Chirwgical Review. CHAPTER X. THE SHAPES OF FLOWERS. § 232. FOLLOWING an order like that of preceding chap ters, let us first note a few typical facts respecting the forms of clusters of flowers, apart from the forms of the flowers them selves. Two kindred kinds of LeguminoscB will serve to show how the members of clusters are distributed in an all-sided man ner or in a two-sided manner, according as the circumstances are alike on all sides or alike on only two sides. In Hippo- erepis, represented in Fig. 226, the flowers growing at the end of a vertical stem, are arranged round it in radial symmetry. Contrariwise in Melilotus, Fig. 227, where the axillary stem bearing the flowers is so placed in relation to the main stem, that its outer and inner sides are differently condi tioned, the flowers are all on the outer side : the cluster is bilaterally symmetrical, since it may be cut into approx imately equal and similar groups by a vertical plane passing through the main axis. Plants of this same tribe furnish clusters of intermediate characters having intermediate conditions. Among these, as among the clusters whicn other types present, may be THE SHAPES OF FLOWERS. 147 found some in which conformity to the general law is not obvious. The discussion of these apparent anomalies would carry us too much out of our course. A clue to the explana tion of them will, I believe, be found in the explanation presently to be given of certain kindred anomalies in the forms of individual flowers. § 233. The radially- symmetrical form is common to all individual flowers that have vertical axes. In plants which are practically if not literally uniaxial, and bear their flowers at the ends of upright stalks, so that the faces open hori zontally, the petals are disposed in an all-sided way. Cro cuses, Tulips, and Poppies are familiar examples of this struc ture occurring under these conditions. A Ranunculus flower, Fig. 228, will serve as a typical one. Similarly, flowers which have peduncles flexible enough to let them hang directly downwards, and are not laterally incommoded, are also radial ; as in the FucJma, Fig. 229, as in Cyclamen, Hyacinth, &c. These rela tions of form to position are, I believe, uniform. Though some flowers carried at the ends of up right or downright stems have oblique shapes, it is only when they have inclined axes or are not equally conditioned all round. No solitary flower having an axis habitually ver tical, presents a bilateral form. This is as we should expect, since flowers which open out their faces horizontally, whether facing upwards or downwards, are, on the average, similarly affected on all sides. At first it seems that flowers thus placed should alone be radial ; but further consideration discloses conditions under which this type of symmetry may exist in flowers otherwise placed. Remembering that the radial form is the primitive form — that, morphologically speaking, it results from the contraction into a whorl, of parts that are originally arranged in the same spiial succession as the leaves; we must expect 1 18 MORPHOLOGICAL DEVELOPMENT. it to continue wherever there are no forces tending to change it. What now must be the forces tending to change it? They must be forces which do not simply affect differently the different parts of an individual flower ; but they must be forces which affect in like contrasted ways the homologous parts of other individual flowers, both on the same plant and on surrounding plants of the same species. A permanent modification can be expected only in cases where, by inherit ance, the effect of the modifying causes accumulates. That it may accumulate the flowers must keep themselves so re lated to the environment, that the homologous parts may generation after generation be subjected to like differentiating forces. Hence, among a plant's flowers which maintain no uni formity in the relations of their parts to surrounding influences, the radial form will continue. Let us glance at the several causes which entail this variability. When flowers are borne on many branches, which have all inclinations from the vertical to the horizontal — as are the flowers of the Apple, the Plum, the Hawthorn — they are placed in countless different attitudes. Consequently, any spontaneous variation in shape which might be advantageous were the attitude constant, is not likely to be advantageous ; and any functionally-produced modification in one flower is likely to be neutralized in off spring by some opposite functionally-produced modification in another flower. It is quite comprehensible, therefore, that irregularly-branched plants should thus preserve their laterally-borne flowers from undergoing permanent devia tions from their primitive radial symmetry. Fig. 230, re- Presenting a blossoming twig of the Blackthorn, will illustrate this. Again, upright panicles such as that of the Saxifrage, shown in Fig. 231, and irregular terminal groups of flowers otherwise named, furnish conditions under which there is similarly an THE SHAPES OF FLOWERS. 149 absence of determinate relations between the parts of the flowers and the incident forces ; and hence an absence of bilateralness. This inconstancy of relative position is produced in various other ways — by extreme flexibility of the peduncles, as in the Blue-bell ; by the tendency of the peduncles to curl to a greater or less extent in different directions, as in Pyrola ; by special twisting of the peduncles, differing in degree in different individuals, as in Convol vulus ; by extreme flexibility of the petals, as in Ly thrum. Elsewhere the like general result arises from a progressive change of attitude ; as in Mi/osotis, the stem of which as it 'anfolds causes each flower to undergo a transition from an upward position of the mouth to a lateral position ; or as in most CrucifercBj where the like effect follows from an altered direction of the peduncle. There are, however, certain seemingly anomalous cases where radial sympathy is maintained by laterally-placed flowers, which keep their parts in relative positions that are tolerably constant. The explanation of these exceptions is not manifest. It is only when we take into account certain incident actions liable to be left un remembered, that we find a probable solution. It will be most convenient to postpone the consideration of these cases until we have reached the general rule to which they are exceptions. § 234. Transitions varying in degree from the radial to wards the bilateral, are common in flowers that are borne at the ends of branches or axes which are inclined in tolerably constant ways. We may see this in sundry garden flowers such as Petunia, or such as Tydaxi and Aclnmenes shown in Figs. 232 and 233. If these plants be examined, it will be perceived that the mode of growth makes the flower unfold in a partially one sided position ; that its parts of attachment have rigidity 150 MORPHOLOGICAL DEVELOPMENT. sufficient to prevent this attitude from being very much interfered with ; and that though the individual flowers yary somewhat in their attitudes, they do not vary to the extent of neutralizing the differentiating conditions — there remains an average divergence from a horizontal unfolding of the flower, to account for its divergence from radial symmetry. We pass insensibly from forms like these, to forms having bilateral symmetry strongly pronounced. Some such forms occur among flowers that grow at the ends of upright stems ; as in Pingiiicula, and in the Violet tribe. But this happens only where in successive generations the flower unfolds its parts sideways in constant relative positions. And in the immense majority of flowers that have well-marked two-sided forms, the habitual exposure of the different parts to different sets of forces, is effectually secured by the mode of placing. As illustrations I may name the genera — Orchis, Utricularia, Salvia, Salix, Delphinum, Mentha, Teucrium, Ajuga, Ballota, Galeopsis, Lamium, Stachys, Glechoma, Ufarrubinm, Cala- mintha, Clinopodiwn, Melittis, Prunella, Scntellaria, Barf- sia, Euphrasia, Ithinanthiis, Mclampt/rum, Pcdicularis, Lin- aria, Digitalis, Orobanche, Fumaria, fyc. ; to which may be added, all the Grasses and all the Papilionacece. In most of these cases the flowers, being sessile on the sides of upright stems, are kept in quite fixed attitudes ; and in the other cases the peduncles are very short, or else stiff enough to secure general uniformity in the positions. A few of the more marked types are shown in Figs. 234 to 241. 234 2,3^ V3ff %37 ?.3S %3$ 2,40 24 J Very instructive evidences here meet us. Sometimes with in the limits of one genus we find radial flowers, bilateral flowers, and flowers of intermediate characters. The genus Begonia may be instanced. In B. rigida the flowers, various THE SHAPES OF FLOWERS. 151 in their attitudes, are in their more conspicuous characters radial : though there is a certain l/ilateralness in the calyx, the five petals are symmetricctlly disposed all round. B. Wagnenana furnishes two forms of flowers : on the same in dividual plant may be found radial flowers like Fig. 242, and others like Fig. 243 that are merging into the bilateral. More decided is the bilateralness in B. albo-coccinea, Fig. 244 ; and still more in B. nitida, Fig. 245. While in B.jatrophoe- 345 2,46 folia, Fig. 246, the change reaches its extreme by the dis appearance of the lateral petals. On examining the modes of growth in these several species, they will be seen to explain these changes in the manner alleged. Even more conclusive are the nearly-allied transformations occur ring in artificially-produced varieties of the same species. Gloxinia may be named in illustration. In Fig. 247 is repre sented one of the ordinary forms, which shows us bilateralness of shape along with a mode of growth that renders the conditions alike on the two sides while different above and below. But in G. erccta, Fig. 248, we have the flower assuming an upright attitude, and at the same time assuming the radial type. This is not to be inter preted as a production of ra dial symmetry out of bilateral symmetry, under the action of the appropriate conditions. It is rather to be taken as a case of what is termed " peloria " — a reversion to the primitive radial type, from which the bilateral modification had been derived. The significant inference to be drawn from it is, that this primitive radial type had an upright attitude; and 243 152 MORPHOLOGICAL DEVELOPMENT. that the derivation of a bilateral type from it, occurred along with the assumption of an inclined attitude. We come now to a group of cases above referred to, in which radial symmetry continues to co-exist with that con stant lateral attitude ordinarily accompanied by the two-sided form. Two examples will suffice : one a very large flower, the Hollyhock, and the other a very small flower, the Agri mony. Why does the radial form here remain unchanged ? and how does its continuaiice consist with the alleged general law ? Until quite recently I have been unable to find any pro bable answers to these questions. When the difficulty first presented itself, I could think of no other possible cause for the anomaly, than that the parts of the Hollyhock-flower, unfolding spirally as they do, might have different degrees of spiral twist in different flowers, and might thus not be unfolded in sufficiently-constant positions. But this seemed a very questionable interpretation ; and one which did not obviously apply to the case of the Agrimony. It was only on inquiring what are the special causes of modifications in the forms of flowers, that a more feasible explanation suggested itself; and this would probably never have suggested itself, had not Mr Darwin's investigations into the fertilization of Orchids led me to take into account an unnoticed agency. The actions which affect the forms of leaves, affect much less decidedly the forms of flowers ; and the forms of flowers are influenced by actions that do not influence the forms of leaves. Partly through the direct action of incident forces and partly through the indirect action of natural selection, leaves get their parts distributed in ways that most facilitate their assimilative functions, under the circumstances in which they are placed ; and their several types of symmetry are thus explicable. But in flowers, the petals and fructifying organs of which do not contain chlorophyll, the tendency to grow most where the supply of light is greatest, is less decided, if not absent ; and a shape otherwise determined is hence less THE SHAPES OF FLOWERS. 153 liable to alter in consequence of altered relations to sun and air. Gravity, too, must be comparatively ineffective in caus ing modifications : the smaller sizes of the parts, as well as their modes of attachment, giving them greater relative rigidity. Not, indeed, that these incident forces of the inor ganic world are here quite inoperative. Fig. -449 2-19, representing a species of Campanula, shows that the developments of individual flow ers are somewhat modified by the relations of their parts to general conditions. But the fact to be observed is, that the extreme trans formations which flowers undergo are not likely to be thus caused : some further cause must be sought. And if we bear in mind the functions of flowers, we shall find in their adaptations to their functions, under conditions that are extremely varied, an adequate cause for the different types of symmetry, as well as for the exceptions to them. Flow ers are parts in which fertilization is effected; and the active agents of this fertilization are insects — bees, moths, butterflies, &c. Mr Darwin has shown in many cases, that the forms and positions of the essential organs of fructifica tion, are such as to facilitate the actions of insects in trans ferring pollen from the anthers of one flower to the pistil of another — an arrangement produced by natural selection. And here we shall find reason for concluding, that the forms and positions of those subsidiary parts which give the gene ral shape to the flower, similarly arise by the survival of individuals which have the subsidiary parts so adjusted as to aid this fertilizing process — the deviations from radial sym metry being among such adjustments. The reasoning is as follows. So long as the axis of a flower is vertical and the conditions are similar all round, a bee or butterfly alight ing on it, will be as likely to come from one side as from another ; and hence, hindrance rather than facilitation would result if the several sides of the flower did not afford it equally 154 MORPHOLOGICAL DEVELOPMENT. free access. In like manner, flowers which are distributed over a plant in such ways that their discs open out on planes of all directions and inclinations, will have no tend ency to lose their radial symmetry ; since, on the average, no part of the periphery is differently related to insect- agency from any other part. But flowers so fixed as to open out sideways in tolerably-constant attitudes, ha\e their petals differently related to insect-agency. A bee or butterfly coming to a laterally-growing flower, does not set tle on it in one way as readily as in another ; but almost of necessity settles with the axis of its body inclined upwards towards the stem of the plant. Hence, the side-petals of a flower so fixed, habitually stand to the alighting insect in relations different from those in which the upper and lower petals stand ; and the upper and lower petals differ from one another in their relations to it. If, then, there so arises an habitual attitude of the insect towards the petals, there must be some particular arrangement of the petals that will be most convenient to the insect — will most facilitate its en trance into the flower. Thus we see in many cases, that a long undermost petal or lip, by enabling the insect to settle in such way as to bring its head opposite to the opening of the tube, aids its fertilizing agency. But whatever be the spe cial modifications of the corolla which facilitate the actions of the particular insects concerned, all of them will conduce to bilateral symmetry ; since they will be alike for the two sides but unlike for the top and bottom. And now we are prepared for understanding the exceptions. Flowers growing sideways can become thus adapted by survival of the fittest, only if they are of such sizes and structures that insect- agency can affect them in the way described. But in the plants named above, this condition is not fulfilled. A Hollyhock- flower is so open, as well as so large, that its petals are not in any appreciable degree differently related to the insects which visit it. On the other hand, the flower of the Agrimony is so small, that unless visited by insects of a THE SHAPES OF FLOWERS. 155 corresponding1 size which settle as bees and butterflies settle, its parts will not be affected in the alleged manner. That all anomalies of this kind can at once be satisfactorily ex plained, is scarcely to be expected : the circumstances of each case have to be studied. But it seems not improbable that they are all due to causes of the kind indicated. § 235. We have already glanced at clusters of flowers for the purpose of considering their shapes as clusters. Wo must now return to them to observe the modifications under gone by their component flowers. Among these occur illus trations of great significance. An example of transition from the radial to the bilateral form in clustered flowers of the same species, is furnished by the cultivated Geraniums , called by florists Pelargoniums. Some of these bearing somewhat small terminal clusters of flowers, wrhich are closely packed together, with their faces almost upwards, have radially-symmetrical flowers. But among other varieties having terminal clusters of which the members are mutually thrust on one side by crowding, the flowers depart very considerably from the radial shape towards the bilateral shape. A like result occurs under like conditions in Rhododendrons and Azaleas. The Verbena, too, furnishes an illustration of radial flowers rendered slightly two-sided by the slight two-sideness of their rela tions to other flowers in the cluster. And among the Cruel- ferce, a kindred case occurs in the cultivated Candytuft. Evidence of a somewhat different kind, is offered us by clustered flowers in which the peripheral members of the clusters differ from the central members ; and this evidence is especially conclusive where we find allied species that do not exhibit the deviation, at the same time that they do not fulfil the conditions under which it may be expected. Thus, in Scabiosa succisa, Fig. 250, which bears its numerous small flowers in a hemispherical knob, the component flowers, similarly circumstanced, are all equal and all radial; but ill 156 MORPHOLOGICAL DEVELOPMENT. Scabiosa arvcnsis, Fig. 251, in which the numerous small flowers form a flattened disk, only the confined central ones are radial : round the edse tlie flowers are much larger, and conspicuously bilateral. But the most remarkable and most conclusive proofs of these relations between forms and positions, are those given by the clustered flowers called UmbellifercB. In some cases, as where the component flowers have all plenty of room, or where the surface of the umbel is more or less globular, the modifications are not conspicuous ; but where, as in Viburnum, C/mrophyllum, Anthriscus, Torilis, Caucalis, Daucus, Tordylium, &c., we have flowers clustered in such ways as to be differently conditioned, we find a num ber of modifications that are marked and varied in propor tion as the differences of conditions are marked and varied. In Chcerophyllum, where the flowers of each umbellule are closely placed so as to form a flat surface, but where the umbellules are wide apart and form a dispersed umbel, the umbellules do not differ from one another ; though among the flowers of each umbellule there are decided differences — the central flowers being small and radial, while the peripheral ones are large and bilateral. But in other genera, where not only the flowers of each umbellule but also the umbellules themselves are closely clustered into a flat surface, the umbel lules themselves become contrasted; and many remarkable secondary modifications arise. In an umbel of Heracleum, for instance, there are to be noted the facts : — first, that the external umbellules are larger than the internal ones ; second, that in each umbellule the central flowers are less developed than the peripheral ones ; third, that this greater development of the peripheral flowers is most marked in the outer umbellules ; fourth, that it is most marked on the outer sides of the outer umbellules ; fifth, that while the interior flowers of each umbellule are radial, the exterior ones are TttK SHAPES OF FLOWERS. 157 bilateral ; sixth, that this bilateralness is most marked in the peripheral flowers of the peripheral umbellules; seventh, that the flowers on the outer side of these peripheral umhellules are those in which the bilateraliiess reaches a maximum ; and eighth, that where the outer urnbellules touch each other, the flowers, being unsymmetrically placed, are unsymmetrically bilateral.* The like modi fications are displayed, though not in so clearly- trace able a way, in an umbel of Tordyliiun, Fig. 252. Considering how obviously these various forms are related to the vari ous conditions, we should be scarcely able, even in the absence of all other facts, to resist the conclusion that the differences in the conditions are the causes of the differ ences in the forms. Composite flowers furnish evidence so nearly allied to that which clustered flowers furnish, that we may fitly glance at them under the same head. Such a common type of this order as the Sun-flower, exemplifies the extremely marked differ ence that arises in many of these plants between the closely-packed internal florets, each similarly circumstanced on all sides, and the external florets, not similarly circumstanced on all sides. In Fig. 253, representing the inner and outer florets of a Daisy, the contrast is marked between the small radial corolla of the one and the larger bilateral corolla of the other. In many cases, how ever, this contrast is less marked : the inner florets having * I had intended here to insert a figure exhibiting these differences ; but as the Cow-parsnip does not flower till July, and as I can find no drawing of the umbel which adequately represents its details, I am obliged to take another instance. Iow it is a corollary from the instability of the homogeneous, that the rates of growth on all sides of a shoot can never be exactly alike ; and it is to be also inferred from the same general law, that the greatest and least rates of growth will not occur on exactly opposite sides of the shoot, at the same time that equal rates of growth are preserved by the two other sides. Hence, there must almost inevitably arise more or less of twist ; and the appendages of the internodes will so be prevented from occurring perpendicularly one over another. A deviation of this kind, necessarily initiated by physical causes in conformity with the general laws of evolution, is likely to be made regular and decided by natural selection. For under ordinary circumstances, a plant will profit by hav ing its axis so twisted as to bring the appended leaves into positions that prevent them from shading one another. And, manifestly, modifications in the forms, sizes, and insertions of the leaves, may, under the same agency, lead to adapted modifications of the twist. We must therefore ascribe this common characteristic of pha3nogams, primarily to local differ ences of nutrition, and secondarily to survival of the fittest. It is proper to add that there are some Monocotyledons; as Urania speciosa, in which this character does not occur. What conditions of existence they are that here hold this natural tendency in check, it is not easy to see.* * The Natural History Rcn'cw for July, 1865, contained an article on the doc- trine of morphological composition set forth in the foregoing Chaps. I. to III. In this article, which unites exposition and criticism in a way that is unhappily not common with reviewers, it is suggested that the spiral structure may he caused by natural selection. "When this article appeared, the foregoing five pages were Branding over in type, as surplus from No. 14, issued in June, 1855. CHAPTER XIII. MORPHOLOGICAL DIFFERENTIATION IN ANIMALS. § 242. THE general considerations which preluded our in quiry into the shapes of plants and their parts, equally serve, so far as they go, to prelude an inquiry into the shapes of animals and their parts. Among animals, as among plants, the formation of aggregates greater in bulk or higher in degree of composition, or both, is accompanied by changes of form in the aggregates as wholes as well as by changes of form in their parts ; and the processes of morphological differentiation conform to the same general laws in the one kingdom as in the other. It is needless to recapitulate the several kinds of modifi cation to be explained, and the several factors that co operate in working them. In so far as these are common to plants and animals, the preceding chapters have suf ficiently familiarized them. Nor is it needful to specify afresh the several types of symmetry and their descriptive names ; for what is true of them in the one case is true of them in the other. There is, however, one new and all- important factor which we shall have now to take into account ; and about this a few preliminary remarks are requisite. § 243. This new factor is motion — motion of the organism in rolation to surrounding objects, or of the parts of the MORPHOLOGICAL DIFFERENTIATION IN ANIMALS. 167 organism in relation to one another, or both. Though there are plants, especially of the simpler kinds, which move, and though a few of the simpler animals do not move ; yet movements are so exceptional and unobtrusive in the one kingdom, while they are so general and conspicuous in the other, that the broad distinction commonly made is well warranted. What, among plants, is an inappreciable cause of morphological differentiation, becomes, among animals, the chief cause of morphological differentiation. Animals that are rooted or otherwise fixed, of course present traits of structure nearest akin to those we have been latelv studying. The motions of parts in relation to one another and to the environment, being governed by the mode of aggre gation and mode of fixing, we are presented with morphological differentiations similar in their general characters to those of plants, and showing us parallel kinds of symmetry under parallel conditions. But animals which move from place to place are subject to an additional class of actions and re actions. These actions and reactions affect them in various ways according to their various modes of movement. Let us glance at the several leading relations between shape and motion which we may expect to find. If an organism advances through a homogeneous medium with one end always foremost, that end, being exposed to forces unlike those to which the other end is exposed, may be expected to become unlike it ; and supposing this to be the only constant contrast of conditions, we may expect an equal distribution of the parts round the axis of move ment — a radial symmetry. If in addition to this habitual attitude of the ends, one surface of the body is always uppermost and another always lowermost, there arise between the top and bottom dissimilarities of conditions, while the two sides remain similarly conditioned. Hence it is inferable that such an organism will be divisible into similar halves by a vertical plane passing through its axis of motion — will have a bilateral symmetry. We may presume 168 MORPHOLOGICAL DEVELOPMENT. that this symmetry will deviate but little from double bilateralness where the upper and under parts are not exposed to strongly-contrasted influences ; while we may rationally look for single bilateral symmetry of a decided kind, in creatures having dorsal and ventral parts conversant with very unlike regions of the environment : as in all cases where the movement is over a solid surface. If the movement, though over a solid surface, is not constant in direction, but takes place as often on one side as on another, radial symmetry may be again looked for ; and if the motions are still more variously directed — if they are not limited to approximately-plane surfaces, but extend to surfaces that are distributed all around with a regular irregularity — an ap proach of the radial towards the spherical symmetry is to be anticipated. Where the habits are such that the intercourse between the organism and its environment, does not involve an average equality of actions and reactions on any two or more sides, there may be expected either total irregularity or some divergence from regularity. The like general relations between forms and incident forces are inferable in the component parts of animals, as well as in the animals as wholes. It is needless, however, to occupy space by descriptions of these. Let us now pass to the facts, and see how they confirm, a posteriori, the con clusions here reached d CHAPTER XIV. THE GENERAL SHAPES OF ANIMALS. § 244. CERTAIN of the Protozoa are quite indefinite in their shapes, and quite inconstant in those indefinite shapes which they have — the relations of their parts are indeter minate both in space and time. In one of the simpler Khizopods, at least during the active stage of its existence, no permanent distinction of inside and outside is established ; and hence there can arise no established correspondence between the shape of the outside and the distribution of environing actions. But when the relation of inner and outer becomes fixed, either over part of the mass or over the whole of it, we have kinds of symmetry that correspond with the habitual incidence of forces. An Amasba in be coming encysted, which we may regard as the production in it of a differentiation between superficial parts and central parts, passes from an indefinite, ever-changing form into a spherical form ; and the order of symmetry which it thus assumes, is in harmony with the average equality of the actions on all its sides. In Diffliigid. Fig. 134, and still better in Arcctta, we have an indefinitely-radial symmetry occurring where the conditions are different above and below but alike all around. Among the Gi'cgarinida the spherical symmetry and symmetry passing from that into the radial, are such as appear to be congruous with the simple cir cumstances of these creatures in the intestines of insects. 170 MORPHOLOGICAL DEVELOPMENT. But the relations of these lowest types to their environments are comparatively so indeterminate, and our knowledge of their actions so scanty, that little beyond negative evidence can be expected from the study of them. The like may be said of the Infusoria. These are more or less irregular. In some cases where the line of move ment through the water is tolerably definite and constant we have a form that is approximately radial — externally at Least. But usually, as shown in Figs. 137, 138, 139, there is either an imsymmetrical or an asymmetrical shape. And when one of these creatures is watched under the microscope, the congruity of this shape with the incidence of forces is mani fest. For the movements are conspicuously varied and indeterminate — movements which do not expose any two or more sides of the mass to approximately equal sets of actions. § 245. Among aggregates of the second order, as among aggregates of the first order, wo fin:l that of those possessing any definite shapes the lowest are spherical or spheroidal. Such are the Thalassicolke, These gelatinous bodies which float passively in the sea, and present in turn all their sides to the same influences, have their parts disposed with ap proximate regularity all around a centre. In some orders of Fora mini/era, as for instance the Nummulites, we have secondary aggregates the parts of which are spirally ar ranged, approximately in harmony with the radial relations of the society to the environment ; but we have other types in which the congregated units are distributed in ways not easily definable, and having to the environment relations that are obscure. Further, among these secondary aggregates in which the units, only physically integrated, have not had their THE GENERAL SHAPES OF ANIMALS. 171 individualities merged into an individuality of a higher order, must be named the compound Infusoria. The cluster of Vorticellce in Fig. 144, will sufficiently exemplify them ; and the striking resemblance borne by its individuals to those of a radially-arranged cluster of flowers, will show how, under analogous conditions, the general principles of morphological differentiation are similarly illustrated in the two kingdoms. § 246. Hadial symmetry is usual in those aggregates of the second order that have their parts sufficiently differentiat ed and integrated to give individualities to them as wholes. The Ccelenterata offer numerous examples of this. Solitary polypes — hydroid or helianthoid — mostly stationary, and when they move, moving with any side foremost, do not by 1'oomotion subject their bodies to habitual contrasts of con ditions. Seated with their mouths upwards or downwards, or else at all degrees of inclination, the individuals of a species taken together, are subject to no mechanical actions affecting some parts of their discs more than other parts. And this indeterminateness of attitude similarly prevents their relations to prey from being such as subject some of their prehensile organs to forces unlike those to which the rest are subject. The fixed end is differently conditioned from the free end, and the two are therefore different ; but around the axis running from the fixed to the free end the conditions are alike in all directions, and the form therefore is radial. Again, among many of the simple free- swimming Hydrozoa, the same general truth is exemplified under other circumstances. In a common Medusa, advanc ing through the water by the rhythmical contractions of its disc, the mechanical reactions are the same on all sides ; and as, from accidental causes, every part of the edge of the disc comes upwards in its turn, no part is permanently af fected in a different way from the rest. Hence the radial form continues. L72 MORPHOLOGICAL DEVELOPMENT Iii others of this same group, however, there occur forms which show us an incipient bilateralness ; and help us to see how a more decided bilateralness may arise. Sundry of the Medusidce are proliferous, giving origin to gemmae from the body of the central polypite or from certain points on the edge of the disc ; and this budding, unless it occurs equally on all sides, which it does not and is unlikely to do, must lend to destroy the balance of the disc, and to make its attitude less changeable. In other cases the growth of a large process from the edge of the disc on one side, as in Steenstrupia, Fig 257 — a process that is perhaps the morphological equivalent of one of the gemmae just named — constitutes a similar modi fication, and a cause of further modification. The existence of this process makes the animal no longer divisible into any two quite similar halves, except those formed by a plane passing through the process ; and unless the process is exactly of the same specific gravity as the disc, it must tend towards either the lowest or the highest point, and must so serve to increase the bilateralness, by keeping the two sides of the disc similarly conditioned while the top and bottom are differently conditioned. Fig. 258 represents the underside of another Medusa, in which a more decided bi lateralness is produced by tho presence of two such process es. Among the simple free-swimming Actinozoa, occur like deviations from radial sym metry, along with like motions through the water in bilateral attitudes. Of this a Cydippe is a familiar example. Though radial in some of its characters, as in the distribution of its meridional bands of locomotive paddles with their accompany ing canals, this creature has a two-sided distribution of tentacles and various other parts, corresponding with its two- sided attitude in moving through the water. And in othei THE GENERAL SHAPES OF ANIMALS. 173 genera of this group, as in Cesium, Eurhamphoea, and Callianira, that almost equal distribution of parts which characterizes the Bcroe is quite lost. Here seems a fit place to meet the objection which some may feel to this and other such illustrations, that they amount very much to physical truisms. If the parts of a Medusa are disposed in radial symmetry around the axis of motion through the water, there will of course be no means of maintaining one part of its edge upwards more than another ; and the equality of conditions may be ascribed to the radiate- ness, as much as the radiateness to the equality of conditions. Conversely, when the parts are not radially arranged round the axis of motion, they must gravitate towards some one attitude, implying a balance on the two sides of a vertical plane — a bilateralness ; and the two-sided conditions so necessitated, may bo as much ascribed to the bilateralness as the bilateralness to the two-sided conditions. Doubt less the form and the conditions are, in the way alleged, necessary correlates ; and in so far as it asserts this, the ob jection harmonizes with the argument. To the difficulty which it at the same time raises by the implied question- Why make the form the result of the conditions, rather than the conditions the result of the form ? the reply is this :— The radial type, both as being the least differentiated type and as being the most obviously related to lower types, must be taken as antecedent to the bilateral type. The indi vidual variations wrliich incidental circumstances produce in the radial type, will not cause divergence of a species from the radial type, unless such variations give advantages to the individuals displaying them; which there is no reason to sup pose they will always do. Those occasional deviations from the radial type, which the law of the instability of the homo geneous warrants us in expecting to take place, will, however, in some cases be beneficial ; and will then be likely to estab lish themselves. Such deviations must tend to destroy the original indcfiniteness and variability of attitude — must 174 MORPHOLOGICAL DEVELOPMENT. cause gravitation towards an habitual attitude. And gravita tion towards an habitual attitude having once commenced, will continually increase, where increase of it is not negatived by adverse agencies : each further degre e of bilateralness rendering more decided the actions that conduce to bilateral- ness. If this reply be thought insufficient, it may be enforced by the further one, that as, among plants, the incident forces are the antecedents and the forms the consequents (changes of Ibrces being in many cases visibly followed by changes of forms) we are warranted in concluding that the like order of cause and effect holds among animals. § 247. Keeping to the same type but passing to a higher degree of composition, we meet more complex and varied illustrations of the same general laws. In the compound Ccelcnterata, presenting clusters of individuals that are severally homologous with the solitary individuals last dealt with, we have to note both the shapes of the individuals thus united, and the shapes of the aggregates made up of them. Such of the fixed Hydrozoa and Actinozoa as form branched societies, continue radial ; both because their varied attitudes do not expose them to appreciable differences in their rela tions to those surrounding actions which chiefly concern them (the actions of prey), and because such differences, even if they were appreciable, would be so averaged in their effects on the dissimilarly-placed members of each group as to be neutralized in the race. Among the tree-like coral-polypedoms, as well as in such ramified assemblages of simpler poly pes as are shown in Figs. 149, 150, we have, indeed, cases in many respects paral lel to the cases of scattered flowers (§ 233), which though placed laterally remain radial, because no differentiating agency can act uniformly on all of them. Meanwhile, in the groups which these united individuals compose, we see the shapes of THE GENERAL SHAPES OF ANIMALS. 175 plants further simulated under a further parallelism of con ditions. The attached ends differ from the free ends as they do in plants ; and the regular or irregular branches obvious ly stand to environing actions in relations analogous to those in which the branches of plants stand. The members of those compound Ccelenterata which move through the water by their own actions, in attitudes that are approximately constant, show us a more or less distinct two- sidedness. Diphycs, Fig. 259, furnishes an example. Each of the largely-developed and modified polypites forming its swimming sacs is bilateral, in correspondence with the bi- lateralness of its conditions ; and in each of the appended polypites the insertion of the solitary tentacle produces a kindred divergence from the primitive radial type. The aggregate, too, which here very much subordinates its mem bers, exhibits the same conformity of structure to circum stances. It admits of symmetrical bisection by a plane pass ing through its two contractile sacs, or nectocalyces, but not by any other plane ; and the plane which thus symmetrically Disects it, is the vertical plane on the two sides of which its mrts are similarly conditioned as it propels itself through ;he water. Another group of the oceanic Hydrozoa,, the Physophoridw, Burnishes interesting evidence — not so much in respect of the brms of the united individuals, which we may pass over, as n respect of the forms of the aggregates. Some of these vhich are without swimming organs, have their parts sus pended from air-vessels which habitually float on the surface )f the water ; and the distribution of their parts is asyra- 44 176 MURl'HOLOGICAL DEVELOPMENT. metrical. The Physdia, Fig. 152, is an example. Here the relations of the integrated group of individuals to the environ ment are indefinite ; and there is hence no agency tending to change that comparatively irregular mode of growth that is probably derived from a primordial type of the branched Hydrozoa. So various are the modes of union among the compound Coelenterata, that it is out of the question to deal with them all. Even did space permit, it would be impracticable for any one but a professed naturalist, to trace through out this group the relations between shapes and conditions of existence. The above must be taken simply as a few of the most significant and easily-interpret- able cases. § 248. In the sub-kingdom Molina- co id a, we meet with examples not wholly unlike the foregoing. Among the types assembled under this title there are simple individuals or aggregates of the second order, and societies or tertiary aggregates produced by their union. The relations of forms to forces have to be traced in both. Solitary Ascidians, fixed or floating, carry on an inactive and indefinite converse with the actions in the environment. Without power to move about vivaciously, and unable to catch any prey but that contained in the currents of water they absorb and expel, these creatures are not exposed to sets of forces that are equal on two or more sides ; and their shapes consequently remain vague. Though there are in them traces of symmetrical arrangement, probably due to their derivation, yet they are substantially asymmetrical. Fig. 156 is an example. Among the composite Ascidians, floating and fixed, the shape of the aggregate, Tllll GENERAL SHAPES OF ANIMALS. 17? partly determined by the habitual mode of gemmation and partly by the surrounding conditions in each case, is in great measure indefinite. We can say no more about it than that it is not obviously at variance with the laws alleged. Evidence of a more positive kind occurs among those com pound Molluscoida which are most like the compound Coclenterata in their modes of union — the Polyzoa. Many of these form groups that are more or less irregular — spreading as films over solid surfaces, combining into sea- weed-like fronds, budding out from creeping stolons, or growing up into tree-shaped societies ; and besides aggregating ir regularly they are irregularly placed on surfaces inclined in all directions. Merely noting that this asymmetrical dis tribution of the united individuals is explained by the absence of defimteness in the relations of the aggregate to incident forces, it concerns us chiefly to observe that the united individuals severally exemplify the same truth as do similarly-united individuals among the Ccelenterata. While their internal organs, though said to have a trace of bi- lateralness, cannot be said to display any definite symmetry ; their external organs are completely radial. Averaging the members of each society, the ciliated tentacles they protrude are similarly related to prey on all sides ; and therefore remain the same on all sides. This distribution of tentacles is not, however, without exception. Among the fresh-water Polyzoa there are some genera, as Plumatella and Crystatclla, in which the arrangement of these parts is very decidedly bilateral. Some species of them show us such relations of the individuals to one another and to their surface of attach ment, as give a clue to this modification ; but in other species the meaning of this deviation from the radial type is not obvious. § 249. In that somewhat heterogeneous assemblage of animals now classed, perhaps provisionally, as Annuloida, we begin again with simple aggregates of the second order, and 178 MORPHOLOGICAL DEVELOPMENT. ascend to aggregates in which, we have seen reason to suspect a higher degree of composition. Good examples of the con nexions between, forms and forces occur in this group. Among the lower annuloid types, the Planaria exemplifies the single bilateral symmetry which, even in very inferior forms, accompanies the habit of moving in one direction over a solid surface. Humbly organized as are these creatures and their allies the Nemertidce, we see in them just as clearly as in the highest animals, that where the movements subject the body to different forces at its two ends, different forces 011 its under and upper surfaces, and like forces along its two sides, there arises a corresponding form, unlike at its extremi ties, unlike above and below, but having its two sides alike. The Echinodermata furnish us with instructive illustrations — instructive because among types that are nearly allied, we meet with wide deviations of form answering to marked con trasts in the relations to the environment. The facts fall into four groups. The Crinoidea, once so abundant and now so rare, present a radial symmetry answering to an incidence of forces that is equal on every side. In the general attitudes of their parts towards surrounding actions, they are like imiaxial plants or like polypes ; and show, as they do, marked differences between the attached ends and the free ends, along with even distributions of parts all round their axes. In the Ophitiridea, proved to be near akin to the Crinoids, and in the Star-fishes, we have radial symmetry co-existing with very different habits ; but habits which nevertheless account for the maintenance of the form. Holding on to rooks and weeds by its simple or branched arms, or by the suckers borne on the under surface of its rays, one of these creatures moves about not always with one side foremost, but with any side foremost. Consequently, averaging its movements, its arms or rays are equally af fected, and therefore remain the same on all sides. On watching the ways of the common Sea-urchin, we are similarly furnished with an explanation of its spherical, or THE GENERAL SHAPES OF ANIMALS. 179 rather its spheroidal, figure. Here the habit is not to move over any one approximately-flat surface ; but the habit is to hold on by several surfaces on different sides at the same time, Frequenting crevices and the interstices among stones and weeds, the Sea-urchin protrudes the suckers arranged in meridional bands over its shell, laying hold of objects now on this side and now on that, now above and now below : the result being, that it does not move in all directions over one plane but in all directions through space. Hence the approach in general form towards spherical symmetry — an approach which is, however, restrained by the relations Q4 the parts to the mouth and vent : the conditions not bein'v~v-v-v-v-Y>->^--YJ^^ }L '• ' •^^^<^^^^f^^^^^ - ' Cp >/!w^^ A|^ shown in Figs. 267 and 268, the contrast between the tipper and under parts is greater, and the head and tail ends differ THE GENERAL SHAPES OF ANIMALS. 18'5 more obviously. In all the higher Articulata, the unlikeness between the front half and the hind half has become conspicuous : there is in them single bilateral symmetry of so pronounced a kind, that no other resem blance is suggested than that between the two sides. By Figs. 269 and 270, representing a decapodous crustacean divided longitudinally and transversely, this truth is made manifest. On calling to mind the habits of the creatures here drawn and described, it will be seen that they explain these forms. The incidence of forces is the same all around the Earth-worm as it burrows through the compact ground. The Centipede, creeping amid loose soil or debris or beneath stones, insinuates itself between solid sur faces — the interstices being mostly greater in one dimension than in others. And all the higher Annulosa, moving about as they do over exposed objects, have their dorsal and ventral parts as dissimilarly acted upon as are their two ends. One other fact only respecting annulose animals needs to be noticed under this head — the fact, namely, that they become unsymmetrical where their parts are unsymmetrically related to the environment. The common Hermit-crab serves as an instance. Here, in addition to the unlikeness of the two sides implied by that curvature of the body which fits the creature to the shell it inhabits, there is an unlikeness due to the greater development of the limbs, and especially the claws, on the outer side. As in the embryo of the Hermit-crab the two sides are alike ; and as the embryo may be taken to represent the type from which the Hermit-crab has been derived ; we have in this case evidence that a symmetrically-bi lateral form has been moulded into an unsym- metrically-bilateral form, by the action of un- symmetrically-bilateral conditions. A further illustration is supplied by Bopyrus, Fig. 271 : a parasite the habits of which similarly account for its dis- torted shape. 184 MORPHOLOGICAL DEVELOPMENT. § 251. Among the Mottusca we find more varied relations between shapes and circumstances. Some of them arc highly instructive. Mollusks of one order, the Pteropoda, swim in the sea much in the same way that butterflies fly in the air, and have shapes not altogether unlike those of butterflies. Fig. 272 represents one of these creatures. That its bilaterally- sym metrical shape harmonizes with its bilaterally- symmetrical conditions is sufficiently obvious. Among the Lamcllibranchiata, we have diverse forms accompanying diverse modes ol life. Such of them as frequently move about, like the fresh water Mussel, have their two valves and the contained parts alike on. the opposite sides of a vertical plane : they are bilaterally symmetrical in conformity with their mode of movement. The marine Mussel, too, though habitually fixed, and though not usually so fixed that its two valves are similarly conditioned, still retains that bilateral symmetry which is characteristic of the order ; and it does this because in the species considered as a whole, the two valves are not dissimilarly conditioned. If the positions of the various, individuals are averaged, it will be seen that the differenti ating actions neutralize one another. In certain other fixed Lamellibranchs, however, there is a considerable deviation from bilateral symmetry ; and it is a deviation of the kind to be anticipated under the circumstances. Where one valve is always downwards, or next to the surface of attachment, while the other valve is always upwards, or next to the environing water, we may expect to find the two valves become unlike. This we do find : witness the Oyster. In the Oyster, too, we see a further irregularity. There is a great indefiniteness of outline, both in the shell and in the animal — an indefiniteness made manifest by comparing different individuals. We have but to remember that growing clustered together, as Oysters do, they must interfere with THE GENERAL SHAPES OF ANIMALS. 185 one another in various ways and degrees, to see how the mdetermiiiateness of form and the variety of form are accounted for. Among the Gasteropods, modifications of a more definite kind occur. " In all Mollusks," says Professor Huxley, " the axis of the body is at first straight, and its parts are arranged symmetrically with regard to a longitudinal verti cal plane, just as in a vertebrate or an articulate embryo." In some Gasteropods, as the Chiton, this bilateral sym metry is retained — the relations of the body to surround ing actions not being such as to disturb it. But in those more numerous types that have spiral shells, there is a marked deviation from bilateral symmetry, as might be ex pected. " This asymmetrical over- development never affects the head or foot of the mollusk : " only those parts which, by inclosure in a shell, are protected from environing actions, lose their bilateralness ; while the external parts, subjected by the movements of the creatures to bilateral conditions, remain bilateral. Here, however, a difficulty meets us. Why is it that the naked Gasteropods, such as our common slugs, deviate from bilateral symmetry, though their modes of movement are those along with which complete bilateral symmetry usually occurs ? The reply is, that their devia tions from bilateral symmetry are probably inherited, and that they are maintained in such parts of their organiza tion as are not exposed to bilaterally- symmetrical conditions. There is reason to believe that the naked Gasteropods are descended from Gasteropods that liad shells : the evidence being that the naked Gasteropods have shells during the early stages of their development, and that some of them retain rudimentary shells throughout life. Now the shelled Gasteropods deviate from bilateral symmetry in the dis position of both the alimentary system and the reproductive system. The naked Gasteropods, in losing their shells, have lost that immense one-sided development of the alimentary pystem which fitted them to their shells, and have acquired ISfi MORPHOLOGICAL DEVELOPMENT that bilateral symmetry of external figure which fits them for their habits of locomotion ; but the reproductive system remains one-sided, because, in respect to it, the relations to external conditions remain one-sided. The Cephalopods, which are interpretable as higher de velopments on the (raster opod type, show us bilaterally- sym metrical external forms along with habits of movement through the water in two-sided attitudes. At the same time, in the radial distribution of the arms, enabling one of these creatures to take an all- sided grasp of its prey, we see how readily upon one kind of symmetry there may be partially developed another kind of symmetry, where the relations to conditions favour it. § 252. The Vertelrata illustrate afresh the truths which we have already traced among the Annulosa. Flying through the air, swimming through the water, and running over the earth as vertebrate animals do, in common with annulose animals, they are, in common with armulose ani mals, diiferent at their anterior and posterior ends, different at their dorsal and ventral surfaces, but alike along their two sides. This single bilateral symmetry remains constant under the extremest modifications of form. Among fish we see it alike in the horizontally-flattened Skate, in the vertically-flattened Bream, in the almost spherical Diodon, and in the greatly-elongated Syngnathus. Among reptiles the Turtle, the Snake, and the Crocodile all display it. And under the countless modifications of structure displayed by birds and mammals, it remains conspicuous. A less obvious fact which it concerns us to note among the Vertebrate, parallel to one which we noted among the Annulosa, is that whereas the lower vertebrate forms deviate but little from triple bilateral symmetry, the deviation be comes great as we ascend. Figs. 273 and 274 show how, besides being divisible into similar halves by a vertical plane passing through its axis, a Fish is divisible into halves that are not very dissimilar by a horizontal plane passing through THE GENERAL SHAPES OF ANIMALS. 187 its axis, and also into other not very dissimilar halves by a plane cutting it transversely. If, as shown in Figs. 275 and 276, analogous sections be made of a superior Reptile, the divided parts differ more decidedly. When a Mammal and a Bird are treated in the same way, as shown in Figs. 277, 278, and Figs. 279, 280, the parts' marked off by the divid ing planes are unlike in far greater degrees. On considering the mechanical converse between organisms of these several types and their environments — on remembering that the fish habitually moves through a homogeneous medium of nearly the same specific gravity as itself, that the terrestrial reptile either crawls on the surface or raises itself very in completely above it, that the more active mammal, having 188 MORPHOLOGICAL DEVELOPMENT. its supporting parts more fully developed, thereby has the under half of its body made more different from the upper half, and that the bird is subject by its mode of life to yet another set of actions and reactions ; we shall see that these facts are quite congruous with the general doctrine, and fur nish further support to it. One other significant piece of evidence must be named. Among the Annuhsa we found un symmetrical bilateralness in creatures having habits exposing them to unlike conditions on their two sides; and among the Vertebrata we find parallel cases. They are presented by the Pleuroncctidce — the order of distorted flat fishes to which the Sole and the Flounder belong. On the hypothesis of evolution, we must conclude that fishes of this order have arisen from an ordinary bila terally-symmetrical type of fish, which, feeding at the bottom of the sea, gained some advantage by placing itself with one of its sides downwards, instead of maintaining the vertical attitude. Besides the general reason there are speci fic reasons for concluding this. In the first place, the young Sole or Flounder is bilaterally symmetrical — has its eyes on opposite sides of its head, and swims in the usual way. In the second place, the metamorphosis which produces the unsym- inetrical structure sometimes does not take place — there are abnormal Flounders that swim vertically, like other fishes. In the third place, the transition from the symmetrical structure to the unsymmetrical structure may be traced. Almost incredible though it seems, one of the eyes is transferred from the under-side of the head to the upper- side. Until lately it was supposed that the change by which the two_ eyes, originally placed on opposite sides, come to be placed on the same side, is effected by a distortion of the cranium ; but it is now asserted that actual migration of an eye occurs. According to Prof. Steenstrup, the eye passes between the ununited bones of the skull ; but according to Prof. Thomson, it passes under the skin. Be the course of the metamorphosis what it may, however, it furnishes several THE GENERAL SHAPES OF ANIMALS. 189 remarkable illustrations of the way in wliicli forms become moulded into harmony with incident forces. For besides this divergence from bilateral symmetry involved by the presence of both eyes upon the upper side, there is a further divergence from bilateral symmetry involved by the differ entiation of the two sides in respect to the contours of their surfaces and the sizes of their fins. And then, what is still more significant, there is a near approach to likeness be tween the halves that were originally unlike, but are, under the new circumstances, exposed to like conditions. The body is divisible into similarly- shaped parts by a plane cutting it along the side from head to tail: " the dorsal and ventral instead of the lateral halves become symmetrical in outline and are equipoised." § 253. Thus, little as there seems in common between the shapes of plants and the shapes of animals, we yet find, on analysis, that the same general truths are displayed by both. The one ultimate principle that in any organism equal amounts of growth take place in those directions in which the incident forces are equal, serves as a key to the phenomena of morphological differentiation. By it we are furnished with intarpretations of those likenesses and uii- likenesses of parts, which are exhibited in the several kinds of symmetry ; and when we take into account inherited effects, wrought under ancestral conditions contrasted in various ways with present conditions, we are enabled to comprehend, in a general way, the actions by which animals have been moulded into the shapes they possess. To fill up the outline of the argument, so as to make it correspond throughout with the argument respecting vegetal forms, it would be proper here to devote a chapter to the differentiations of those homologous segments out of which animals of certain types are composed. Though, among most animals of the third degree of composition, such as the root- wl Hydrozoa, the Polyzoa, and the Ascidioida, the united 190 MORPHOLOGICAL DEVELOPMENT. individuals are not reduced to the condition of segments of a, 3omposite individual, and do not display any marked differ entiations ; yet there are some animals in which such subordinations, and consequent heterogeneities, occur. The oceanic Hydrozoa form one group ; and we have seen reason to conclude that the Annulosa form another group. It is not worth while, however, to occupy space in detailing these unlikenesses of homologous segments, and seeking specific explanations of them. Among the oceanic Hi/drozoa they are extremely varied ; and the habits and derivations of these creatures are so little known, that there are no adequate data for interpreting the forms of the parts in terms of their relations to the environment. Conversely, among the An- nnlGsa those differentiations of the homologous segments which accompany their progressing integration, have so much in common, and have general causes which are so ob vious, that it is needless to deal with them at any length. They are all explicable as due to the exposure of different parts of the chain of segments to different sets of actions and re actions : the most general contrast being that between the anterior segments and the posterior segments, answering to the most general contrast of conditions to which annulose animals subject their segments; and the more special con trasts answering to the contrasts of conditions entailed by their more special habits. Were an exhaustive treatment of the subject practicable, there should here, also, come a chapter devoted to the internal structures of animals — meaning, more especially, the shapes and arrangements of the viscera. The relations between forms and forces among these inclosed parts, are, however, mostly too obscure to allow of interpretation. Protected aa the viscera are in great measure from the incidence of ex ternal forces, we are not likely to find much correspondence between their distribution and the distribution of external forces. In this case the influences, partly mechanical, partly physiological, which the organs exercise on one another, THE GENERAL SHAPES OF ANIMALS. 191 become the chief causes of their changes of figure and ar rangement ; and these influences are complex and indefinite. One general fact may, indeed, be noted — the fact, namely? that the divergence towards asymmetry which generally characterizes the viscera, is marked among those of them which are most removed from mechanical converse with the environment, but not so marked among those of them which are less removed from such converse. Thus while, through out the Vcrtebrata, the alimentary system, with the exception of its two extremities, is asymmetrically arranged, the re spiratory system, which occupies one end of the body, ge nerally deviates but little from bilateral symmetry, and the reproductive system, partly occupying the other end of the body, is in the main bilaterally symmetrical : such deviation from bilateral symmetry as occurs, being found in its most interiorly-placed parts, the ovaries. Just indicating these facts as having a certain significance, it will be best to leave this part of the subject as too involved for detailed treat ment. Internal structures of one class, however, not included among the viscera, admit of general interpretation — struc tures which, though internal, are brought into tolerably- direct relations with the environing forces, and are therefore subordinate in their forms to the distribution of those forces. These internal structures it will be desirable to deal with at some length ; both because they furnish important illustra tions enforcing the general argument, and because an inter pretation of them which we have seen reason to reject, cannot be rejected without raising the demand for souic other interpretation. 45 CHAPTER XV. THE SHAPES OF VERTEBRATE SKELETONS. § 254. WHEN an elongated mass of any substance is transversely strained, different parts of the mass are ex posed to forces of opposite kinds. If, for example, a bar of metal or wood is supported at its two ends, as shown in Fig. 281, and has to bear a weight on its centre, its lower 281 part is thrown into a state of tension, while its upper part is thrown into a state of compression. As will be manifest to any one who has observed what happens on breaking a stick across his knee, the greatest degree of tension falls on the fibres that form the convex surface, while the fibres forming the concave surface are subject to the greatest degree of compression. Between these extremes the fibres at different depths are subject to different forces. Progressing upwards from the under surface of the bar shown in Fig. 281, the tension of the fibres becomes less ; and progressing down wards from the upper surface, the compression of the fibres becomes less; until, at a certain distance between the two surfaces, there is a place at which the fibres are neither ex tended nor compressed. This, shown by the dotted lino in THE SHAPES OF VERTEBRATE SKELETONS. 193 the figure, is called in mechanical language the " neutral axis." It varies in position with the nature of the substance strained : being, in common pine-wood, at a distance of about five eighths of the depth from the upper surface or three eighths from the under surface. Clearly, if such a piece of wood instead of being subject to a downward force is secured at its ends and subject to an upward force, the distribution of the compressions and tensions will be reversed, and the neutral axis will be nearest to the upper surface. Fig. 282 represents these opposite attitudes of the bar and the changed position of its neutral axis : the arrow indicating the direc tion of the force producing the upward bend, and the faint dotted line a, showing the previous position of the neutral axis. Between the two neutral axes will be seen a central space • and it is obvious that when the bar has its strain from time to time reversed, the repeated changes of its molecular con dition must affect the central space in a wav different from that in which they affect the two outer spaces. Fig. 283 is a diagram conveying some idea of these contrasts in molecular condition. If A B C D be the middle part of a bar thus treated, while G II and K L are the alternating neutral axes ; then the forces to which the bar is in each case subject, may be readily shown. Supposing the deflecting force to be acting in the direction of the arrow E, then the tensions to which the fibres between G and F are exposed, will be represented by a series of lines increasing in length as the distance from G increases ; so that the triangle G F M, will express the amount and distribution of all the moleculai tensions. But the molecular compressions throughout the space from G to E, must balance the molecular tensions ; and hence, if the triangle G E N be made equal to tho tri- 194 MORPHOLOGICAL DEVELOPMENT. an gle G F M, the parallel lines of which it is composed (here dotted for the sake of distinction) -will express the amount and distribution of the compressions between E and G. Similarly, when the deflecting force is in the direction of the arrow F, the compressions and tensions will be quantitatively symbolized by the triangle K F 0, and K E P. And thus the several spaces occupied by full lines and by dotted lines and by the two together, will represent the different actions to which different parts of the transverse section are subject by alternating transverse strains. Here then it is made manifest to the eye that the central space between G and K, is differently conditioned from the spaces above and below it; and that the difference of condition is sharply marked off. The fibres forming the outer surface C D, are subject to violent tensions and violent compressions. Pro gressing inwards the tensions and compressions decrease — the tensions the more rapidly. As we approach the point G, the tensions to which the fibres are alternately subject, bear smaller and smaller ratios to the compressions, and disappear at the point G. Thence to the centre occur compressions THE SHAPES OF VERTEBRATE SKELETONS. 195 only, of alternating intensities, becoming at the centre small and equal ; and from the centre we advance, through a reverse series of changes, to the other side. Thus it is demonstrable that any substance in which the power of resisting compression is unequal to the power of resisting tension, cannot be subject to alternating transverse strains, without having a central portion differentiated in its conditions from the outer portions, and consequently dif ferentiated in its structure. This conclusion may easily be verified by experiment. If something having a certain tough ness but not difficult to break, as a thick piece of sheet lead, be bent from side to side till it is broken, the surface of frac ture will exhibit an unlikeness of texture between the inner and outer parts. § 255. And now for the application of this seemingly-irre levant truth. Though it has no obvious connection with the interpretation of vertebral structure, we shall soon see that it fundamentally concerns us. The simplest type of vertebrate animal, the fish, has a mode of locomotion which involves alternating transverse strains. It is not, indeed, subjected to alternating transverse strains by some outer agency, as in the case we have been investigating : it subjects itself to them. But though the strains are here internally produced instead of externally produced, the case is not therefore removed into a wholly different category. For sup- posing Fig. 284 to represent the outline of a fish when bent on one side (the dotted lines representing its outline when the bend is reversed), it is clear that part of the sub stance forming the convex half must be in a state of tension. This state of tension implies the existence in the other half of some counter-balancing compression. And between the two there must be a neutral axis. The way in which this conclusion is reconcilable with the fact that there is tension 196 MORPHOLOGICAL DEVELOPMENT. somewhere in the concave side of a fish, since the curve is caused by muscular contractions on the concave side, will be made clear by the rude illustration which a bow supplies. A bow may be bent by a thrust against its middle (the two ends being held back), or it may be bent by contracting a string that unites its ends ; but the distributions of me chanical forces within the wood of the bow, though not quite alike in the two cases, will be very similar. Now while the muscular action on the concave side of a fish differs from that represented by the tightened string of a bow, the difference is not such as to destroy the applicability of the illustration : the parallel holds so far as this, that within that portion of the fish's body which is passively bent by the contracting muscles, there must be, as in a strung bow, a part in com pression, a part in tension, and an intermediate part which is neutral. Recognizing the fact that even in the developed fish with its complex locomotive apparatus, this law of the transverse strain holds in a qualified way, we shall understand how much more it must hold in any form that may be supposed to initiate the vertebrate type — a form devoid of that seg mentation by which the vertebrate type is more or less cha racterized. We shall see that assuming a rudimentary animal still simpler than the Amphioxus, to have a feeble power of moving itself through the water by the undulations of its body, or some part of its body, there will necessarily come into play certain reactions that must affect the median portion of the undulating mass in a way unlike that in which they affect its lateral portions. And if there exists in this median portion a tissue that keeps its place with any constancy, we may expect that the differential conditions produced in it by the transverse strain, will initiate a dif ferentiation. It is true that the distribution of the viscera in the Amphiozus, Fig. 191, and in the type from which we may suppose it to arise, is such as to interfere with this THE SHAPES OF VERTEBRATE SKELETONS. 197 process. It is also true that the actions and reactions de scribed would not of themselves give to the median portion a cylindrical shape, like that of the cartilaginous rod run ning along the back of the Amphioxus. But what we have here to note in the first place is, that these habitual alternate flexions have a tendency to mark off from the outer parts an unlike inner part, which may be seized hold of, main tained, and further modified, by natural selection, should any advantage thereby result. And we have to note in the second place, that an advantage is likely to result. The con tractions cannot be effective in producing undulations, un less the general shape of the body is maintained. External muscular fibres unopposed by an internal resisteiit mass, would cause collapse of the body. To meet the require ments there must be a means of maintaining longitudinal rigidity without preventing bends from side to side ; and such a means is presented by a structure initiated as described. In brief, whether we have or have not the actual cause, we have here at any rate " a true cause." Though there are difficulties in tracing out the process in a specific way, it may at least be said that the mechanical genesis of this rudiment ary vertebrate axis is quite conceivable. And even the difficulties may, I think, be much more fully met than would at first sight seem possible. What is to be said of the other leading trait which the bimplest vertebrate animal has in common with all higher vertebrate animals — the segmentation of its lateral mus« 198 MORPHOLOGICAL DEVELOPMENT. cular masses ? Is this, too, explicable on the mechanical hypothesis ? Have we, in the perpetual transverse strains, a cause for the fact that while the rudimentary vertebrate axis is without any divisions, there are definite divisions of the substance forming the animal's sides ? I think we have. A glance at the distribution of forces under the transverse strain, as represented in the foregoing diagrams, will show how much more severe is the strain on the outer parts than on the inner parts ; and how, consequently, any modifications of structure eventually necessitated, will arise peripherally before they arise centrally. The perception of this may be enforced by a simple experiment. Take a stick of sealing-wax and warm it slowly and moderately before the fire, so as to give it a little flexibility. Then bend it gently until it is curved into a semi-circle. On the convex surface small cracks will be seen, and on the concave sur face wrinkles ; while between the two the substance remains uudistorted. If the bend be reversed and re-reversed, time after time, these cracks and wrinkles will become fissures which gradually deepen. But now, if changes of this class, en tailed by perpetual transverse strains, commence superficially, as they manifestly must ; there arise the further questions — • What will be the special modifications produced under these special conditions? and through what stages will these modifi cations progress ? Every one has literally at hand an example of the way in which a flexible external layer that is now ex tended and now compressed, by the bending of the mass it covers,, becomes creased ; and a glance at the palms and the fingers will show that the creases are near one another where the skin is thin, and far apart where the skin is thick, Between this familiar case and the case of the rhinoceros- hide, in which there are but a few large folds, various grada tions may be traced. Now the like must happen with the increasing layers of contractile fibres forming the sides of the musciilar tunic in such a type as that supposed. The bendirg« will produce in them small wrinkles while they are THE SHAPES OF VERTEBRATE SKELETONS. 199 thin, but more decided and comparatively distant fissures aa they become thick. Fig 289, which is a horizontal longitudinal section, shows how these thickening layers will adjust themselves on the convex and the con cave surfaces, supposing the fibres of which they are composed to be oblique, as their function requires ; and it is not difficult to see that when once definite divisions have been established, they will advance inwards as the layers develop ; and will so produce a series of muscular bundles. Here then we have something like the myocommata which are traceable in the Amphioxus, and are conspicuous in all superior fishes. § 256. These speculative conceptions I have ventured to present with the view of showing that the hypothesis of the mechanical genesis of vertebrate structure, is not wholly at fault when applied to the most rudimentary vertebrate ani mal. Lest it should be alleged that the question is begged if we set out with a type which, like the Amphioxus, already displays segmentation throughout its muscular system, it seemed needful to indicate conceivable modes in which there may have been mechanically produced those leading traits that distinguish the Amphioxus. It seemed needful to assign an origin for the iiotochord ; and to this we see a clue in the differentiating effects of the transverse strain. It seemed needful to account for the existence of muscular divisions while yet there are no vertebral divisions ; and for this, also, the transverse strain furnishes a feasible reason. But now, having shown that the actions and reactions in volved by its mode of locomotion, are possible causes of those rudimentary structures which the simplest vertebrate animal presents, let us return to the region of established fact, and consider whether such actions and reactions as we actually witness, are adequate causes of those observed differentiations and integrations which distinguish the more-developed ver« 200 MORPHOLOGICAL DEVELOPMENT. tebrate animals. Let us see whether the theory of mechani cal genesis afford us a deductive interpretation of the in ductive generalizations. Before proceeding, we must note a process of functional adaptation which here co-operates with natural selection. I refer to the habitual formation of denser tissues at those parts of an organism which are exposed to the greatest strains — either compressions or tensions. Instances of hard ening under compression are made familiar to us by the skin. We have the general contrast between the soft skin covering the body at large, and the indurated skin covering the inner surfaces of the hands and the soles of the feet. We have the fact that even within these areas the parts on which the pressure is habitually greatest, have the skin habitually thickest ; and that in each person special points exposed to special pressures become specially dense — often as dense as horn. Further, we have the converse fact, that the skin of little-used hands becomes abnormally thin — even losing, in places, that ribbed structure which distinguishes skin subject to rough usage. Of increased density directly following increased tension, the skeletons, whether of men or animals, furnish abundant evidence. Anatomists easily discriminate between the bones of a strong man and those of a weak man, by the greater development of those ridges and crests to which the muscles are attached ; and naturalists, on comparing the remains of domesticated animals with those of wild animals of the same species, find kindred differences. The ftrst of these facts shows unmistakably the immediate etfect of function on structure, and by obvious alliance with it the second may be held to do the same — both implying that the deposit of dense substance capable of great resist ance, habitually takes place at points where the tension is excessive. Taking into account, then, this adaptive process, con tinually aided by the survival of individuals in which it bas taken place most rapidly, we may expect, on tracing up THE SHAPES OF VERTEBRATE SKELETONS. 201 the evolution of the vertebrate axis, to find that as the mus cular power becomes greater there arise larger and harder masses of tissue, serving the muscles as 'points d'appid ; and that these arise first in those places where the strains are greatest. jSTow this is just what we do find. The myocom- oiata are so placed that their actions are likely to affect first that upper coat of the notochord, where there are found " quadrate masses of somewhat denser tissue," which " seem faintly to represent neural spines," even in the Ampliioxus. It is by the development of the neural spines, and after them of the haemal spines, that the segments of the vertebral column are first marked out ; and under the increasing strain of more-developed myocommata, it is just these peripheral appendages of the vertebral segments that must be most subject to the forces which cause the formation of denser tissue. It follows from the mechanical hypothesis that as the muscular segmentation must begin externally and pro gress inwards, so, too, must the vertebral segmentation. Besides thus finding reason for the fact that in fishes with wholly cartilaginous skeletons, the vertebral segments are indicated by these processes, while yet the notochord is uii- segmented ; we find a like reason for the fact that the tran sition from the less-dense cartilaginous skeleton to the more- dense osseous skeleton, pursues a parallel course. In. the existing Lepidosiren, which by uniting certain piscine and amphibian characters betrays its close alliance with primitive types, the axial part of the vertebral column is unossified, vvnile there is ossification of the peripheral parts. Similarly with numerous genera of fishes classed as paloeozoic. The fossil remains of them show that while the neural and haemal spines consisted of bone, the central parts of the vertebros were not bony. It may in some cases be noted, too, both in extant and in fossil forms, that while the ossification is com plete at the outer extremities of the spines it is incomplete at their inner extremities — thus similarly implying centri petal development. 202 MORPHOLOGICAL DEVELOPMENT. § 257. After these explanations the process of eventual segmentation in the spinal axis itself, will be readily under stood. The original cartilaginous rod has to maintain longi tudinal rigidity while permitting lateral flexion. As fast as it becomes definitely marked out, it will begin to concentrate within itself a great part of those pressures and tensions caused by transverse strains. As already said, it must be acted upon much in the same manner as a bow, though it is bent by forces acting in a more indirect way ; and like a bow, it must, at each bend, have the substance of its convex side extended and the substance of its concave side compressed. So long as the vertebrate animal is small or inert, such a cartilaginous rod may have sufficient strength to withstand the muscular strains ; but, other things equal, the evolution of an animal that is large, or active, or both, implies mus cular strains that must tend to cause modification in such a cartilaginous rod. The results of greater bulk and of greater vivacity may be best dealt with separately. As the animal increases in size, the rod will grow both longer and thicker. On looking back at the diagrams of forces caused by transverse strains, it will be seen that as the rod grows thicker, its outer parts must be exposed to more severe ten sions and pressures, if the degree of bend is the same. It is doubtless true that when the fish or reptile, advancing by lateral undulations, becomes longer, the curvature assumed by the body at each movement becomes less ; and that from this cause the outer parts of the notochord are, other things equal, less strained — the two changes thus partially neutral izing one another. But other things are not equal. For while, supposing the shape of the body to remain con stant, the force exerted in moving the body increases as the cubes of its dimensions, the sectional area of the notochord, on which fall the reactions of this exerted force, increases only as the squares of the dimensions : whence results an in tenser stress upon its substance. Merely noting that the other varying factor — the resistance of the water — may here THE SHAPES OF VERTEBRATE SKELETONS. 203 be left out of the account (since for similar masses moving with equal velocities the resistances increase but little faster than the squares of the dimensions, which is the rate at which the sectional areas of the notochords increase) we see that aug menting bulk, taken alone, involves but a moderate residuary increase of stain, on each portion of the notochord ; and this is probably the reason why it is possible for a large slug gish fish like the Sturgeon, to retain the notochordal struc ture. But now, passing to the effects of greater ac tivity, a like dynamical inquiry at once shows us how rapidly the violence of the actions and reactions rises as the move ments become more vivacious. In the first place, the resist ance of a medium such as water increases as the square ol the velocity of the body moving through it ; so that to -maiii- tam double the speed, a fish has to expend four times the energy. But the fish has to do more than this — it has to initiate this speed, or to impress on its mass the force implied by this speed. ]Sow the vis viva of a moving body varies as the square of the velocity ; whence it follows that the energy required to generate that vis viva is measured by the square of the velocity it produces. Consequently, did the fish put itself in motion instantaneously, the expenditure of energy in generating its own vis viva and simultaneously overcoming the resistance of the water, would vary as the fourth power of the velocity. But the fish cannot put itself in motion instantaneously — it must do it by increments ; and thus it results that the amounts of the forces expended to give itself different velocities must be represented by some series of numbers falling between the squares and the fourth powers of those velocities. Were the increments slowly accumulated, the ratio of increasing effort would but little exceed the ratio of the squares ; but whoever observes the sudden, convulsive action with which an alarmed fish darts out of a shallow into deep water, will see that the velocity is very rapidly gener ated, and that therefore the ratio of increasing effort probably exceeds the ratio of the squares very considerably. At any 204 MORPHOLOGICAL DEVELOPMENT. rate it will be clear that the efforts made by fish in rushing upon prey or escaping enemies (and it is these extreme efforts which here concern us) must, as fish become more active, rapidly exalt the strains to be borne by their motor organs ; and that of these strains, those which fall upon the noto- chord must be exalted in proportion to the rest. Thus the development of locomotive power, which survival of the fittest must tend in most cases to favour, involves such in crease of stress on the primitive cartilaginous rod as will tend, other things equal, to cause its modification. What must its modification be ? Considering the compli cation of the influences at work, conspiring, as above indi cated, in various ways and degrees, we cannot expect to do more than form an idea of its average character. The nature of the changes which the notochord is likely to undergo, where greater bulk is accompanied by higher activity, is rudely indicated by Figs. 291, 292, and 293. The successively 4& thicker lines represent the successively greater strains to which the outer layers of tissue are exposed ; and the widen ing inter-spaces represent the greater extensions which they have to bear when they become convex, or else the greater gaps that must be formed in them. Had these outer layers to undergo extension only, as on the convex side, continued natural selection might result in the formation of a tissue elastic enough to admit of the requisite stretching. But at each alternate bend, these outer layers, becoming concave, are subject to increased compression — a compression which they cannot withstand if they have become simply more extensible. To withstand this greater compression they must become harder as well as more extensible. How are these two requirements to be reconciled ? If, as facts war rant us in supposing, a formation of denser substance occurs THE SHAPES OF VERTEBRATE SKELETONS. 205 nt those pa.rts of the notochord where the strain is greatest ; it is clear that this formation cannot so go on as to produce a continuous mass : the perpetual flexions must prevent this. If matter that will not yield at each bend, is deposited while the bendings are continually taking place, the bendings will maintain certain places of discontinuity in the deposit — places aC which the whole of the stretching consequent on each bend will be concentrated. And thus the tendency will be to form segments of hard tissue capable of great resistance to compression, with intervals filled by elastic tissue capable of great resistance to extension — a vertebral column. And now observe how the progress of ossification is just such as conforms to this view. That centripetal develop ment of segments which holds of the vertebrate animal as a whole, as, if caused by transverse strains, it ought to do, and which holds of the vertebral column as a whole, as it ought to do, holds also of the central axis. On the mechanical hypothesis, the outer surface of the notochord should be the first part to undergo induration, and that division into seg ments that must accompany induration. And accordingly, in a vertebral column of which the axis is beginning to ossify, the centrums consist of bony rings inclosing a still continuous rod of cartilage. § 258. Sundry other general facts which the comparative morphology of the Vcrtcbmta discloses, supply further con firmation. Let us take first the structure of the skull. On considering the arrangement of the muscular flakes, or myocommata, in any ordinary fish that comes to table — an arrangement already sketched out in the Amphioxus — it is not difficult to see that that portion of the body out of which the heal of the vertebrate animal becomes developed, is a por tion which cannot subject itself to bendings in the same degree as the rest of the body. The muscles developed there \nust be comparatively short, "and much interfered with by the pre-existing orifices. Hence the cephalic part will not 20C MORPHOLOGICAL DEVELOPMENT. partake in any considerable degree of the lateral undula« tioiis ; and there will not tend to arise in it any such distinct segmentation as arises elsewhere. We have here, then, an explanation of the fact, that from the beginning the develop ment of the head follows a course unlike that of the spinal column ; and of the fact that the segmentation, so far as it can be traced in the head, is most readily to be traced in the occipital region and becomes lost in the region of the face. Still more significantly, we have an explanation of the fact that the base of the skull, answering to the front end of the notochord, never betrays any sign of segmentation. This, which is absolutely at variance with the hypothesis of the transcendental anatomists, is in complete harmony with the foregoing hypothesis. For if, as we have seen, the segmenta tion consequent on mechanical actions and reactions must progress from without inwards, affecting last of all the axis ; and if, as we have seen, the region of the head is so circum stanced that the causes of segmentation act but feebly even on its periphery ; then, it is to be expected that its axis will not be segmented at all : that portion of the primitive notochord which is included in the head, having to un dergo no lateral bendings, may ossify without division into segments. Of other incidental evidences supplied by comparative morphology, let me next refer to the supernumerary bones, which the theory of Goethe and Oken as elaborated by Prof. Owen, has to get rid of by gratuitous suppositions. In many fishes, for example, there are what have been called inter- neural spines and inter-haemal spines. These cannot by any ingenuity be affiliated upon the archetypal vertebra, and they are therefore arbitrarily rejected as bones belonging to the exo- skeleton ; though in shape and texture they are similar to the spines between which they are placed. On the hy pothesis of evolution, howrever, these additional bones are accounted for as arising under actions like those that gave origin to the bones adjacent to them. And similarly witti THE SHAPES OF VERTEBRATE SKELETONS 207 tmch bones as those called sesamoid; together with ethers too numerous to name. Again, in the course of evolution, both as displayed in the Vertebrata generally and in each vertebrate embryo, three skeletons succeed one another — the membranous, the car* tilaginous, and the osseous. These substitutions take place variously and unsystematically. While one part of a skele ton retains the membranous character, another part of the same skeleton has become cartilaginous. At the same time that certain components have become partially or completely ossified, other components continue cartilaginous or mem branous. Further, though there is a general succession of these stages, the succession is not regularly maintained ; for in many cases bones are formed by the deposit of osseous matter in portions of the membranous skeleton, which thus do not pass through the cartilaginous stage. " Nor," says Prof. Huxley, " does any one of these states ever completely obliterate its predecessor ; more or less cartilage and mem brane entering into the composition of the most completely ossified skull, and more or less membrane being discoverable in the most completely chondrified skull." And then, too, the processes of chondrification and ossification often proceed with but little respect for the pre-existing divisions ; but severally may result in the establishment of two parts wher<; forces. CHAPTER IV. DIFFERENTIATIONS AMONG THE INNER TISSUES OF PLANTS* § 277. In passing from plants formed of threads or thin laminae, to plants having some massiveness, we find that after the external and internal parts have become distinguished from one another, there arise dictinctions among the internal parta themselves, as well as among the external parts themselves: the primarily-differentiated parts are both re-differentiated. From types of very low organisation illustrations of this may be drawn. In the thinner kinds of Laminaria there exists but the single contrast between the outer layer of cells and an inner layer ; but in larger species of the same genus, as L. digital a t there are three unlike layers on each side of a central layer differing from them — augmentation of bulk is accompanied by multiplication of concentric internal struc tures, having their unlikenesses obviously related to unlike- nesscs in their conditions. In Furcelhiria and various *AlgcB of similarly swollen forms, the like relation may be traced. Just indicating the generality of this contrast, but not * Students of vegetal physiology, familiar with the controversies respecting sundry points dealt with in this chapter, will probably be surprised to find taken for granted in it, propositions which they have habitually regarded as open to doubt. Hence it seems needful to say that the conclusions here set forth, have resulted from investigations undertaken for the purpose of forming opinions on several unsettled questions which I had to treat, but which I could find in books no adequate data for treating. The details of these inves tigations, and the entire argument of which this chapter is partly an abstract, will be found in Apmjndix C, 49 256 PHYSIOLOGICAL DEVELOPMENT. attempting to seek in these lower types for any more specific interpretation of it, let us pass to the higher types. The argument will be amply enforced by the evidence obtained from them. "We will look first at the conditions which they have to fulfil ; and then at the way in which the functions and structures adapting them to these conditions arise. § 278. A terrestrial plant that grows vertically needs no marked modification of its internal tissues, so long as the height it reaches is very small. As we before saw, the spiral or cylindrical rolling up of a simple cellular frond, or the more bulky growth of a simple cellular axis, may give the requisite strength ; and the requisite circulation may be carried on through the unchanged cellular tissue. But in proportion as the height to be attained and the mass to be supported increase, the supporting part must acquire greater bulk or greater density, or both ; and some modification that shall facilitate the transfer of nutritive liquids must take place. Hence, in the inner tissues of plants we may expect to find that structural changes answering to these requirements become marked, as the growth of the aerial part becomes great. Facts correspond with these expectations. Among the humbler Acrogens, which creep over, or raise themselves but little above, the surfaces they flourish upon, there is scarcely any internal differentiation : the vascular and woody structures, if not in all cases absolutely un represented, are rarely and very feebly indicated. But among the higher Acrogens — the Ferns and Lycopodiums — which raise their fronds to considerable heights, there are vascular bundles and hard tissues like wood; and by the Tree- Ferns massive axes are developed. That the relation which thus shows itself among Cryptogams is habitual among Phaenogams, scarcely needs saying. Phsenogams, however, are not universally thus charac terized in a. decided way. Besides the comparative want ol woody substance in flowering plants of humble growth, and THE INNER TISSUES OF PLANTS. 257 besides the paucity of vessels in ordinary water-plants, there are cases of much more marked divergence from this typical internal structure. These exceptional cases occur under exceptional conditions, and are highly instructive. They are of two kinds. One group of them is furnished by certain plants that arc parasitic on the exposed roots of trees — parasitic not partially, as the Mistletoe, but to the extent of subsisting wholly on the sap they absorb. Fungus- like in colour and texture, and having scales for leaves, these BalanophorcB and Rafflexiacece are recognizable as Phoenogams by scarcely any other traits than their fructifications, Along with their abortive leaves and absence of chlorophyll, there is a great degradation of those internal tissues by which Phaenoganis are commonly distinguished. Though Dr. Hooker has shown that they are not, as some botanists thought, devoid of spiral vessels ; yet, as shown by the mistake previously made in classifying them, their appliances for circulation are rudimentary. And this trait goes along with a greatly-simplified distribution of nutriment. In the absence of leaves there can be but little down-current of nutriment, such as leaves usually supply to roots : there cannot be much beyond an upward current of the absorbed juices. The other cases occur where circulation is arrested or checked in a different way ; namely, in plants that are wholly submerged. These are the Podo- stemones, which are aquatic even to the extent of flowering under water. Clothing as they do the submerged rocks in tropical rivers, their roots, like those of the Alyce, servo only for attachment ; their foliar expansions, frond-like in shape, are everywhere bathed by the water; and their organs of fructification never exposed to the air, but perhaps aided in their functions by water-insects instead of air-insects, are the only marked signs of kinship to other Phconogams. Observe then the connexion of facts. One of these Podcstemones needs rio internal stiffening substance, for it exists in a medium of its own specific gravity ; and having no unlikeness between 258 PHYSIOLOGICAL DEVELOPMENT. the materials assimilated at its fixed and its free ends, it lias no need for a circulation — nor, indeed, in the absence of evaporation from any part of its surface, could any active circulation take place. Here, accordingly, the ordinary internal structures are undeveloped : though spiral vessels are not entirely absent, yet they are so rare as to do no more than verify the inference of phaBiiogamic relationship drawn from the flowers. The method of agreement, the method of difference, and the method of concomitant variations, thus unite in proving a direct relation between the demand for support and cir culation, and the existence of these vascular woody bundles which the higher plants habitually possess. The question which we have to consider is — Under what influences are these structures, answering to these requirements, developed ? How are these internal differentiations caused ? The inquiry may be conveniently divided. Though the supporting tissues and the tissues concerned in the circulation of liquids are closely connected, and indeed entangled, with one another, we may titly deal with them apart. Let us take first the supporting tissue. § 279. Many common-place facts indicate that the me chanical strains to which upright-growing plants are exposed, themselves cause increase of the dense deposits by which such plants are enabled to resist such strains. There is the fact that the massiveness of a tree-trunk varies according to the stress habitually put upon it. If the contrast between the slender stem of a tree growing in a wood and the bulky stem of a kindred tree growing in the fields, be ascribed to differ ence of nutrition rather than difference of exposure to winds ; there is still the fact that a tree trained against a wall has a less bulky stem than a tree of the same kind growing un supported ; and that between the long weak branches of the one and the stiff ones of the other there are decided contrasts. If it be objected that a tree so trained and branches so borne THE INNV.R TISSUES OF PLANTS. 259 have relatively less foliage, and that therefore these unlike- nesses also are due to unlikenesses of general nutrition, which may in part be true ; there are still such cases as those of garden plants, which when held up by tying- them to sticks have weaker steins than when they are unpropped, and sink down if their props are taken away. Again, there is the evidence supplied by roots. Though the contrast between the feeble roots of a sheltered tree and the strong roots of an exposed tree, may, like the contrast of their stems, be mainly due to difference of nutrition, and therefore supplies but doubtful evidence, we get tolerably clear evidence where trees growing on inclined rocky surfaces, send into crevices that afford little moisture or nutriment, roots which never theless become thick where they are so directed as to bear great strains. Suspicion thus raised is strengthened into conviction by special evidences occurring in the places where they are to be expected. The Cactuses, with their succulent growths that pass into woody growths slowly and irregularly, give us the opportunity of tracing the conditions under which the wood is formed. Good examples occur in the genus Cereus, and especially in forms like C. crenu/atns. Here, from a massive vertically-growing rod of fleshy tissue, two inches or more in diameter, there grow at intervals lateral rods similarly bulky, which, quickly curving themselves, take vertical directions. One of these heavy branches puts great strains on its own substance and that of the stem at their point of junction ; and here both of them become brown and hard, while they continue green and succulent all around. Such differentiations may be traced internally before they are visible on the surface. If a joint of an Opuntia be sliced through longitudinally, the greater resistance to the knife all around the narrow neck, indicates there a larger deposit of lignin than elsewhere ; and a section of the tissue placed under the microscope, exhibits at the narrowest part a con centration of the woody and vascular bundles. Clear evidence of another kind has been noted by Mr. Darwin, in the i{60 PHYSIOLOGICAL DEVELOPMENT. org-ans of attachment of climbing plants. Speaking of Solamim iasminoides he says : — " When the flexible petiole of half- or a quarter- grown leaf has clasped any object, in three or four days it increases much in thickness, and after several weeks becomes wonderfully hard and rigid ; so that I could hardly remove one from its support. On comparing a thin transverse slice of this petiole with one from the next or older leaf beneath, which had not clasped anything, its diameter was found to be fully doubled, and its structure greatly changed. * * * This clasped petiole had actually become thicker than the stem close beneath ; and this was chiefly due to the greater thickness of the ring of wood, svhich presented, both in transverse and longitudinal sections, a closely similar structure in the petiole and axis. The assumption by a petiole of this structure is a singular morphological fact ; but it is a still more singular physio logical fact that so great a change should have been induced by the mere act of clasping a support." If there is a direct relation between mechanical stress and the formation of wood, it ought to explain for us the internal distribution of the wood. Let us see whether it does this. When seeking in mechanical actions and reactions the cause of that indurated structure which forms the verte brate axis (§§ 254-7), it was pointed out that in a transversely- strained mass, the greatest pressures and tensions are thrown on the molecules of the concave and convex surfaces. Hence, supposing the transversely-strained mass to be a cylinder, bent backwards and forwards not in one plane but now in this plane and now in that, its peripheral layers will be those on which the greatest stress falls. An ordinary exogenous axis is such a cylinder so strained. The main tenance of its attitude either as a lateral shoot or a vertical shoot, implies subjection to the bendings caused by its own weight and by the ever- varying wind. These bendings imply tensions and pressures falling most severely first on one side of its outer layers and then on another. And if the THE INNP:R TISSUES OF PLANTS. 261 dense substance able to resist these tensions and pressures is deposited most where they are greatest, we ought to find it taking the shape of a cylindrical casing. This is just what we do find. On cutting across a shoot in course of formation, we see its central space either unoccupied or occupied only by soft tissue. That the layer of hard tissue surrounding this is not the outermost layer, is true : there lies beyond it the cambium layer, from which it is formed. But outside of the cambium there is another layer of dense tissue, the liber, having frequently a tenacity greater even than that of the wood — a layer which, while it protects the cambium and oilers additional resistance to the transverse strain, admits of being fissured as fast as the cylinder of wood thickens. That is to say, the deposit of resisting substance is as completely peripheral as the exogenous mode of growth permits. So, too, in general arrangement is it with the endogenous stem. Different as is here the mode of growth, and different as is the internal structure, there yet holds the same general dis tribution of tissues, answering to the same mechanical con ditions. The vascular woody bundles, more abundant towards the outside of the stem than near the centre, produce a harder casing surrounding a softer core. In the supporting structures of leaves we rind significant deviations from this arrangement. While axes are on the average exposed to equal strains on all sides, most leaves, spreading out their surfaces horizontally, have their petioles subject to strains that are not alike in all directions ; and in them the hard lissue is differently arranged. Its transverse section is not ring-shaped but crescent-shaped : the two horns being directed towards the upper surface of the petiole. That this arrangement is one which answers to the mechanical con ditions, is not easy to demonstrate : we must satisfy ourselves by noting that here, where the distribution of forces is different, the distribution of resisting tissue is different. And then, showing conclusively the connexion between these differ ences, we have the fact that in petioles growing vertically 262 PHYSIOLOGICAL DEVELOPMENT. and supporting peltate leaves — petioles which are therefore subject to equal transverse strains on all sides — the vascular bundles are arranged cylindrically, as in axes. Such, then, are some of the reasons for concluding that the development of the supporting tissue in plants, is caused by the incident forces which this tissue has to resist. The individuals in which this direct balancing of inner and outer actions progresses most favourably, are those which, other things equal, are most likely to prosper ; and by habitual survival of the fittest, there is established a systematic and constant distribution of a deposit adapted to the circumstances of each type. § 280. The function of circulation may now be dealt with. We have to consider here by what structures this is dis charged ; and what connexion exists between the demand for them and the genesis of them. The contrast between the rates at which a dye passes through simple cellular tissue and cellular tissue of which the units have been elongated, indicates one of the structural changes required to facilitate circulation. If placed with its cut surface in a coloured liquid, the parenchyma of a potato or the medullary mass of a cabbage-stalk, will absorb the liquid with extreme slowness ; but if the stalk of a fungus be similarly placed, the liquid runs up it, and especially up its loose central substance, very quickly. On comparing the tissues which thus behave so differently, we find that whereas in the one case the component cells, packed close together, have deviated from their primitive sphericity only as much as mutual pressure necessitates, in the other case, they are drawn out into long tubules with narrow spaces among them — the greatest dimensions of the tubules and the spaces being in the direction which the dye takes so rapidly. That which we should infer, then, from the laws of capillary action, is experimentally shown : liquid moving through tissues follows the lines in which the elements of the tissues are most THE INNER TISSUES OF PLANTS. elongated. It does this for two reasons. That narrowing of the cells and intercellular spaces which accompanies their elongation, facilitates capillarity ; and at the same time fewer of the septa formed by the joined ends of the cells have to be passed through in a given distance. Hence the general fact that the establishment of a rudimentary vascular system, is the formation of bundles of cells lengthened in the direction which the liquid is to take. This we see very obviously among the lower Acrogens. In one of the lichen- like Liverworts, the veins which, branching through its frond, serve as communications with its scattered rootlets, are formed of cells longer than those composing the general tissue of the frond : the lengths of these cells corresponding in their directions with the lengths of the veins. So, too, is it with the midribs of such fronds as assume more definite shapes ; and so, too, is it with the creeping stems which unite many such fronds. That is to say, the current which sets towards the growing part from the part which supplies the materials for growth, sets through a portion of the tissues composed of units that are longer in the line of the current than at right angles to that line. The like is true of Phaenogams. Omitting all other characteristics of those parts of them through which chiefly the currents of sap flow, we find the uniform fact to be that they consist of cells and intercellular spaces distinguished from others by their lengths. It is thus with veins, and midribs, and petioles; and if we wish proof that it is thus with stems, we have but to observe the course taken by a coloured solution into which a stem is inserted. What is the original cause of this differentiation ? Is it possible that this modification of cell-structure which favours the transfer of liquid towards each place of demand, is itself caused by the current which the demand sets up ? Does the stream make its own channel ? There are various reasons for thinking that it claes. In the first place, the simplest and earliest channels, such as we see in the Liverworts, do not 264 PHYSIOLOGICAL DEVELOPMENT. develop in any systematic way, but branch out irregularly, following everywhere the irregular lobes of the frond aa these spread ; and on examining under a magnifier the places at which the veins are lost in the cellular tissue, it will be seen that the cells are there slightly longer than those around : suggesting that the lengthening of them which produces an extension of the veins, takes place as fast as the growth of the tissue beyond causes a current to pass through them. In the second place, a disappearance of the granular contents of these cells accompanies their union into a vein — a result which the transmission of a current may not improbably bring about. But be the special causes of this differentiation what they may, the evidence favours very much the conclusion that the general cause is the setting up of a current towards a place where the sap is being consumed. In the histological development of the higher plants we find confirmation The more finished distributing canals in Phsenogams are formed of cells previously lengthened. At parts of which the typical struc ture is fixed, and the development direct, this fact is not easy to trace ; the cells rapidly take their fibrous structures in antici pation of their pre-determined functions. But in places where new vessels are required in adaptation to a modify ing growth, we may clearly trace this succession. The swelling root of a turnip, continually having its vascular system further developed, and the component vessels lengthened as well as multiplied, gives us an opportunity of watching the process. In it we see that the reticulated cells which unite to form ducts, arise in the midst of bundles of cells that have previously become elongated, and that they arise by transformation of such elongated cells ; and we also see that these bundles of elongated cells have an arrangement quite suggestive of their formation by passing currents. Are there grounds for thinking that these further trans formations by which strings of elongated cells pass into THE INNER TISSUES OF PLANTS. 265 vessels lined with spiral, annular, reticulated, or other frameworks, are also in any way determined by the currents of sap carried ? There are some such grounds. As just indicated, the only places where we may look for evidence with any rational hope of finding it, are places where some local requirement for vessels has arisen, in consequence of some local development which the type does not involve. In these cases we find such evidence. Good illustrations occur in those genera of the CactacefB, which simulate leaves, like Epiphyllum and Phyllocactus. A branch of one of these is outlined in Fig. 256. As before explained, this is a flattened axis ; and the notches along its edges are the seats of the axillary buds. Most of these axillary buds are arrested ; but occasionally one of them grows. Now if, taking an Epiphyllum-shQQi which bears a lateral shoot, we compare the parts of it that are near the abortive axillary buds with the part that is near the developed axillary bud, we find a conspicuous difference. In the neighbourhood of an abortive axillary bud there is no external sign of any internal differentiation ; and on holding up the branch against the light, the uniform trans- lucency shows that there is no greater amount of dense tissue near it than in other parts of the succulent mass. But where an axillary bud has developed, a prominent rounded ridge joins the midrib of the lateral branch with the midrib of the parent branch. In the midst of this rounded ridge an opaque core may be seen. And on cutting through it, this opaque core proves full of vascular bundles imbedded in woody deposits. Clearly, these clusters of vessels imply transformations of the tissues, caused by the passage of increased currents of sap. The vessels were not there when the axillary bud was formed ; they would not have de veloped had the axillary bud proved abortive ; but they arise as fast as growth of the axillary bud draws the sap along the lines in which they lie. Verification is obtained by examining the internal structures. If longitudinal 266 PHYSIOLOGICAL DEVELOPMENT. sections be made through a growing bud of Opuntia or Cereits, it will be found that the vessels in course of for mation converge towards the point of growth, as they would do if the sap-currents determined their formation ; that they are most developed near their place of convergence, which they also would be if so produced ; and that their terminations in the tissue of the parent shoot are partially- formed lines of irregular fibrous cells, like those out of which the vessels of a leaf or bud are developed. Concluding, then, that sap-vessels arise along the lines of least resistance, through, which currents are drawn or forced, the question to be asked is — What physical process produces them ? Their component cells, united end to end more or less irregularly in ways determined by their original positions, form a channel much more permeable, both longitudinally and laterally, than the tissue around. How is this greater permeability caused ? The idea, first propounded I believe by Wolff, that the adjoined ends of the cells are perforated or destroyed by the passing current, is one for which much is to be said. Whether these septa are dissolved by the liquids they transmit, or whether they are burst by those sudden gushes which, as we shall hereafter see, must frequently take place along these canals, needs not be discussed : it is sufficient for us that the septa do, in many cases, disappear, leaving internal ridges showing their positions ; and, in other cases, become extremely porous. Though it is manifest that this is not the process of vascular development in tissues that unfold after pre-determined types, since, in these, the dehi- eccnces or perforations of septa occur before such direct actions can have come into play ; yet it is still possible that the disappearances of septa which now arise by repe tition of the type were established in the type by such direct actions. Be this as it may, however, a simultaneous change undergone by these longitudinally - united cells must be otherwise caused. Frame-works are formed in them — frame- works which, closely fitting their inner THE 1XXEH 'J1SSUES OF PLANTS. 2G7 surfaces, may consist either of successive rings, or continuous spiral threads, or networks, or structures between spirals and networks, or networks with opening's so far diminished that the cells containing them are distinguished as fenestrated. Their differences omitted, however, these structures have the common character that, while supporting the coats of the vessels and serving to restore their diameters after they have been com pressed, they also give special facilities for the passage of liquids, both through the sides cf the transformed cells and through their united ends, where these are not destroyed. For one of these internal frame-works is not, as usually stated, produced by the deposition of substance on the cell-mem brane, in the shape which the frame-work eventually assumes. Wore it so, this frame- work would have a thickness additional to that of the cell- wall as previously existing, which it has not. On comparing one of these cells longitudinally cut through, with an adjacent cell of the kind to which it was originally similar, we see that over every opening in the frame-work, the wall of the cell is far thinner than the walls of the adjacent cells: the cell-membrane at each of these openings being quite bare, instead of being, as in adjacent cells, covered bv a layer of deposit. Hence this transformation of cells into sap-channels, is in pare the arrangement or re-arrangement of their sub stance in such ways as greatly to diminish the resistance to the passage of liquid, both longitudinally and laterally. To attempt any physical interpretation of this change is scarcely safe : the conditions are so complex. There are many reasons for suspecting, however, that it arises from a vacuolation of the substance deposited on the cell wall. Tt rapidly deposited, as it is likely to be along lines where sap is freely supplied, this may, in passing from the state of a soluble colloid to that of an insoluble colloid, so contract as to leave uncovered spaces on the cell-membrane ; and this change, originally consequent on a physico-chemical action, may be so maintained and utilized by natural selection, as to result in structures of a definite kind, regularly formed in 268 PHYSIOLOGICAL DEVELOPMENT. growing parts in anticipation of functions to be afterwards discharged. But, without alleging any special cause for this metamorphosis, there is good evidence that it is in some way consequent upon the carrying of sap. If we examine tissues such as that in the interior of a growing turnip that hcis not yet become stringy, we may, in the first place, find bundles of elongated cells not having yet developed in them those fenestrated or reticulated structures by which the ducts are eventual!}7 characterized. Along the centres of adjacent bundles we may find incomplete lines of such cells — some that are partially or wholly transformed, with some between them that are not transformed. In other bundles, completed chains of such transformed cells are visible. And then, in still older bundles, there are several complete chains running side by side. All which facts imply a metamorphosis of the elongated cells, caused by the continued action of the currents carried. § 281. Here, however, presents itself a further problem. Taking it as manifest that there is a typical distribution of supporting tissue adapted to meet the mechanical strains a plant is exposed to by its typical mode of growth, and also that there goes on special adaptation of the supporting tissue to the special strains the individual plant has to bear ; and taking it as tolerably evident that the sap channels are originally determined by the passage of currents along lines of least resistance; there still remains the ultimate question — Through what physical actions are established these general and special adjustments of supporting tissue to the strains borne, and these distributions of nutritive liquid required to make possible such adjustments? Clearly, if the external actions produce internal reactions; and if this play of actions a*id reactions results in a balancing of the strains by the resistances; we may rationally suspect that the incident forces are directly conducive to the structural changes by which they are met. Let us consider how they must work. THE INNER TISSUES OF PLANTS. 269 When any part of a plant is bent by the wind, the tissues on its convex surface are subject to longitudinal tension, and Inese extended outer layers comjress the layers beneath them. Such of the vessels or canals in these subjacent layers as contain sap, must have some of this sap expelled. Part of h will be squeezed through the more or less porous walls of the canals into the surrounding tissue, thus supplying it with assimilable materials ; while part of it, and probably the larger part, will be thrust along the canals longitudinally upwards and downwards. When the branch or twig or leaf stalk recoils, these vessels, relieved from pressure, expand to their original diameters. As they expand, the sap rushes back into them from above and below. In whichever of these directions least has been expelled by the compression, from that direction most must return during the dilation ; seeing that the force which more efficiently resisted the thrusting back of the sap is the same force which urges it into the expanded vessels again, when they are relieved from pressure. At the next bend of the part a further portion of sap will be squeezed out, and a further portion thrust for wards along the vessels. This rude pumping process thus serves for propelling the sap to heights which it could not reach by capillary action, at the same time that it incident ally serves to feed the parts in which it takes place. It strengthens them, too, just in proportion to the stress to be borne ; since the more severe and the more repeated the strains, the greater must be the exudation of sap from the vessels or ducts into the surrounding tissue, and the greater the thickening of this tissue by secondary deposits. By this same action the movement of the sap is determined either upwards or downwards, according to the conditions, While the leaves are active and evaporation is going on from them, these oscillations of the branches and petioles urge forward the sap into them ; because so long as the vessels of the leaves are being emptied, the sap in the compressed vessels of the oscillating parts will meet with less resistance 270 PHYSIOLOGICAL DEVELOPMENT. iji the direction of the leaves than in the opposite direction But when evaporation ceases at night, this will no longer be the case. The sap drawn to the oscillating parts, to supply the place of the exuded sap, must come from the directions of least resistance. A slight breeze will bring it back from the leaves into the gently-swaying twigs, a stronger breeze into the bending branches, a gale into the strained stem and roots — roots in which longitudinal tension produces, in another way, the same effects that transverse tension does in the branches. Two possible misinterpretations must be guarded against. It must not be supposed that this force-pump action causes movement of the sap towards one point rather than another : it is simply an aid to its movement. From the stock of sap distributed through the plant, more or less is everywhere being abstracted — here by evaporation ; here by the unfolding of the parts into their typical shapes ; here by both. The result is a tension on the contained liquid columns, that is greatest now in this direction and now in that. This tension it is which must be regarded as the force that determines the current upwards or downwards; and all which the mechanical actions do is to facilitate the transfer to the places of greatest demand. Plence it happens that in a plant prevented from oscillating, but having a typical tendency to assume a certain height and bulk, the demands set up by its unfolding parfs \vill still cause currents; and there will still be alternate ascents and descents, according as the varying conditions change the direction of greatest demand — the only difference being, that in the absence of oscillations the the growth will be less vigorous. Similarly, it must not be supposed that mechanical actions are here alleged to be the sole causes of wood- formation in the individual plant. The tendency of the individual plant to form wood at places where wood has been habitually formed by ancestral plants, is manifestly a cause, arid, indeed, the chief cause. In this, as in all other cases, inherited structures repeat themselves THE INNER TISSUES OF PLANTS. 271 irrespective of the circumstances of the individual : absence of the appropriate conditions resulting simply in imperfect repetition of the structures. Hence the fact that in trained trees and hothouse shrubs, dense substance is still largely deposited ; though not so largely as where the normal me chanical strains have acted. Hence, too, the fact, that in such plants as the Elephants -foot or the Welwitschla niirabilis, which for untold generations can have undergone no oscillations, there is an extensive formation of wood (though not to any considerable height above the ground), in repetition of an ancestral type : natural selection having here maintained the habit as securing some other advantage than that of support. Still, it must be borne in mind that though intermittent mechanical strains cannot be assigned as the direct causes of these internal differentiations in plants that are artificially sheltered or supported, they are assignable as the indirect causes ; since the inherited structures, repeated apart from such strains, are themselves interpretable as accumulated results of such strains acting on successive generations of ancestral plants. This will become clear on combining the several threads of the argument and bringing it to a close, which we may now do. § 282. To put the co-operative actions in their actual order, would require us to consider them as working on individuals small modifications that become conspicuous and definite only by inheritance and gradual increase ; but it will aid our comprehension without leading us into error, if we suppose the whole process resumed in a single continuously-existing plant. As the plant erects the integrated series of fronds whose united parts form its rudimentary axis, the increasing area of frond-surface exposed to the sun's rays entails an increasing draught upon the liquids contained in the rudimentary axis. The currents of sap so produced, once established along certain lines of cells that offer least resistance, render them 50 272 PHYSIOLOGICAL DEVELOPMENT. by their continuous passage more and more permeable. This establishment of channels is aided by the wind. Each bend produced by it while yet the tissue is undifferentiated, squeezes towards the place of growth and evaporation the liquids that are passing by osmose from cell to cell; and when the lines of movement become defined, each bend helps, by forcing the liquid along these lines, to remove obstructions and make continuous canals. As fast as this transfer of sap is facilitated, so fastis the plant enabled further to raise itself, and add to its assimilating surfaces ; and so fast do the transverse strains, becoming greater, give more efficient aid. The channels thus formed can be neither in the centre of the rudimentary axis nor at its surface ; for at neither of these places can the transverse strains produce any considerable compressions. They must arise along a tract between the outside of the axis and its core — a tract along which there occur the severest squeezes between the ex tended outer layers and the internal mass. Just that dis tribution which we find, is the distribution which these me chanical actions tend to establish. As the plant gains in height, and as the mass of its foliage accumulates, the strains thrown upon its axis, and especially the lower part of its axis, rapidly increase. Supposing the forms to remain similar, the strains must increase in the ratio of the cubes of the dimensions ; or even in a somewhat higher ratio. One consequence must be, that the compressions to which the vessels at the lower part of the stem are subject, become greater as fast as the height to which the sap has to be raised becomes greater ; and another consequence must be, that the local exudation of sap produced by the pressure is proportionately augmented. Hence the materials for nutri tion of the surrounding tissues being there supplied more abundantly, we may expect thickening of the surrounding tissues to show itself there first: in other words, wood will be formed round the vessels of the lower part of the stem. The resulting greater ability of this lower THE INNER TISSUES OF PLANTS. 273 part of the stem to bear strains, renders possible an increase of height ; and while after an increase of height the lowest part becomes still further strained, and still further thickens, the part above it, exposed to like actions, undergoes a like thickening. This induration, while it spreads upwards, also spreads outwards. As fast as the rude cylinder of dense matter formed in this way, begins to inclose the original vessels, it begins to play the part of a resistant mass, between which and the outer layers the greatest compression occurs at each bend. While, therefore, the original vessels become useless, the peripheral cells of the developing wood become those which have their liquid contents squeezed out longitu dinally and laterally with the greatest force; and, consequently, amid them are formed new sap-channels, from which there is the most active local exudation, producing the greatest deposit of dense matter. Thus fusing together, as it were, the individualities of successive generations of plants, and letting that facilitation of the process which natural selection has all along given, be represented by the most favourable working together of these mechanical processes, we are enabled to interpret the leading internal differentiations of plants as consequent on a direct equilibration between inner and outer forces. Here, indeed, we see illustrated in a way more than usually easy to follow, the eventual balancing of outer actions by inner reactions. The relation between the demand for liquid and the formation of channels that supply liquid, as well as that between the incidence of strains and the deposit of substance that resists strains, are among the clearest special examples of the general truth that the moving equilibrium of an organism, if not overthrown by an incident force, must eventually be adjusted to it. The processes here traced out are, of course, not to be taken as the only differentiating processes to which the inner tissues of plants have been subject. Besides the chief changes we have considered, various less conspicuous changes 274 PHYSIOLOGICAL DEVELOPMENT. have taken place. These must be passed over as arising in ways too involved to admit of rpecific interpreta tions ; even supposing them to have been produced by causes of the kind assigned. But the probability, or rather indeed the certainty, is, that some of them have not been so produced. Here, as in nearly all other cases, in- diect requilibration has worked in aid of direct equilibration ; and in many cases indirect equilibration has been the sole agency. Besides ascribing to natural selection the rise of various internal modifications of other classes than those above treated, we must ascribe some even of these to natural selection. It is so with the dense deposits which form thorns and the shells of nuts : these cannot have resulted from any inner reactions immediately called forth by outer actions ; but must have resulted mediately through the effects of such outer actions on the species. Let it be understood, therefore, that the differentiations to which the foregoing interpretation applies, are only those most conspicuous ones which are directly related to the most conspicuous in cident forces. They must be taken as instances on the strength of which we may conclude that other internal differentiations have had a natural genesis, though in ways that we cannot trace. CHAPTER V. PHYSIOLOGICAL INTEGRATION IN PLANTS. § 283. A good deal has been implied on this topic in the preceding chapters. Here, however, we must for a brief space turn our attention immediately to it. Plants do not display integration in such distinct and multiplied ways as do animals. 13 at its advance may be traced both directly and indirectly — directly in the increas ing co-ordination of actions, and indirectly in the effect of this upon the powers and habits. Let us group the facts under these heads : ascending in both cases from the lower to the higher types, § 284. The inferior Alga, along with little unlikeness of parts, show us little mutual dependence of parts. Having surfaces similarly circumstanced everywhere, much physio logical division of labour cannot arise ; and therefore there cannot be much physiological unity. Among the superior Alga, however, the differentiation between the attached part and the free part is accompanied by some integration. There is evidently a certain transfer of materials, which is doubtless facilitated by the elongated forms of the cells in the stem, and probably leads to the formation of dense tissue at the places of greatest strain, in a way akin to that recently ex plained in other cases. And where there is this co-ordina tion of actions, the parts are so far mutually dependent that each dies if detached from the other. That though the 276 PHYSIOLOGICAL DEVELOPMENT. organization is so low neither part can reproduce the other and survive by so doing, is probably due to the circumstance that neither part contains any considerable stock of untrans- formed protoplasm, out of which new tissues may be pro duced. Fungi and Lichens present no very significant advances of integration. "We will therefore pass at once to the Acrogens. In those of them which, either as single fronds or strings of fronds, spread over surfaces, and which, rooting- themselves as they spread, do not need that each part should receive aid from remote parts, there is no developed vascular system serving to facilitate transfer of nutriment : the parts being little differentiated there is but little integration. But along with assumption of the upright attitude and the ac companying specializations, producing vessels for distribu ting sap and hard tissue for giving mechanical support, there arises a decided physiological division of labour ; rendering the aerial part dependent on the imbedded part and the im bedded part dependent on the aerial part. Here, indeed, as elsewhere, these concomitant changes are but two aspects of the same change. Always the gain of power to discharge a special function involves a loss of power to perform other functions ; and always, therefore, increased mutual dependence constituting physiological integration, must keep pace with that increased fitting of particular parts to particular duties which constitutes physiological differentiation. Making a great advance among the Acrogens, this physio logical integration reaches its climax among Endogens and Exogens. In them we see interdependence throughout masses that are immense. Along with specialized appli ances for support and transfer, we find an exchange of aid at great distances. We see roots giving the vast aerial growth a hold tenacious enough to withstand violent winds, and supplying water enough even during periods of drought ; we see a stem and branches of corresponding strength for up holding the assimilating organs under ordinary and extraor- PHYSIOLOGICAL INTEGRATION IN PLANTS. 277 dinary strains ; and in these assimilating organs we set elaborate appliances for yielding to the stem and roots the materials enabling them to fulfil their offices. As a con sequence of which greater integration accompanying the greater differentiation, there is ability to maintain life over an immense period under marked vicissitudes. Even more conspicuously exemplified in Phaenogams, is that physiological integration which holds together the functions not of the individual only but of the species as a whole. The organs of reproduction, both in their relations to other parts of the individual bearing them and in their relations to corresponding parts of other individuals, show us a kind of integration conducing to the better preservation of the race ; as those already specified conduce to the better preservation of the individual. In the first place, this greater co-ordination of functions just described, itself enables Phaenogams to be queath to the germs they cast off, stores of nutriment, pro tective envelopes, and more or less of organization : so giving them greater chances of rooting themselves. In the second place, certain differentiations among the parts of fructification, the meaning of which Mr. Darwin has so admirably explained, give to the individuals of the species a kind of integration that makes possible a mutual aid in the production of vigorous offspring. And it is interesting to observe how, in that dimorphism by which in some cases this mutual aid is made more efficient, the greater degree of integration is dependent on the greater degree of differentiation — not simply differentiation of the fructifying organs from other parts of the plant bearing them, but differentiation of these fructifying organs from the homologous organs of neighbouring indi viduals of the same race. Another form of this co-ordination of functions that conduces to the maintenance of the species, may be here named — partly for its intrinsic interest. I refer to the strange processes of multiplication that occur in the genus Bnjophylhtm. It is well known that the succulent leaves of B. calycinum, borne on foot-stalks 278 PHYSIOLOGICAL DEVELOPMENT eo brittle that they are easily snapped by the wind, send forth from their edges when they fall to the ground, buds that root themselves and grow into independent plants. The correlation here obviously furthering the preservation of the race, is more definitely established in another species of the genus — B. proliferum. This plant, shooting up to a consider able height, and having a stem containing but little woody fibre, habitually breaks near the bottom while still in flower ; and is thus generally prevented from ripening its seeds. The multiplication is, however, secured in another way. Before the stem is broken young plants have budded out from the pedicels of the flowers, and have grown to considerable lengths ; and on the fall of the parent they forthwith commence their separate lives. Here natural selection has established a remarkable kind of co-ordination between a special habit of growth and decay, and a special habit of proliferation. § 285. The advance of physiological integration among plants as we ascend to the higher types, is implied by their greater constancy of structure, as well as by the stricter limi tation of their habitats and modes of life. " Complexity of structure is generally accompanied with a greater tendency to permanence in form," says Dr. Hooker ; or, conversely, " the least complex are also the most variable." This is the second aspect under which we have to contemplate the facts. The differences between the simpler Algce and Fungi, and between them and the Lichens, are so feebly marked that botanists have been unable to frame satisfactory definitions of these classes. " Linnaeus, for instance, and Jussieu, con- sideied Lichens as forming a part of Algce, in which they are followed by Fries." Mr. Berkeley, however, quoting the admission of Fries " that there is no certain distinction be tween Lichens and Fungi, except the presence in the former of green globules, resembling grains of chlorophyll," him self prefers to unite Fungi and Lichens under the general head of Mycetales. This structural indeflniteuess is accom- PHYSIOLOGICAL INTEGRATION IN PLANTS. 279 panied by functional indefiniteness. The ugh, considered collectively, these Thallogens form " three very natural groups, according as they inhabit the water, the earth, or the air ;" yet if, instead of their higher members we look at their lower members, we find these distinctions of habitat very undecided. A/gce, which are mostly aquatic, include many small forms that frequent the damp places preferred by Lichens and Fungi. Among Lichens, as among Fungi, there are kinds that lead submerged lives like the Atyte. While terrestrial Lichens and Fungi compete for the same places, as well as simulate one another's modes of growth. Besides this indistinctness of the classes, there is great variability in the shapes and modes of life of their species — a variability so great that what were at first taken to be different species, or different genera, or even different orders, have proved to be merely varieties of one species. So inconstant in struc ture are the Algce that Schleiden quotes with approval the opinion of Kutzing, that " there are no species but merely forms of Algce" In all which groups of facts we see that these lowest types of plants, little differentiated, are also but little integrated. Acrogens present a parallel relation between the small specialization of functions which constitutes physiological differentiation, and the small combination of functions which constitutes physiological integration. " Mosses," says Mr. Berkeley, "are no less variable than other cryptogams, and are therefore frequently very difficult to distinguish. Not only will the same species exhibit great diversity in the size, mode of branching, form and nervation of the leaves, but the characters of even the peristome itself are not constant." And concerning the classification of the remaining group, FiUcales, he says: — "Not only is there great difficulty in arranging ferns satisfactorily, but it is even more difficult to determine the limits of species." After this vagueness of separation as well as inconstancy of structure and habit among the lower plants, the stability 280 PHYSIOLOGICAL DEVELOPMENT. 4k of structure and habit and divisibility of groups among the higher plants, appear relatively marked. Though Phaenogams are much more variable than most botanists have until recently allowed, yet the definitions of species and genera may be made with far greater precision and are far less capable of change than among Cryptogams. And this comparative fixity of type, implying, as it does, a closer combination of the component functions, we see to be the accompaniment of the greater differentiation of those functions and of the structures performing them. That these characters are correlatives is further shown by the fact that the higher plants are more restricted in their habitats than the lower plants, both in space and time. " The much narrower delimitation in area of animals than plants," sEVELOPMENT. Another case — a very interesting one, somewhat allied to this — is presented by the ruminating animals. Here several dilatations of the alimentary canal precede the true stomach ; and in these, large quantities of unmasticated food are stored, to be afterwards returned to the mouth and masticated at leisure. What conditions have made this specialization advantageous ? and by what process has it been established? To both these questions the facts indicate answers which are not unsatisfactory. Creatures that obtain their food very irregularly — now having more than they can consume, and now being for long periods without any — must, in the first place, be apt, when very hungry, to eat to the extreme limits of their capacities ; and must, in the second place, profit by peculiarities which enable them to compensate themselves for long fasts, past and future. A perch which, when its stomach is full of young frogs, goes on filling its oesophagus also ; or a trout which, rising to the fisherman's fhr, proves when taken off the hook to be full of worms and insect-larvae up to the very mouth, gains by its ability to take in such unusual supplies of food when it meets with them — obviously thrives better than it would do could it never eat more than a stomachf'ul. That this ability to feed greatly in excess of immediate requirement, is one that varies in indi viduals of the same race, we see in the marked contrast between our own powers in this respect, and the powers of uncivilized men ; whose fasting and gorging are to us so astonishing. Carrying with us these considerations, we shall not be surprised at finding dilatations of the oesophagus in vultures and eagles, which get their prey at long intervals in large masses ; and we may naturally look for them too in birds like pigeons, which, coming in flocks upon occasional supplies of grain, individually profit by devouring the greatest quantity in a given time. Now where the trituration of the food is, as in these cases, carried on in a lower part of the alimentary canal, nothing further is required than the %toring-chamber ; but for a mammal, having its grinding THE INNER TISSUES OF ANIMALS. 315 apparatus in its mouth, to gain by the habit of hurriedly swallowing unmasticated food, it must also have the habit of regurgitating the food for subsequent mastication. This correlation of habits with their answering structures, may, as we shall see, arise in a very simple way. The starting point of the explanation is a familiar fact — the fact that indigestion, often resulting from excess of food, is apt to cause that reversed peristaltic action known as vomiting. From this we pass to the fact, also within the experience of most persons, that during slight indigestion the stomach some times quietly regurgitates a small part of its contents as far as the back of the mouth — giving an unpleasant acquaintance with the taste of the gastric juices. Exceptional facts of the same class help the argument a step further. " There are certain individuals who are capable of returning, at will, a greater or smaller portion of the contents of the digesting stomach into the cavity of the mouth. * * * In some of these cases, the expulsion of the food has required a violent effort. In the majority, it has been easily evoked or suppressed. While in others, it has been almost uncontrollable ; or its non- occurrence at the habitual time has been followed by a painful feeling of fulness, or by the act of vomiting.'' Here then we have a certain physiological action, occa sionally happening in most persons and in some developed into a habit more or less pronounced : indigestion being the habitual antecedent. Suppose then that gregarious animals, living on innutritive food such as grass, are subject to a like physiological action, and are capable of like varia tions in the degree of it. What will naturally happen ? They wander in herds, now over places where food is scarce and now coming to places where it is abundant. Some mas ticate their food completely before swallowing it ; while some masticate it incompletely. If an oasis, presently bared by their grazing, has not supplied the whole herd a full meal, then the individuals which masticate completely will have had less than those which masticate incompletehT — will not PHYSIOLOGICAL DEVELOPMENT. have had enough. Those which masticate incompletely and distend their stomachs with food difficult to digest, will be liable to these regurgitalions ; but if they re-masticate what is thus returned to the mouth (and we know that animals often eat again what they have vomited), then the extra quantity of food taken, eventually made digestible, will yield them more nourishment than is obtained by those which masticate completely at first. The habit initiated in this natural way, and aiding survival when food is scarce, will be apt to cause modifications of the alimentary canal. We know that dilatations of canals readily arise under habitual distensions. We know that canals habitu ally distended become gradually more tolerant of the contained masses that at first irritated them. And we know that there commonly take place adaptive modifications of their surfaces. Hence if a habit of this kind and the structural changes resulting from it, are in any degree inheritable, it is clear that, increasing in successive generations, both imme diately by the cumulative effect of repetitions and mediately by survival of the individuals in which they are most decided, they may go on until they end in the peculiarities which Ruminants display. § 298. There are structures belonging to the same group which cannot, however, be accounted for in this way. They are the organs that secrete special products facilitating digestion — the liver, pancreas, and various smaller glands. All these appendages of the alimentary canal, large and independent as some of them seem, really arise by differen tiations from its coats. The primordial liver, as we see it in a simple animal such as the Planaria, consists of nothing more than bile-cells scattered along a tract of the intestinal surface. Accumulation of these bile-cells is accompanied by increased growth of the surface which bears .them — a growth which at first takes the form of a cul-de-sac, having an outside that projects from the intestine into the peri- visceral cavity THL; INNER TISSUES OF ANIMALS. 317 As the mass of bile-cells becomes greater, there arise w> condary lateral cavities opening into the primary one, and through it into the intestine; until eventually these cavities with their coatings of bile-cells, become ramifying ducts dis* tributed through the solid mass we know as a liver. How is this differentiation caused ? Before attempting any answer to this question, it is requisite to inquire the nature of bile. Is that which the liver throws into the intestines a waste product of the organic actions ? or is it a secretion aiding digestion ? or is it mixture of these ? Modern investigations imply that it is most likely the last. The liver is found to have a compound function. Bernard has proved to the satisfaction of physiologists, that there goes on in it a formation of glycogen — a substance that is trans formed into sugar before it leaves the liver and is afterwards carried away by the blood to eventually disappear in the lungs. It is also shown, experimentally, that there are generated in the liver certain biliary acids ; and by the aid either of these or of some other compounds, it is clear that bile renders certain materials more absorbable : its effect on fat is demonstrable out of the body ; and the greatly diminished absorption of fat from the food when the discharge of bile into the intestine is prevented, is probably one of the causes of that pining away that results. But while recognizing the fact that the bile consists in part of a solvent, or solvents, aiding digestion, there is abundant evidence that one element of it is an effete product ; and probably this is the primary element. The yellow-green substance called biliverdine, which gives its colour to bile, is found in the blood before it reaches the liver ; which is not the case with the glycogen or the biliary acids. " As soon as the biliary secretion is in abeyance," says Dr. Harley, the most recent authority on the subject, " biliverdine' accumu lates in the blood (until the serum is as it were completely saturated with the pigment), from which it exudes and staina the tissues, and produces the colour we term jaundice;* 318 PHYSIOLOGICAL DEVELOPMENT. * * * " the urine assumes a saffron tint in consequence of the elimination of the colouring master by the kidneys ;" and afterwards " the sweat, the milk, the tears, the sputa" become yellow. We have clear proof, then, that biliverdine is an excrementitious matter, which, if not got rid of through the liver, makes its way out, to some extent, through other or gans, producing in them more or less derangement — itching of the skin, and sometimes, in the kidneys, a secondary disease. That of the bile discharged into the intestine, only some components are re- absorbed, is demonstrated by the fact that when injected into the blood, bile destroys life in less than twenty-four hours ; and that biliverdine is not among the re-absorbed components, is shown both by the persistence of the colour which it gives to the substances in the intestine, and by the absence of that jaundice which, if re- absorbed, it would produce. Hence we are warranted in classing bili verdine as a waste product. And considering that the bile- cells, where they first make their appearance among animals, are distinguished by the colour ascribable to this substance, we may fairly infer that the excretion of biliverdine is the original function of the liver. One further preliminary is requisite. We must for a moment return to those physico-chemical data, set down in the first chapter of this work (§§ 7 — 8.) We there saw that the complex and large-atomed colloids which mainly compose living organic matter, have extremely little molecular mo bility ; and, consequently, extiemely little power of diffusing tli em selves. Whereas we saw not only that those absorbed matters, gaseous and liquid, which further the decomposition of living organic matter, have very high diffusibilities ; but also that the products of the decomposition are much more diffusible than the components of living organic matter. And we saw that, as a consequence of this, the tissues give ready entrance to the substances that decompose them, and ready exit to the substances into which they are decomposed. Hence it follows that, prirnarity, the escape of effete matters from the THE 1NNEK TISSUES OF ANIMALS. 319 organism, is a physical action parallel to that which goes on among mixed colloids and crystalloids that are dead or even inorganic. Excretion is simply a specialized form of this spontaneous action ; and what we have to inquire is, — how the specialization arises. Two causes conspire to establish it. The first is that these products of decomposition are diffusible in widely different degrees. While the carbonic acid and water permeate the tissues with ease in all directions, and escape more or less from all the exposed surfaces, urea, and other waste substances incapable of being vaporized, cannot escape thus readily. The second is that the different parts of the organism, being subject to different physical conditions, are from the outset sure severally to favour the exit of these various products of decomposition in various degrees. How these causes must have co-operated in localizing the excretions, we shall see on remembering how they now co-operate in localizing the sepa ration of morbid materials. The characteristic substances of gout and rheumatism have their habitual places of deposit. Tuberculous matter, though it may be present in various organs, gravitates towards some much more than towards others. Certain products of disease are habitually got rid of by the skin, instead of collecting internally. Mostly, these have special parts of the skin which they affect rather than the rest ; and there are those which, by breaking out sym metrically on the two sides of the body, show how definitely the places of their excretion are determined by certain favour ing conditions, which corresponding parts may be presumed to furnish in equal degrees. Further, it is to be observed of these morbid substances circulating in the blood, that having once commenced segregating at particular places, they tend to continue segregating at those places. As suming, then, as we may fairly do, that this localization of excretion, which we see continually commencing afresh with morbid matters, has always gone on with the matters produced by the waste of the tissues, let us take a further 53 320 PHYSIOLOGICAL DEVELOPMENT. step, and ask how localizations become fixed. Other things equal, that which from its physical conditions is a place of least resistance to the exit of an effete product, will tend to become established as the place of excretion ; since the rapid exit of an effete product will profit the organism. Other things equal, a place at which the excreted matter produces least detrimental effect will become the established place. If at any point the excreted matter produces a beneficial effect, then, other things equal, natural selection will determine it to this point. And if facility of escape anywhere goes along with utilization of the escaping substance, then, other things equal, the excretion will be there localized by survival of the fittest. Such being the conditions of the problem, let us ask what will happen with the lining membrane of the alimentary canal. This, physiologically considered, is an external sur face ; and matters thrown off from it make their way out of the body. It is also a surface along which is moving the food to be digested. Now, among the various waste products continually escaping from the living tissues, some of the more complex ones, not very stable in composition, are likely, if added to the food, to set up changes in it. Such changes may either aid or hinder the preparation of the food for absorption. If an effete matter, making its exit through the wall of the intestine, hinders the digestive process, the enfeeblement and disappearance of individuals in which this happens, will prevent the intestine from becoming the esta blished place for its exit. While if it aids the digestive process, the intestine will, for converse reasons, become more and more the place to which its exit is limited. Equally manifest is it that if there is one part of this alimentary canal at which, more than at any other part, the favourable effect results, this will become the place of excretion. If from this general statement we pass to the special case before us, we find our data to be these : — The substance to be excreted, biliverdine, a waste product of the organic actions, THE INNER TISSUES OF ANIMALS. 321 is, as jaundice shows us, capable of escaping out of the body through all its surfaces, even in so differentiated a type as the highest mammal ; and in the un differentia ted types we may infer that the facility of escape is nearly the same through all the surfaces. For the gradual localization of its escape at a particular part of the intestinal surface, it is requisite only that either some disadvantage consequent on its escape elsewhere should be avoided, or some advantage due to its effect on digestion should be gained ; and this advantage may be either direct or indirect. It is not necessary that the biliverdine should itself act on the food : it is enough if it aids in the elaboration of other matters, either nutritive or solvent. If its presence causes or furthers the formation of glycogen from other components of the blood ; or if it sets up the complex reactions which generate the biliary acids ; these effects will suffice to establish, as the place of its excretion, the place where these products are useful. And once this place of excretion having been established, the development of a liver is simply a question of time and natural selection. Whether in this case, as well as in the cases of the exclu sively secreting glands formed along the alimentary canal (to which a modification of the foregoing argument is applicable), any tendency to localization results from the immediate action of the local conditions, is an interesting question. It is possible that the contrasts between the intra-vascular and extra- vascular liquids at these places may be a factor in the differentiation, as in a case already dealt with. (§ 292.) But this possibility must be left undiscussed. § 299. A differentiation of another order occurring in the alimentary canal, is that by which a part of it is developed into a lateral chamber or chambers, through which carbonic acid exhales and oxygen is absorbed. Comparative anatomy and embryology unite in showing that a lung is formed, just as a liver or other appendage of the alimentary canal ia formed, by the growth of a hollow bud into the peri- visceral 322 PHYSIOLOGICAL DEVELOPMENT. cavity, or space between the alimentary canal and the wall oi the body. The interior of this bud is simply a cul-de-sac of the alimentary canal, with the mucous lining of which its own mucous lining is continuous. And the development of this cul-de-sac into an air-chamber, simple or compound, is merely a great extension of area in the internal surface of the cul-de-stic, along with that specialization which fits it for excreting and absorbing substances different from those which other parts of the mucous surface excrete and absorb. These lateral air-chambers, universal among the higher Vertebrata and very general among the lower, and everywhere attached to the alimentary canal between the mouth and the stomach, have not in all cases the respiratory function. In most fishes that have them they are what we know as swim-bladders. In some fishes the cavities of these swim-bladders are completely shut off from the alimentary canal : nevertheless showing, by the communi cations which they have with it during the embryonic stages, that they are originally direrticula from it. In other fishes there is a permanent diicttts pneumaticus, uniting the cavity of the swim-bladder with that of the gullet — the function, however, being still not respiratory in an appreciable degree, if at all. But in certain still extant representatives of the sauroid fishes, as the Lepidosteus, the air-bladder is "divided into two sacs that possess a cellular structure/' and "the trachea which proceeds from it opens high-up in the throat, and is surrounded with a glottis/' In the Amphibia the corresponding organs are chambers over the surfaces of which there are saccular depressions, indicating a transition towards the air-cells characterizing lungs ; and accompanying this advance we see, as in the common Triton, the habit of coming up to the surface and taking down a fresh supply of air in place of that discharged. How are the internal air-chambers, respiratory or non- respiratory, developed ? Upwards from the amphibian stage, in which they are partially refilled at long intervals, there ia THE INNER TISSUES OF ANIMALS. 323 no difficulty in understanding how, by infinitesimal steps, they pass into complex and ever-moving lungs. But how is the differentiation that produces them initiated ? How comes a portion of the internal surface to be specialized for converse with a medium to which it is not naturally exposed ? The problem appears a difficult one ; but there is a not unsatisfactory solution of it. When many gold-fish are kept in a small aquarium, as with thoughtless cruelty they frequently are, they swim close to the surface, so as to breathe that water which is from instant to instant absorbing fresh oxygen. In doing this they often put their mouths partly above the surface, so that in closing them they take in bubbles of air; and sometimes they may be seen to continue dcing this — the relief due to the slight extra aeration of blood so secured, being the stimulus to continue. Air thus taken in may be detained. If a fish that has taken in a bubble turns its head down wards, the bubble will ascend to the back of its mouth, and there lodge; and coming within reach of the contractions of the oesophagus, it may be swallowed. If, then, among fish thus naturally led upon occasion to take in air-bubbles, there are any having slight differences in the alimentary canal that facilitate lodgment of the air, or slight nervous differences such as in human beings cause an accidental action to be come " a trick," it must happen that if an advantage accrues from the habitual detention of air-bubbles, those individuals most apt to detain them, will, other things equal, be more likely than the rest to survive ; and by the survival of descendants inheriting their peculiarities in the greatest degrees, and increasing them, an established structure and an established habit may arise. And that they do in some way arise we have proof : the common Loach is well known to swallow air, which it afterwards discharges loaded with carbonic acid. From air thus swallowed the advantages that may be derived are of two kinds. In the first place, the fish is made 324 PHYSIOLOGICAL DEVELOPMENT. specifically lighter, and the muscular effort needed to keep it from sinking is diminished — or, indeed, if the bubble is of the right size, is altogether saved. The contrast between the movements of a Goby, which, after swimming up towards the surface falls rapidly to the bottom on ceasing its exertions, and the movements of a Trout, which remains suspended just balancing itself by slight undulations of its fins, shows how great an economy results from an internal float, to fishes which seek their food in mid-water or at the surface. Hence the habit of swallowing air having been initiated in the way described, we see why natural selection will, in certain fishes, aid modifications of the alimentary canal favouring its lodgment — modifications constituting air-sacs. In the second place, while from air thus lodged in air-sacs thus developed, the advantage will be that of flotation only if the air is infrequently changed or never changed ; the advantage will be that of supplementary respiration if the air-sacs are from time to time partially emptied and refilled. The re quirements of the animal will determine which of the two functions predominates. Let us glance at the different sets of conditions under which these divergent modifications may be expected to arise. The respiratory development is not likely to take place in fishes that inhabit seas or rivers in which the supply of aerated water never fails : there is no obvious reason wrhy the established branchial respiration should be replaced by a pulmonic respiration. Indeed, if a fish's branchial respiration is adequate to its needs, a loss would result from the effort of coming to the surface for air ; especially during those first etages of pulmonic development when the extra aeration achieved was but small. Hence in fishes so circumstanced, the air-chambers arising in the way described would naturally become specialized mainly or wholly into floats. Their con tained air being infrequently changed, no advantage would arise from the development of vascular plexuses over their surfaces ; nothing would be gained by keeping open the com- THE INNER TISSUES OF ANIMALS. 325 munication between them and the alimentary canal ; and there might thus eventually result closed chambers the gaseous contents of which, instead of being obtained from without, were secreted from their walls, as gases often are from mucous membranes. Contrariwise, aquatic vertebrata in which the swallowing of air-bubbles, becoming habitual, had led to the formation of sacs that lodged the bubbles ; and which continued to inhabit waters not always supplying them with sufficient oxygen ; might be expected to have the sacs further developed, and the practice of chang ing the contained air made regular, if either of two advan tages resulted — either the advantage of being able to live in old habitats that had become untenable without this modifi cation, or the advantage of being able to occupy new habitats. Now it is just where these advantages are gained that we see the pulmonic respiration coming in aid of the branchial respiration, and in various degrees replacing it. Shallow waters are liable to three changes which conspire to make this supplementary respiration beneficial. The summer's sun heats them, and raising the temperatures of the animals they contain, accelerates the circulation in these animals, exalts their functional activities, increases the production of car bonic acid, and thus makes aeration of the blood more needful than usual. Meanwhile the heated water, instead of yielding to the highly carbonized blood brought to the branchiae the usual quantity of oxygen, yields less than usual; for as the heat of the water increases, the quantity of air it contains diminishes. And this greater demand for oxygen joined with smaller supply, pushed to an extreme where the water is nearly all evaporated, is at last still more intensely felt in consequence of the excess of carbonic acid discharged by the numerous creatures congregated in the muddy puddles that remain. Here, then, it is, that the habit of taking in air-bubbles is likely to become established, and the organs for utilizing them developed ; and here it is, accordingly, that we find all stages of the transition to aerial respiration. The Loach before- 326 PHYSIOLOGICAL DEVELOPMENT. mentioned, which swallows air, frequents small waters liable to be consideraoly warmed ; and the Cuchia, an anomalous eel shaped fish, which has vascular air-sacs opening out at the back of the mouth, "is generally found lurking in holes and crevices, on the muddy banks of marshes or slow-moving rivers." Still more significant is the fact that the Lepidoslrcn, or "mud-fish" as it is called from its habits, is the only true fish that has lungs. But it is among the Amphibia that we see most conspicuously this relation between the development of air-breathing organs, and the peculiarities of the habitats. Pools, more or less dissipated annually, and so rendered unin habitable by most fishes, are very generally peopled by these transitional types. Just as we see, too, that in various climates and in various kinds of shallow waters, the supple mentary aerial respiration is needful in different degrees ; so do we find among the Amphibia many stages in the substi tution of the one respiration for the other. The facts, then, are such as give to the hypothesis a vraisemblance greater than could have been expected. The relative effects of direct and indirect equilibration in establishing this further heterogeneity, must, as in many other cases, remain undecided. The habit of taking in bubbles is scarcely interpretable as a result of spontaneous variation : we must regard it as arising accidentally during the effort to obtain the most aerated water ; as being persevered in because of the relief obtained ; and as growing by repetition into a tendency bequeathed to offspring, and by them, or some of them, increased and transmitted. The formation of the first slight modifications of the alimentary canal favouring the lodgment of bubbles, is not to be thus explained. Some favourable variation in the shape of the passage must her^e have been the initial step. But the gradual increase of this structural modification by the survival of individuals in which it is carried furthest, will, I think, be all along aided by immediate adaptation. The part of the alimentary canal previously kept from the air, but now habitually in contact THE INKER TISSUES OF ANIMALS, '321 v/itli the air, must be in some degree modified by the action of the air; and the directly-produced modification, increasing in the individual and in successive individuals, cannot cease until there is a complete balance between the actions of the changed agency and the changed tissue. It i? indeed probable that the growth as well as the differentiation of the pulmonic surface, when once commenced, will bt furthered by the direct process. The reasoning befor* used in the case of branchias (§ 292) applies in the cast of lungs. If exchange between the plasma in the blood vessels and the plasma in the tissues surrounding thenv goes on with a rapidity that becomes greater where the difference between them becomes greater ; if, consequently, at some place where the carbonized plasma inside the blood-vessels is brought close to an unusually decarbonized or much oxygenated plasma outside of the blood-vessels, the exchange of these liquids becomes unusually active ; if, as a result, the circulation in the part is augmented ; then it is to be inferred that the extra nutrition will cause extra growth. The surface of the rudimentary lung will increase in area so long as the capillary osmose is much greater than in other parts of the body ; and it will continue to be greater until, by the extension of the aerating surface, the respiratory exchange has been rendered so efficient as to bring down the contrast between the intra- vascular and extra-vascular liquids to a level with the contrasts between the iiitra-vascular and extra-vascular liquids in other organs. That is to say, the growth which this direct action produces, will go on until the functional efficiency of the lungs is in equilibrium with the functional efficiencies of other parts throughout the organism. § 300. We come now to differentiations among the truly inner tissues — the tissues which have direct converse neither with the environment nor with the foreign substances taken into the organism from the environment. These, speaking broadly, are the tissues which lie between the double layer PHYSIOLOGICAL DEVELOPMENT. forming the integument with its appendages, and the double layer forming the alimentary canal with its dicerticula. We will take first the differentiation which produces the vascular system. Certain forces producing and aiding distribution of liquids in animals, come into play before any vascular system exists ; and continue to further circulation after the development of a vascular system. The first of these is osmotic exchange, acting locally and having an indirect general action ; the second is osmotic distension, acting generally and having an indirect local action ; the third is local variation of pressure which movement of the body throws on the tissues and their contained liquids. A few words are needed in elucidation of each. If in any creature, however simple, different changes are going on in parts that are differently conditioned — if, as in a Hydra, one surface is exposed to the surrounding medium while the other surface is exposed to dissolved food ; then between the unlike liquids which the dissimilarly-placed parts contain, osmotic currents must arise ; and a movement of liquid through the intermediate tissue must go on as long as an unlikeness between iho liquids is kept up. This primary cause of re-distribution remains one of the causes of re-distri bution in every more-developed organism : the passage of matters into and out of the capillaries is everywhere thus set up. And obviously in producing these local currents, osmose must also indirectly produce general currents, or aid them if otherwise produced. Osmose, however, still further. aids circulation b}^ the liquid pressure which it esta blishes throughout the organism. More marked than the contrasts between the liquids in some parts and those in other parts, is the contrast between the whole mass of liquid in the animal and the liquid bathing its surfaces — either the water in which it is immersed, or the water taken into its .alimentary canal. Its blood and all its juices being denser than water, the result is an osmotic absorption tend ing ever to distend all its permeable parts— its tissues, THE INNER TISSUES OF ANIMALS. 329 and its vessels when it has them. But these vessels and tissues are elastic; and if distended must everywhere com press their contents — must tend, therefore, to squeeze out their contents where there is least resistance. Consequently, if at any place there is an abstraction of nutritive liquid, either for growth or function, more nutritive liquid will be forced towards that place. TMs cause of currents, which cannot fail to work throughout the distended tissues even of animals that are without blood-vessels, comes more actively into play where the body is everywhere traversed by these branching tubes with elastic walls. When we learn that the pressure of blood within the arteries and veins of a mammal varies from some 3 Ibs. to J of a Ib. per square inch, we see, on averaging this pressure, that the coats of the vascular system exert considerable force on the blood. This average pressure cannot be due to the heart's action ; since if, in the absence of the heart's action, the whole mass of the blood in the vascular system were not above atmospheric pressure, the heart's action could not produce a pressure above that of the atmosphere in one part of the vascular system without lowering the pressure below that of the atmo sphere in another part of the vascular system. Hence it follows that irrespective of the heart's action, the dis tended walls of the vascular system must so compress the blood, as to cause a flow of it towards places whore its escape is least resisted — towards places, that is, where it is most rapidly abstracted by function or growth. This is a cause of distribution which is at work before any central organ of circulation exists. Though in the rudimen tary vascular systems of the simpler animals, the osmotic distension is probably nothing like so great, there must be some of it ; and in the absence of a pumping organ, this force is probably an important aid to that move ment of the blood which the functions set up. How the third cause — the changes of internal pressure which an animal's movements produce — furthers circulation, will be 330 PHYSIOLOGICAL DEVELOPMENT. sufficiently manifest. That parts which are bent or strained necessarily have their contained vessels squeezed, has been before shown (§ 281) ; and whether the bend or strain 13 caused, as in a plant, by an external force, or, as usually in an animal, by an internal force, there must be a thrusting of the liquids towards plates of least resistance — that is, towards places of greatest consumption. This which, in animals with out hearts is a main agent of circulation, continues to further it very considerably even among the highest animals. There is experimental proof of the fact. The pressure in the jugu lar vein of a horse, which is about f of a pound per square inch while the muscles are at rest, rises to 2J Ibs. per square inch when the muscles are contracted to raise the head. Such, then, are the several forces we have to take into account in studying the genesis of the vascular system. Let us now7 pass to the facts to be interpreted. Even in such simple types as the Hydrozoa, cavities in the sarcode faintly indicate a structure that facilitates the transfer of nutritive matters. These vacuoles, possibly caused by the contraction of colloid substance in passing from the soluble to the insoluble state, become reservoirs filled with the plasma that slowly oozes through the sarcode ; and every movement of the animal, accompanied as it must be by changed pressures and tensions on these reservoirs, tends here to fill them and there to squeeze out their contents in that or the other direction — possibly aiding to produce, by union of several vacuoles, those lacunse or irregular canals which the sarcode in some cases presents. Irregular canals of this kind, not lined with any mem branes but being simply cavities running through the flesh, mainly constitute the vascular system in Mollmcoida and many Mollusca In the simplest of these types the nutritive liquid, absorbed into the cavity of the peri- visceral sac, is thrust hither and thither through this sac with every change in the creature's attitude, and simultaneously fills some of the sinuses which open out of this sac and run through the sub- THE INNER TISSUES OF ANIMALS. 3-31 stance of the body. This distribution of the plasma, which muscular movement and osmotic distension here combine to aid, is, in somewhat more developed types, further aided by a rudimentary heart : in the peri-visceral sac is seated an open-mouthed tube, along which a wave of contraction pro ceeds, first for a while in one direction and then again in the opposite direction. The higher orders of Mollusca have this simple contractile tube developed into a branched system of vessels or arteries, which run into the substance of the body and end in lacunae or simple fissures. This ending in lacunae takes place at various distances from the vascular centre. In some genera the arterial structure is carried to the periphery of the blood-system, while in others it stops short midway. Throughout most orders of the Mollusca the back current of blood continues to be carried by channels of the original kind : there are no true veins, but the blood having been delivered into the tissues, finds its way back to the peri- vis ceral cavity through inosculating sinuses. Among the Ce- phalopods, however, the afferent blood-canals, as well as the efferent ones, acquire distinct walls ; but even here the shut ting off of the vascular system from the general cavity of the body is not complete; since there are still certain veins which empty themselves into the peri- visceral sac. Put ting together these facts we may see pretty clearly the stages of vascular development. From the original reservoir of nutritive liquid between the alimentary canal and the wall of the body, a portion is partially shut off ; and by the ver micular contraction of the open tube thus formed, there is produced a more rapid transfer of the nutritive liquid from one part of the peri- visceral sac to another, than was origi nally produced by the motions of the animal. Clearly, the extension of this contractile tube and the development from it of branches running hither and thither into the tissues, must, by defining the channels of the blood throughout a part of its course, render its distribution more regular and active. As fast as this centrifugal growth of definite channels advances. 332 PHYSIOLOGICAL DEVELOPMENT. so fast are the efferent currents of blood, prevented from escaping laterally, obliged to move from the centre towards the circumference ; and so fast also does the less- developed set of channels become, of necessity, occupied by afferent currents, When, by a parallel increase of definiteness, the lacunae and irregular sinuses through which the afferent cur rents pass, become transformed into veins, the accompanying disappearance of all stagnant or slow -moving collections of blood, implies a further improvement in the circulation. By what agency is effected this differentiation of a definite vascular system from the indefinite peri- visceral sac ? No sufficient reply is obvious. The genesis of the primordial heart is not comprehensible as a result of direct equilibration ; and we cannot readily see our way to it as a result of in direct equilibration ; for it is difficult to imagine what favour able variation natural selection could have seized hold of to produce such a structure. A contractile tube that aided the distribution of nutritive liquid, being once established, survival of the fittest would suffice for its gradual extension and its successive modifications. But what were the early stages of the contractile tube, while it was yet not sufficiently formed to help circulation, and while it must nevertheless have had some advantage without which no selective process could go on ? This part of the question we must leave as at present insoluble. To another part of the question, how ever, an answer may be ventured. If we ask the origin of those ramifying channels which, first appearingas simple chan nels, eventually become vessels having definite walls, a reply admitting of considerable justification, is, that the currents of nutritive liquid forced and drawn hither and thither through the tissues themselves initiate these channels. We know that etreams running over and through solid and quasi-solid inor ganic matter, tend to excavate definite courses. We saw reason for concluding that the development of sap-channels in plants conforms to this general principle. May we not then suspect that the nutritive liquid contained in the tissue THE INNER TISSUES OF ANIMALS. 33cS of a simple animal, made to oozo now in this direction and now in that by osmotic distension and bjr the changes of pressure which the animal's movements cause, comes to have certain lines along which it is thrust backwards and forwards more than along other lines ; and must by repeated passings make these more and more permeable, until they become lacunae ? Such actions will inevitably go on; and such actions appear competent to produce some, at least, of the observed effects. The leading facts which indicate that this is a part cause of vascular development, are these. Growths normally recurring in certain places at certain intervals, are accompanied by local formations of blood-vessels. The periodic maturation of ova among the Mammalia, supplies an instance. Through the stroma of an ovarium are dis tributed innumerable minute vesicles, which, in their early stages, are microscopic. Of these, severally contained in their minute ovi-sacs, any one may develop : the determining cause being probably some slight excess of nutrition. When the development is becoming rapid, the capillaries of the neighbouring stroma increase and form a plexus on the walls of the ovi-sac. Now since there is no typical distribution of the developing ova ; and since the increase of an ovum to a certain size precedes the increase of vascularity round it ; we can scarcely help concluding that the setting up of currents towards the point of growth determines the formation of the blood-vessels. It may be that having once commenced, this local vascular structure completes itself in a typical manner; but it seems clear that this greater development of blood-vessels around the growing ovum is initiated by the draught towards it. Ab normal growths show still better this relation of cause and ef fect. The false membranes sometimes found in the bronchial tubes in croup, may perhaps fairly -be held abnormal in but a partial sense : it may be said that their vascular systems are formed after the type of the membranes to which they are akin. But this can scarcely be said of the morbid growths 334 PHYSIOLOGICAL DEVELOPMENT. classed as malignant. The blood-vessels in an encephaloid cancer, are led to enlarge and ramify, often to an immense extent, by the unfolding of the morbid mass to which they carry blood. Alien as is the structure as a whole to the type of the organism ; arid alien in great measure as is its tissue to the tissue on which it is seated ; it nevertheless happens that the growth of the alien tissue and accompanying ab straction of materials from the blood-vessels, determine a corresponding growth of these blood-vessels. Unless, then, we say that there is a providentially-created type of vascular structure for each kind of morbid growth (and even this would not much help us, since- the vascular structure has no constancy within the limits of each kind), we are com pelled to admit that in some way or other the currents of blood are here directly instrumental in forming their own channels. One more piece of evidence, before cited as exemplifying adaptation (§ 67), may be called to mind. When any main channel for blood, leading to or from a certain part of the body, has been rendered impervious, others among the channels leading to or from this same part, enlarge to the extent requisite for fulfilling the extra func tion that falls upon them : the enlargement being caused, as we must infer, by the increase of the currents carried. Here then are facts warranting inductively the deduction above drawn. It is true that we are le,ft in the dark respect ing the complexities of the process. How the channels for blood come to have limiting membranes, and many of them muscular coats, the hypothesis does not help us to say. But the evidence assigned goes far to warrant the belief that vascu lar development is initiated by direct equilibration ; though in direct equilibration may have had the larger share in establish ing the structures which distinguish finished vascular systems § 301. Of the inner tissues which remain let us next take bone. In what manner is differentiated this dense substanco serving in most cases for internal support ? THE INNER TISSUES OF ANIMALS. 335 Already when considering the vertebrate skeleton under its morphological aspect (§ 256') it was pointed out that the formation of dense tissues, internal as well as external, is, in some cases at least, brought about by the mechanical forces to be resisted. Through what process it is brought about we could not then stay to inquire : this question being not morphological but physiological. Answers to some kindred questions have since been attempted. Certain actions to which the internal dense tissues of plants may be ascribed, have been indicated ; and more recently, analogous actions have been assigned as causes of some external dense tissues of animals. We have now to ask whether actions of the same nature have produced these internal dense tissues of animals. The problem is an involved one. Bones have more than one. stage: they are membranous or cartilaginous before they be come osseous ; and their successive component substances so far differ that the effects of mechanical actions upon them differ. And having to deal with transitional states in which bone is formed of mixed tissues, having unlike physical properties and unlike minute structures, the effects of strains become too complicated to follow with precision. Anything in the way of interpretation must therefore be regarded as tentative. If analysis and comparison show that the phenomena are not inconsistent with the hypothesis of mechanical genesis, it is as much as can be expected. Let us first observe more nearly the mechanical conditions to which bones are subject. The endo-skeleton of a mammal with the muscles and liga ments holding it together, may be rudely compared to a structure built up of struts and ties ; of which, speaking generally, the struts bear the pressures and the ties bear tiie tensions. The framework of ail ordinary iron roof will give an idea of the functions of these two elements, and of the mechanical characters required by them. Such a framework consists partly of pieces that have each to bear a thrust in the direction of its length, and. partly of pieces that have each 54 336 PHYSIOLOGICAL DEVELOPMENT. to bear a pull in the direction of its length ; and these struts and ties are differently formed to adapt them to those different strains. Further, it should be remarked that though the rigidity of the framework depends on the ties which are flexible, as much as on the struts which are stiff, yet the ties help to give the rigidity simply by so holding the struts in position that they cannot escape from the thrusts which fall on them. Now the like relation holds with a difference among the bones and muscles — the difference being, that hero the ties admit of being lengthened or shortened and the struts of being moved about upon their joints. The mechanical re lations are not altered by this however. The actions are of essentially the same kind in an animal that is standing, or keeping itself in a strained attitude, as in one that is changing its attitude — the same in so far that we have in each a set of flexible parts that are pulling and a set of rigid parts that are resisting. It needs but to remember the sudden collapse and fall that take place when the muscles are paralyzed, or to remember the inability of a bare skeleton to support itself, to see that the struts without the ties cannot suffice. And we have but to think of the formless mass into which a man wrould sink when deprived of his bones, to see that the ties without the struts cannot suffice. To trace the way in which a particular bone has its particular thrust thrown upon it, may not always be practicable. Though it is easy to perceive how a flexor or extensor of the arm causes by its tension a re active pressure along the line of the humerus, and is enabled to produce its effect only by the rigidity of the humerus ; yet it is not so easy to perceive how such bones as tho^e of a horse's haunch are similarly acted upon. Still, as the weight of the hind quarters has to be transferred from the pelvis to the feet, and must be so transferred through the bones, it is manifest that though these bones form a very crooked line, the weight must produce a pressure along the axis of each : the muscles and ligaments concerned serving here, as in other cases, so to hold the bones that they bear the pressure instead THE INNER TISSUES OF ANIMALS. J337 of being displaced by it. Not forgetting that many processes of the bones have to bear tensions, we may then say that generally, though by no means universally, bones are in ternal dense masses that have to bear pressures — pressures which in the cylindrical bones become longitudinal thrusts. Leaving out exceptional cases, let us consider bones as masses thus circumstanced. When giving reasons for the belief that the vertebrate skeleton is mechanically originated, one of the facts put in evidence was, that in the vertebrate series the transition from the cartilaginous to the osseous spine begins peripherally (§ 257) : each vertebra being at first a ring of bone sur rounding a mass of cartilage. And it was pointed out that this peripheral ossification is ossification at the region ot greatest pressures. Now it is not vertebra) only that follow this course of development. In a cylindrical bone, though it is differently circumstanced, the places of commencing ossi fication are still the places on which the severest stress fulls. Let us consider how such a bone that has to bear a longitu dinal pressure is mechanically affected. If the end of a walking-cane be thrust with force against the ground, the cane bends ; and partially resuming its straightncss when relieved, again bends, usually towards the same side, when the thrust is renewed. A bend so caused acts on the fibres of the cane in nearly the same way as does a bend caused by supporting the cane horizontally at its two ends and suspending a weight from its middle. In either case the fibres on the con vex side are extended and the fibres on the concave side com pressed. Kindred actions occur in a rod that is so thick as not to yield visibly under the force applied. In the absence of complete homogeneity of its substance, complete symmetry in its form, and an application of a force exactly along its axis, there must be some lateral deflection ; and therefore some distribution of tensions and pressures of the kind indi cated. And then, as the tact which here specially concerns us, We have U> note that- IMC Mi-oiiirest tensions and pressures are 338 PHYSIOLOGICAL DEVELOPMENT. borne by the outer layers of fibres. Now the shaft of a long bone, subject to mechanical actions of this kind, similarly has its outer layer most strained. In this layer, therefore, on the mechanical hypothesis, ossification should commence, and here it does commence — commences, too, midway between the ends 'where the bends produce on the superficial parts their most intense effects. But we have not in this place simply to observe that ossification commences at the places of greatest stress, but to ask what causes it to do this. Can we trace the physical actions which set up this deposit of dense tissue ? It is, I think, possible to indicate a " true cause " that is at work ; though whether it is a sufficient cause may be questioned. We concluded that in certain other cases, the formation of dense tissue indirectly results from the alternate squeezing and relaxation of the vessels running through the part ; and the inquiry now to be made is, whether, in developing bone, the same actions go on in such ways as to produce the ob served effects. At the outset we are met by what seems a fatal difficulty — cartilage is a non-vascular tissue: this sub stance of which unossiried bones consist is not permeated by minute canals carrying nutritive liquid, and cannot, there fore, be a seat of actions such as those assigned. This ap parent difficulty, however, furnishes a confirmation. For cartilage that is wholly without blood-vessels does not ossify : ossification takes place only at those parts of it into which the capillaries penetrate. Hence, we get additional reason for suspecting that bone- formation is due to the alleged cause ; since it occurs where mechanical strains can produce the actions described, but does not occur wh -re mechanical strains cannot produce them. Let us consider more closely what the factors are, and how they will cooperate under the particular conditions. It seems possible that these canals that exist in the superficial layer of a cartilagin ous bone before it begins to ossify, are themselves produced by the mechanical actions. For every time a mass of carti lage is strained and its superficial layers more especially THE INNER TISSUES OF ANIMALS. 339 subject to tensions and pressures, the nutritive liquid diffused through the substance of the cartilage, compressed as it must be, will tend to ooze from the surface of the cartilage, and to return again when the stress is taken off. Such alternate movements of the nutritive liquid, perpetually repeated, will be apt to form channels. These, at first quite superficial and inappreciable, will become more appreciable ; since, when they are once commenced, any further additions of substance to the surface will be prevented from closing their openings by the alternate rushes of liquid ; and so a vascular layer of appreciable thickness may gradually be formed. But without doing more than hint this, it will suffice for the argument if we commence with the external vascular layer as already existing, and consider what will take place in it. Cartilage is elastic — is somewhat extensible, and spreads out laterally under pressure, but resumes its form when relieved. How, then, will the capillaries traversing such a substance be affected at the places where it is strained by a bend ? Those on the convex side will be laterally squeezed, in the same way that we saw the sap-vessels on the convex side of a bent branch are squeezed ; and as exudation of the sap into the adjacent prosenchyrna will be caused in the one case, so, in the other, there will be caused exudation of serum into the adjacent cartilage : extra nutrition and increase of strength resulting in both cases. The parallel ceases here, however. In the shoot of a plant, bent in various directions by the wind, the side which was lately compressed, is now extended ; and hence that squeezing of the sap-vessels which results from extension, suffices to feed and harden the tissue on all sides of the shoot. But it is not so with a bone. Having yielded on one side under longitudinal pressure, and resumed as nearly as may be its previous shape when the pressure is taken off, the bone yields again towards the same side when again longitudinally pressed. Hence the substance of its concave side, never rendered convex by a bend in the opposite direction, would 340 PHYSIOLOGICAL DEVELOPMENT. not receive any extra nutrition did no other action come into play. But if we consider how intermittent pressures must act on cartilage, we shall see that there will result extra nutrition of the concave side also. Squeeze between two pieces of glass a thin bit of caoutchouc that has a hole through it. While the caoutchouc spreads out away from the centre, it also spreads inwards, so as partially to close the hole. Everywhere its molecules move away in directions of least resistance ; and for those near the hole, the direction of least resistance is towards the hole. Let this hole stand for the transverse section of one of the capillaries passing through cartilage, and it will be manifest that on the side of the unossified bone made concave in the way described, the compressed cartilage will squeeze the capillaries traversing it ; and in the absence of perfect homogeneity in the cartilage, the squeeze will cause extra exudation from the capillaries into the cartilage. Thus every additional strain will give to the cartilage it falls upon, an additional supply of the materials for growth. So that presently the side which, by yielding more than any other, proves itself to be the weakest, will cease to be the weakest. What further will happen ? Some other side will yield a little— the bends will take place in some other plane; and the portions of cartilage on which repeated tensions and pressures now fall will be strengthened. Thus the rate of nutrition, greatest at the place where the bending is greatest, and changing as the incidence of forces changes, will bring about at every point a balance between the resistances and the strains. Thus, too, there will be determined that peripheral induration which we see in bones so circumstanced. As in a shoot we saw that the woody deposit takes place towards the outside of the cylinder, where, according to the hypothesis, it ought to take place ; so, here, we see that the excess of exudation and hardening, occurring where the strains are most intense, will form a cylinder having a dense outside and a porous or hollow inside. These processes will be essentially the same THE INNER TISSUES OF ANIMALS. 341 in bones subject to more complex mechanical actions ; such us sundry of the flat bones and others that serve as internal fulcra. Be the strains transverse or longitudinal, be they torsion strains or mixed strains, the outer parts of the bone will be more affected by them than its inner parts. They will therefore tend everywhere to produce resisting masses having outer parts more dense than their inner parts. And by causing most growth where they are most intense, will call out reactive forces adequate to balance them — forms and thicknesses of bone offering resistances equal to the- strains, however numerous and varied. There are doubt less obstacles in the way of this interpretation. It may be said that the forces acting on the outer layers in the manner described, would compress the capillaries too little to produce the alleged effects ; and if evenly distributed along the whole lengths of the layers, they would probably be so. But it needs only to bend a flexible mass and observe the tendency to form creases on the concave surface, to feel assured that along the surface of an ossifying bone, the yielding of the tissue when bent will not be uniform. In the absence of complete homogeneity, the interstitial yielding will take place at some points more than others, and at one point above all others. At these weakest points, and especially at one, the action on the capillaries will be concentrated. "When, at the weakest point — the centre of commencing ossification — an extra amount of deposit has been caused, it will cease to be the weakest ; and adjacent points, now the weakest, will become the places of yielding and induration. And in pro portion as the layer becomes filled with unyielding matter, the remaining compressible parts of it, arid their contained capillaries, will be more severely compressed. It may be further objected that the hypothesis is incompatible with the persistence of cartilage for so long a time between the epiphyscs of bones and the bony masses which they ter minate. But there is the reply that the places occupied by this curtilage, being places at which the bone lengthens, the PHYSIOLOGICAL DEVELOPMENT. non-ossification is in part apparent only — it is rather that new cartilage is formed as fast, as the pre-existing cartilage ossifies ; and there is the further reply that the slowness of the ultimate ossification of this part, is due to its non- vascularity, and to mechanical conditions that are unfavour able to its acquirement of vascularity. Once more, the de murrer that in the epiphyses ossification does not begin at the surface but within the mass of the cartilage, is met by an explanation parallel to that before given (§ 293, note) of the deep-seated induration produced by an external pressure which, during long intervals, does not intermit completely; as in a bunion, a node on the instep, and what is called " housemaid's knee." Of course it is not meant that this osseous development by direct equilibration, takes place in the individual. Though it is a corollary from the argument that in each individual the process must be furthered and modified by the particular actions to which the particular bones are exposed; yet the leading traits of structure assumed by the bones are assumed in conformity with the inherited type. This, however, is no difficulty. The type itself is to be regarded as the accumulated result of such modifications, transmitted and increased from generation to generation. The actions above described as taking place in the bone of an individual, must be understood as producing their total effect little by little in the corre sponding bones of a long series of individuals. Even if but a small modification can be so wrought in the individual, yet if such modification, or a part of it, is inheritable, we may readily understand how, in the course of geologic epochs, the observed structures may arise by the assigned way. Here may fitly come in a strong confirmation. If we find cases where individual bones, subject in exceptional degrees to the actions described, present in exceptional amounts the modifications attributed to them, we are greatly helped in understanding how there may be produced in the race that Aggregate of modifications which the hypothesis implies THE INNER TISSUES OF ANIMALS. 343 StN.'h cases occur in ricketty children. I am indebted to Mr. Busk for pointing- out these abnormal formations of dense tissue, that are not apparently explicable as results ol mechanical actions and re-actions. It was only on tracing out the processes here at work, that there suggested itself the specific interpretation of the normal process, as above set forth. When, from constitutional defect, bones do not ossify with due rapidity, and are meanwhile subject to the ordinary strains, they become distorted. Ptemembering how a mass which has been made to yield in any direction by a force it cannot withstand, is some little time before it recovers completely its previous form, and usually, indeed, undergoes what is called a "permanent set;" it is inferable that when a bone is repeatedly bent at the same time that the liquid contained in its capillaries is poor in the materials for forming dense tissue, there will not take place a propor tionate strengthening of the parts most strained ; and these parts will give way. This happens in rickets. But this having happened, there goes on what, in teleological language, we call a remedial process. Supposing the bone to be one commonly affected — a femur; and supposing a permanent bend to have been caused in it by the weight of the body ; the subsequent result is an unusual deposition of cartilaginous and osseous matter on the concave side of the bone. If the bone is represented by a strung bow, then the deposit occurs at the part represented by the space between the bow and the string. And thus occurring where its resistance is most effective, it increases until the approximately-straight piece of bone formed within the arc, has become strong enough to bear the pressure without appreciably yielding. Now this direct adaptation, seeming so like a special provision, and furnishing so remarkable an instance of \vhat, in medical but unscientific language, is called the ris mcdkatrix natnrce, is simply a result of the above- described mechanical actions and re-actions, going on under the exceptional conditions. Each time such a bent bone is subject to a force which again 344 PHYSIOLOGICAL DEVELOPMENT. bends it, the severest compression falls on the substance of its concave side. Each time, then, the capillaries running through this part of its substance are violently squeezed — far more squeezed than they or any other of the capillaries would have been, had the bone remained straight. Hence, on every repetition of the strain, these capillaries near the concave surface have their contents forced out in more than normal abundance. The materials for the formation of tissue are supplied in quantity greater than can be a.^-simi- lated by the tissue already formed ; and from the excess of exuded plasma, new tissue arises. A layer of organizable material accumulates between the concave surface and the periosteum ; in this, according to the ordinary course of tissue- growth, new capillaries appear ; and the added layer presently assumes the histological character of the layer from which it has grown. What next happens ? This added layer, further from the neutral axis than that which has thrown it out, is now the most severely compressed, and its capillaries are the most severely squeezed. The place of greatest exudation and most rapid deposit of matter, is there fore transferred to this new layer ; and at the same time that active nutrition increases its density, the excess of organizable material forms another layer external to it : the successive layers so added, encroaching on the space between the concave surface of the bone and the chord of its arc. What limits the encroachment on this space ? — what stops the pro cess of filling it up ? The answer to this question will be manifest on observing that there comes into play a cause which gradually diminishes the forces falling on each new layer. For the transverse sectional area is step by step increased ; and an increase of the area over which the weight borne is distributed, implies a relatively smaller pressure upon each part of it. Further, as the transverse dimensions of the bone increase, the materials composing its convex and concave layers, becoming further from the neutral axis, become bitter placed for resisting the strains to be borne THE INKER TISSUES OF ANIMALS. So that both by the increased quantity of dense matter and by its mechanically more-advantageous position, the bendings of the bone are progressively decreased. But as they ara decreased, each new layer formed on the concave surface, has its substance and its capillaries less compressed ; and the resulting growth and induration are rendered less rapid. Evidently, then, the additions, slowly diminishing, will eventually cease ; and this will happen when the bone no longer bends. That is to say, the thickening of the bone will reach its limit when there is equilibrium between the inci dent forces and the forces which resist them. Here, indeed, we may trace with great clearness the process of direct equilibration — may see how an unusual force, falling on the moving equilibrium of an organism and not overthrowing it, goes on working modifications until the re-action balances the action. That, however, which now chiefly concerns us, is to note how this marked adaptation supports the general argument. Unquestionably bone is in this case formed under the influ ence of mechanical stress, and formed just where it most effectually meets the stress. This result, not otherwise explained, is explained by the hypothesis above set forth. And when we see that this special deposit of bone is ac counted for by actions like those to which bone-formation in general is ascribed, the probability that these are the actions at work becomes very great. Of course it is not alleged that osseous structures arise in this way alone. The bones of the skull and various dermal bones cannot be thus interpreted. Here the natural selec tion of favourable variations appears the only assignable cause — the equilibration is indirect. We know that ossific deposits now and then occur in tissues where they are not usually found ; and such deposits, originally abnormal, if they occurred in places where advantages arose from them, might readily be established and increased by survival of the fittest. Especially might we expect this to happen when a 346 PHYSIOLOGICAL DEVELOPMENT constitutional tendency to form bone had been established b^ actions of the kind described ; for it is a familiar fact that differentiated types of tissue, having once become elements of an organism, are apt occasionally to arise in unusual places, and there to repeat all their peculiar histological cha racters. And this may possibly be the reason why the bones of the skull, though not exposed to forces such as those which produce, in ether bones, dense outer layers including less dense interiors, nevertheless repeat this general trait of bony structure. While, however, it is beyond doubt that some bones are not due to the direct influence of mechanical stress, we may, I think, conclude that mechanical stress initiates bone-formation. § 302. What is the origin of nerve ? In what way do its properties stand related to the properties of that protoplasm whence the tissues in general <->nse? and in what way is it differentiated from protoplasm simultaneously with the other tissues ? These are profoundly interesting questions ; but questions to which positive answers cannot be expected. All that can be done is to indicate answers which seem feasible. That the property specially displayed by nerve, is a pro perty which protoplasm possesses in a lower degree, is mani fest. The sarcode of a Rhizopod and the substance of an unimpregnated ovum, exhibit movements that imply a propa gation of stimulus from one part of the mass to another ; and through the nerveless body of a polype, we see slowly travelling and spreading a contraction set up by touching a tentacle — a contraction which implies the passage from part to part of some stimulus causing the contraction. We have not far to seek for a probable origin of this phenomenon. There is good reason for ascribing it to the extreme insta bility of the organic colloids of which protoplasm consists, These, in common with colloids in general, assume different isomeric forms with great facility ; and they display not THE INNER TISSUES OF ANIMALS. 347 simply isomerism but polymerisra. Further, this readiness to undergo molecular re- arrangement, habitually shows itself in colloids by the rapid propagation of the re-arrangement from part to part. As Prof. Graham has shown, matter in this state often " pectizes " almost instantaneously — a touch will transform an entire mass. That is to say, the change of molecular state once set up at one end, spreads to the other end— there is a progress of a stimulus to change ; and this is what we see in a nerve. So much being understood, let us re-state the case more completely. Molecular change, implying as it does motion of molecules, communicates motion to adjacent molecules ; be they of the same kind or of a different kind. If the adjacent molecules, either of the same kind or of a different kind, be stable in composition, a temporary increase of oscillation in them as wholes, or in their parts, may be the only result ; but if they are unstable there are apt to arise changes of arrangement "among them, or among their parts, of more or less permanent kinds. Especially is this so with the complex molecules which form colloidal matter, and with the organic colloids above all. Hence it is to be inferred that a molecular dis turbance in any part of a living animal, set up by either an external or internal agency, will almost certainly disturb and change some of the surrounding colloids not originally im plicated — will diffuse a wave of change towards other parts of the organism : a wave which will, in the absence of per fect homogeneity, travel further in some directions than in others. Let us ask next what will determine the differences of distance travelled in different directions. Ob viously an}r molecular agitation spreading from a centre, will go furthest along routes that offer least resistance. What routes will these be? Those along which there He most molecules Uiat are easily changed by the diffused molecular motion, and which yet do not take up much molecular motion in assuming their new states. Molecules which are tolerably stable will not readily propagate the agitation ; for they will absorb it 348 PHYSIOLOGICAL DEVELOPMENT. in the increase of their own oscillations, instead of passing "t on. Molecules which are unstable but which, in assuming isomeric forms, absorb motion, will not readily propagate it ; since it will disappear in working the changes in them. But unstable molecules which, in being isomerically transformed, do not absorb motion, and still more those which, in being so transformed, give out motion, will readily propagate any molecular agitation ; since they will pass on the impulse either undiminished, or increased, to adjacent molecules. If then we assume, as we are not only warranted in doing but are obliged to do, that protoplasm contains two or more colloids, either mingled or feebly combined (since it cannot consist of simple albumen or fibrin or casein, or any allied proximate principle) ; it may be concluded that any mole cular agitation set up by what we call a stimulus, will diffuse itself further along some lines than along others, if the com ponents of the protoplasm are rot quite homogeneously dis persed, and if some of them are isomerically transformed more easily, or with less expenditure of motion, than others ; and it will especially travel along spaces occupied chiefly by those molecules which give out molecular mo tion during their metamorphoses, if there should be any such. But now let us ask what structural effects will be wrought along a tract traversed by this wrave of molecular disturbance. As is shown by those transforma tions that so rapidly propagate themselves through colloids, molecules that have undergone a certain change of form, are apt to communicate a like change of form to ad jacent molecules of the same kind — the impact of each overthrow is passeel on and produces another overthrow. Probably the proneness towards isochronism of molecular movements necessitates this. If any molecule has had its components re-arranged, and their oscillations conse quently altered, there result movements not concordant with the movements in adjacert untransformed molecules, but which, impressing themselves on the parts of such untrans TKE INNER TISSUES OF ANIMALS. 349 formed molecules, tend to generate in them concordant move ments — tend, that is, to produce the re-arrangements involved by these concordant movements. Is this action limited to strictly isoraeric substances ? or may it extend to substances that are closely allied ? If along with the molecules of a compound colloid there are mingled those of some kindred colloid ; or if with the molecules of this compound colloid there are mingled the components out of which other such molecules may be formed ; then there arises the question — does the same influence which tends to propagate the iso meric transformations, tend also to form new molecules of the same kind out of the adjacent components ? There is reason to suspect that it does. Already when treating of the nutrition of parts (§ 64), it was pointed out that we are obliged to recognize a power possessed by each tissue to build up, out of the materials brought to it, molecules of the same type as those of which it is formed. This building up of like mole cules seems explicable as caused by the tendency of the new components which the blood supplies, to acquire move ments isochronous with those of the like components in the tissue ; which they can do only by uniting into like com pound molecules. Necessarily they must gravitate towards a state of equilibrium ; such state of equilibrium — moving equilibrium of course — must be one in which they oscillate in the same times with neighbouring molecules ; and so to oscillate they must fall into groups identical with the groups around them. If this be a general principle of tissue-growth and repair, we may conclude that it will apply in the case before us. A wave of molecular disturbance passing along a tract of mingled colloids closely allied in com position, and isomerically transforming the molecules of one of them, will be apt at the same time to form some new mole cules of the same type, at any place where there exist the proximate components, either uncornbined or feebly combined in some not very different way. And this will be most likely to occur where the molecules of the colloid that are under- 350 PHYSIOLOGICAL DEVELOPMENT. going the isomeric change, predominate, but have scattered through them the other molecules out of which they mav be formed, either by composition or modification. That is to say, a wave of molecular disturbance diffused from a centre, and travelling furthest along a line where lie most molecules that can be isomerically transformed with facility, will be likely at the same time to further differentiate this line, and make it more characterized than before by the easy-trans- formability of its molecules. One additional step, and the interpretation is reached. Analogy shows it to be not improbable that these organic colloids, isomerically trans formed by slight molecular impact or increase of molecular motion, will some of them resume their previous molecular structures after the disturbance has passed. We know that what are stable molecular arrangements under one degree of molecular agitation, are not stable under another degree ; and there is evidence that re-arrangements of an inconspicuous kind are occasionally brought about by very slight changes of molecular agitation. Water supplies a case. Prof. Graham infers that water undergoes a molecular re-arrange ment i.t about 32° — that ice has a colloid form as well as a crystalloid form, dependent on temperature. Send through it an extra wave of the molecular agitation we call heat, and its molecules aggregate in one way. Let the wave die away, and its molecules resume their previous mode of aggregation. And obviously such transformations may bo repeated back wards and forwards within narrow limits of temperature. Now among the extremely unstable organic colloids, such a phenomenon is far more likely to happen Suppose, then, that the nerve-colloid is one of which the molecules are changed in form by a passing wave of extra agitation, but re>ume their previous form when the wave has passed : the previous form being the most stable under the conditions which then recur. What follows P It follows that these molecules will be readv again to undergo isomeric transformation when there again occurs the stimulus ; will, as before, propagate the transforma THE IN^ER TISSUES OF ANIMALS. 351 tion most along the tract where they are most abundant; will, as before, simultaneously tend to form new molecules of their own type ; will, as before, make the line along which they lie one of easier transfer for the molecular agitation. Every repetition will help to increase, to integrate, to define more completely, the course of the escaping molecular motion — • extending its remoter part while it makes its nearer part more permeable — will help, that is, to form a line of discharge, a line for conducting impressions, a nerve. Such seems to me a not unfair series of deductions from the known habitudes of colloids in general and the organic colloids in particular. And I think that the implied nature and properties of nerve, correspond better with the observed phenomena than do the nature and properties implied by other hypotheses. Of course the speculation as it here stands is but tentative, and leaves much unexplained. It gives no obvious reply to the questions — what causes the formation of nerves along some lines rather than others ? what determines their appropriate connexions ? — questions, however, to which, when we come to deal with physiological integration, we may find not unsatisfactory answers. Moreover it says nothing about the genesis of ganglia. A ganglion, it is clear, must consist of a colloidal matter equally unstable, or still more unstable, which, when disturbed, falls into some different molecular arrangement, perhaps chemically simpler, and gives out in so doing a large amount of molecular motion — serves as a reservoir of molecular motion which may be suddenly discharged along an efferent nerve or nerves, when excite ment of an afferent nerve has disengaged it. How such a structure as this results, the hypothesis does not show But admitting these shortcomings it may still be held that we are, in the way pointed out, enabled to form an idea of the actions by which nervous tissue is differentiated. § 303. A speculation akin to, and continuous with, the last, is suggested by an inquiry into the origin of muscular tissue 55 352 PHYSIOLOGICAL DEVELOPMENT. Contractility as well as irritability is a property of protoplasm or sarcode ; and, as before suggested (§ 22), is not improbably due to isomeric change in one of its component colloids. It is a feasible supposition that of the several isomeric changes simultaneously set up among these component colloids, some may be accompanied by decided change of bulk and some not. Clearly the isomeric change undergone by the colloid which we suppose to form nerve, must be one not accompanied by appreciable change of bulk ; since change of bulk implies " internal work/' as physicists term it, and therefore ex penditure of force. Conversely, the colloid out of which muscle originates, may be one that readily passes into an iso meric state in which it occupies less space : the molecular disturbance causing this contraction being communicated to it from adjacent portions of nerve- substance that are mole- cularly disturbed ; or being otherwise communicated to it by direct mechanical or chemical stimuli ; as happens where nerves do not exist, or where their influence has been cut off. This interpretation seems, indeed, to be directly at variance with the fact that muscle does not diminish in bulk during contraction but merely changes its shape. That which we see take place with the muscle as a whole, is said also to take place with each fibre — while it shortens it also broadens. There is, however, a possible solution of this difficulty. A contracting colloid yields up its water ; and the contracted colloid plus the free water, may have the same bulk as before though the colloid has less. If it be replied that in this case the water should become visible between the substance of the fibre and its sarcolemma or sheath, it may be rejoined that this is not necessary — it may be deposited interstitially. Possibly the striated structure is one that facilitates its exudation and subsequent re-absorption ; and to this may be due the superiority of striated muscle in rapidity of contrac tion. Granting the speculative character of this interpretation, let us see how lar it agrees with the facts. If \he actions are as here supposed, the contracted or more inte« THE INNER TISSUES OF ANIMALS. 353 grated state of the muscular colloid will be that which it tends continually to assume — that into which it has an in creasing aptitude to pass when artificial paralysis has been produced, as shown by Dr. Norris — that into which it lapses completely in rigor mortis. The sensible motion generated by the contraction can arise only from the transformation of insensible motion. This insensible motion suddenly yielded up by a contracting mass, implies the fall of its com ponent molecules into more stable arrangements. And there can be no such fall unless the previous arrangement is un stable. From this point of view, too, it is pos sible to see how the hydro-carbons and oxy-hydro-carbons consumed in muscular action, may produce their effects. It was said, when exposing The Data of Biology, that non-nitro genous substance might evolve heat only when transformed in the circulating fluids, " but partly heat, and partly another force, when transformed in some active tissue that has ab sorbed it: just as coal, though producing little else but heat as ordinarily burnt, has its heat partially transformed into mechanical motion if burnt in a steam-engine furnace " (§ 18) ; and recent inquiries make it clear that some such relation exists.* Here a feasible modus operandi becomes manifest. For these non-nitrogenous elements of food when consumed in the tissues, give out lar^e amounts of molecular 7 O C motion. They do this in presence of the muscular colloids that have lost molecular motion during their fall in the stable or contracted state. And from the molecular motion they give out, may be restored the molecular motion lost by the contracted colloids : these contracted colloids may so have their molecules raised to that unstable state from which, again falling, they can again generate mechanical motion. * See account of experiments made by Profs. Fick and Wislicenus, trans lated hy Prof. Wanklyn in the Phil. Mag. for May or June, 1866. See also an article by Prof. FraLklund ill the September number of the samt journal. 354 PHYSIOLOGICAL DEVELOPMENT. This conception of the nature and mode of action of muscle, while it is suggested by known properties of colloidal matter and conforms to the recent conclusions of organic chemistry and molecular physics, establishes a comprehensible relation between the vital actions of the lower and the higher animals. Tf we contemplate the movements of cilia, of a Rhizopod's pseudo-podia, of a Polype's body, or of the long pendant ten tacles of a Medusa, we shall see great congruity between them and this hypothesis. Bearing in mind that the con tractile substance of developed muscle is affected not by nervous influence only, but, where nervous influence is destroyed, is made to contract by mechanical disturbance and chemical action, we may infer that it does not differ intrin sically from the primordial contractile substance, which, in the lowest animals, changes its bulk under other stimuli than the nervous. We shall see significance in the fact ascer tained by Dr. Ransom, that various agents which excite and arrest nervo-muscular movements in developed animals, excite and arrest the protoplasmic movements in ova. We shall understand how tissues not yet differentiated into muscle and nerve, have this joint irritability and contractility ; how muscle and nerve may arise by the segregation of their mingled colloids, the one of which, not appreciably altering its bulk during isomeric change, readily propagates molecular disturbance, while the other, contracting when isomerically changed, less readily passes on the molecular disturbance ; and how by this differentiation and integration of the con ducting and the contracting colloids, the one ramifying through the other, it becomes possible for a whole mass to contract suddenly, instead of contracting gradually, as it does when undifferentiated. The question remaining to be asked is — What causes the specialization of contractile substance ? — What causes the growth of colloid masses which monopolize this contractility, and leave kindred colloids to monopolize other properties ? Has natural selection gradually localized and increased THE INNER TISSUES OF 4NIMATA 355 the primordial muscular substance ? or has the frequent recur rence of irritations and consequent contractions at particular parts done it ? We have, I think, reason to conclude that direct equilibration rather than indirect equilibration has been chiefly operative. The reasoning that was used in tlie case of nerve applies equally in the case of muscle. A portion of undifferentiated tissue containing a predominance of the colloid that contracts in changing, will, during each change, tend to form new molecules of its own type from the other colloids diffused through it : the tendency of these entangled colloids to fall into unity with those around them, will be aided by every shock of isomeric transformation. Hence, repeated contractions will further the growth of the contracting mass, and advance its differentiation and. integration. If, too, we remember that the muscular colloid is made to contract by mechanical disturbance, and that among me chanical disturbances one which will most readily affect it simultaneously throughout its mass is caused by stretching, we shall be considerably helped towards understanding how the contractile tissues are developed. If extension of a mus cular colloid previously at rest, produces in it that molecular disturbance that leads to isomeric change and decrease of bulk, then there is no difficulty in explaining the movements of cilia. The formation of a contractile layer in the vascular system becomes comprehensible : each dilatation of a blood vessel caused by a gush of blood, will be followed by a con striction ; the heart will pulsate violently in proportion as it is violently distended ; arteries will develop in power as the stress upon them becomes greater. And we shall simi larly have an explanation of the increased muscularity of the alimentary canal that is brought about by increased distension of it. That the production of contractile tissue in certain localities, is due to the more frequent excitement in those localities of the contractility possessed by undift'erentiated tissue in general, is a view harmonizing with facts which the difFe- 356 PHYSIOLOGICAL DEVELOPMENT. rentiated contractile tissues exhibit. These are the rela tions between muscular exercise, muscular power, and mus cular structure ; and it is the more needful for us here to notice them because of certain anomalies they present, which, at first sight, seem inconsistent with the belief that the functionally- determined modifications of muscle are in heritable. Muscles disagree greatly in their tints — all gradations between white and deep red being observable. Contrasts are visible between the muscles of different animals, be tween the muscles of the same animal at different ages, and between different muscles of the same animal at the same age. We will glance at the facts under these heads : noting under each of them the connexion which here chiefly con cerns us — that between the activity of muscle and its depth of colour. The cold-blooded Vertebmta are, taken as a group, distinguished from the warm-blooded by the whiteness of their flesh ; and they are also distinguished by their comparative inertness. Though a fish or a reptile can exert considerable force for a short time, it is not capable of prolonged exertion. Birds and mammals show greater en durance along with darker-coloured muscles. If among birds themselves or mammals themselves we make comparisons, we meet with kindred contrasts — especially between wild and domestic creatures of allied kinds. Barn-door fowls are lighter-fleshed than most untamed gallinaceous birds ; and among these last the pheasant, moving about but little, is lighter-fleshed than the partridge and the grouse which are more nomadic. The muscles of the sheep are not on the average so dark as those of the deer ; and it is said that the flesh of the wild-boar is darker than that of the pig Perhaps, however, the contrast between the hare and the rabbit affords, among familiar animals, the best example of the alleged relation : the dark- fleshed hare having no retreat and making wide excursions, while the white-fleshed rabbit, passing a great part of its time in its burrow, rarely wanders THE INNER TISSUES OF ANIMALS. 357 far from home. The parallel contrast between young and old animals has a parallel meaning. Veal is much whiter than beef, and lamb is of lighter colour than mutton. Though at first sight these facts may not seem to furnish confirmatory evidence, since lambs in their play appear to expend more muscular force than their sedate dams ; yet the meaning of the contrast is really as alleged. For in consequence of the law that the strains which animals have to overcome, increase as the cubes of the dimensions, while their powers of overcoming them increase only as the squares (§ 46), the movements of an adult animal cost very much more in muscular effort than do those of a young animal : the result being that the sheep and the cow exercise their muscles more vigorously in their quiet movements, than the lamb and the calf in their lively movements. It may be added as significant, that the domestic animal in which no very marked darkening of the flesh takes place along with increasing age, namely the pig, is one which, ordinarily kept in a sty, leads so quiescent a life that the assigned cause of darkening does not come into action. But perhaps the most conclusive evidences are the contrasts that exist between the active and inactive muscles of the same animal. Between the leg-muscles of fowls and their pectoral muscles, the difference of colour is familiar ; and we know that fowls exercise their leg-muscles much more than the muscles which move their wings. Similarly in the turkey, in the guinea fowl, in the pheasant. And then, adding much to the force of this evidence, we see that in partridges and grouse, which belong to the same order as our domestic fowls, but use their wings as habitually as their legs, little or no difference is visible between the colours of these two groups of muscles. Special contrasts like these do not, however, exhaust the proofs ; for there is a still more significent general contrast. The muscle of the heart, which is the most active of all muscles, is the darkest of all muscles. The connection of phenomena thus shown, in so many ways, 358 PHYSIOLOGICAL DEVELOPMENT. implies that the bulk of a muscle is by no means the 8v>le measure of the quantity of force it can evolve. It would seem that, other things equal, the depth of colour varies with the constancy of action ; while, other things equal, the bulk varies with the amount of force that has to be put forth upon oc casion. These of course are approximate relations. More correctly we may say that the actions of pale muscles are either relatively feeble though frequent (as in the massive flanks of a fish), or relatively infrequent though strong (as in the pectoral muscles of a common fowl) ; while the actions of dark muscles are both frequent and strong. Some such dif ferentiation may be anticipated by inference from the respec tive physiological requirements. A muscle which has upon occasion to evolve considerable force, but which has thereafter a long period of rest during which repair may restore it to efficiency, requires neither a large reserve of the contrac tile substance that is in some way deteriorated by action, nor highly-developed appliances for bringing it nutri tive materials and removing effete products. Where, con trariwise, an exerted muscle that has undergone much molecular change in evolving mechanical force, has soon again to evolve much mechanical force, and so on continually ; it is clear that either the quantity of contractile substance present must be great, or the apparatus for nutrition and depuration must be very efficient, or both. Hence we may look for marked unlikenesses of minute structure between muscles that are markedly contrasted in activity. And we may suspect that these conspicuous contrasts of colour between active and inactive muscles, are due to these implied diffe rences of minute structure — partly differences between the numbers of blood-vessels and partly differences between the quantities of sarcous matter. Here, then, we have a key to the apparent anomaly above hinted at — the maintenance of bulk by certain muscles which have been rendered comparatively inactive by changed habits of life. That the pectoral muscles of those domestic birds THE INNER TISSUES OF ANIMALS. 359 which fly but little, have not dwindled to any great extent, has been thought a fact at variance with the conclusion that functionally-produced adaptations are inheritable. It has been argued that if parts which are exercised increase, not only in the individual but in the race, while parts which become less active decrease ; then a notable difference of size should exist between the muscles used for flight in birds that fly much, and those in birds of an allied kind that fly little. But, as we here see, this is not the true implication. The change in such cases must be chiefly in vascularity and abun dance of contractile substance ; and cannot be, to any great extent, in bulk. For a bird to fly at all, its pectoral muscles, bones of attachment, and all accompanying appliances, must be kept up to a certain level of power. If the parts dwindle much, the creature will be unable to lift itself from the ground. Bearing in mind that the force which a bird ex pends to sustain itself in the air during each successive instant of a short flight, is, other things equal, the same as it ex pends in each successive instant of a long flight, we shall see that the muscles employed in the two cases must have some thing like equal intensities of contractile power ; and that the tructural differences between them must have relation mainly to the lengths of time during which they can continue to re peat contractions of like intensity. That is to sav, while the power of flight is retained at all, the muscles and bones can not greatly dwindle; but the dwindling, in birds whose flights are short or infrequent or both, will be in the reserve stock of the substance that is incapacitated by action, or in the appliances that keep the apparatus in repair, or in both. Only where, as in the strut hious birds, the habit of flight is lost, can we expect atrophy of all the parts concerned in flight ; and here we find it. Are such differentiations among the muscles functionally produced ? or are they produced by the natural selection of variations distinguished as spontaneous ? We have, I think, good grounds for concluding that they are functionally pro- 860 PHYSIOLOGICAL DEVELOPMENT. duced. "We know that in individual men and animals, the power of sustained action in muscles is rapidly adaptable to the amount of sustained action required. We know that being "out of condition," is usually less shown by the inability to put out a violent effort than by the inability to continue making violent efforts ; and we know that the result of train ing for prize-fights and races, is more shown in the prolonga tion of energy than in the intensification of energy. At the same time, experience has taught us that the structural change which accompanies this functional change, is not so much a change in the bulk of the muscles as a change in their inter nal state : instead of being soft and flabby they become hard. We have inductive proof, then, that exercise of a muscle causes some interstitial gro\vth along with the power of more sus tained action ; and there can be no doubt that the one is a condition to the other. What is this interstitial growth ? There is reason to suspect that it is in part an increased deposit of the sarcous substance and in part a development of blood-vessels. Microscopic observation tends to confirm the conclusions before drawn, that repetition of contractions fur thers the formation of the matter which contracts, and that greater draughts of blood determine greater vascularity. And if the contrasts of molecular structure and the contrasts of vascularity, directly caused in muscles by contrasts in their activities, are to any degree inheritable ; there results an explanation of those constitutional differences in the colours and textures of muscles, which accompany constitutional differences in their degrees of activity. It may be added that if we are warranted in so ascribing the differentiations of muscles from one another to direct equilibration, then we have the more reason for thinking that the differentiation of muscles in general from other structures is also due to direct equilibration. That unlike- nesses between parts of the contractile tissues having unlike functions, are caused by the unlikenesses of their functions, renders it the more probable that the unlikenesses between THE INNER TISSUES OF ANIMALS. 361 contractile tissue and other tissues, have been caused by ana logous unlikenesses. § 304. These interpretations, which have already occupied too large a space, must here be closed. Of course out of phenomena so multitudinous and varied, it has been imprac ticable to deal \\ith any but the most important ; and it has been practicable to deal with these only in a general way. Much, however, as remains to be explained, I think the possi bility of tracing, in so many cases, the actions to which these internal differentiations may rationally be ascribed, makes it likely that the remaining internal differentiations are due to kindred actions. We find evidence that in more cases than seemed probable, these actions produce their effects directly on the individual ; and that the unlikenesses are produced by accumulation of such effects from generation to generation. While for the remaining unlikenesses, we have, as an adequate cause, the indirect effects wrought by the sur vival, generation after generation, of the individuals in which favourable variations have occurred — variations such as those of which human anatomy furnishes endless instances. Thus accounting for so much, we may not unreasonably presume that these co-operative processes of direct and indirect equili bration will account for what remains. Though not strictly included under the title of the chap ter, there is a subject on which a few words may here be added, because of the elucidations yielded to it by some parts of the chapter. I refer to the repair and growth of the differentiated tissues. When treating inductively of that resto ration which takes place in \vorn organs, it was admitted that little in the way of deductive interpretation is apparent — nothing beyond the harmony between the facts and the general principle of segregation (§ 64). And it was further admitted that it is not obvious why, within certain limits, an organ grows in proportion as it is exercised. Certain of the foregoing considerations, however, help us towards a partial 362 PHYSIOLOGICAL DEVELOPMENT. rationale of these phenomena. When treating of the de velopment of respiratory surfaces, external or internal, at places where the greatest contrast exists between the oxy genated plasma outside the vessels and the carbonized blood inside them, reference was made to the truth that the ex change of liquids must, other things equal, be rapid in pro portion as the contrast between them is great. Now this truth holds generally. In every tissue the rate of osmotic exchange must vary as this contrast varies ; and where the contrast is produced by composition or decomposition going forward in the tissue, the amount of exchange must be pro portionate to the amount of composition or decomposition. If the blood is circulating through an inactive organ, there is nothing to disturb, in any gre it degree, the proximate equilibrium between the plasma within the blood-vessels and the plasma without them. But if the tissue is functionally excited — if it is made to yield up and expend part of the force latent in its molecules or the molecules of the oxy-hydro- carbons permeating it, its contained liquid necessarily becomes charged with molecules of another order — simpler molecules ; and the greater the amount of function the more different is it made from the liquid contained in the blood-vessels. Hence the osmotic exchange mu>t be most rapid where the metamorphosis of substance is most rapid —the materials for consumption and for re-integration of tissue, must be supplied in proportion to the demand. This, however, is not the sole process by which waste and repair are equilibrated. There is the osmotic distension above pointed out as one of the causes of circulation — a force tending ever to thrust most blood to the places where there is the greatest escape for it ; (hat is— the greatest consumption of it. For since in an active tissue, the plasma passing out of its capillaries into its sub stance is continually yielding up its complex molecules, either to be assimilated or to be decomposed ; and since the products of decomposition, whether of the nitrogenous tissue or of its contained hydro-carbons, are simpler than, the THE INNER TISSUES OF ANIMAI5. 363 substances from which they arise, and therefore have greater molecular mobility ; it follows that the liquid contained in an active tissue has a greater average molecular mobility than the liquids elsewhere; and therefore makes its way through the channels of excretion faster than elsewhere : the t\vo chief products, carbonic acid and water, escaping with especial facility. Hence the place becomes a place of least resistance, through \vhich the distended walls of the elastic vascular system tend continually to force out an extra quantity of plasma. The argument carried a step further, yields us an idea of the way in which not only repair but also growth of the exercised tissue may be caused — at least, where this tissue is one which evolves force. Assuming it to be established that the force generated by muscle does not result from the consumption of its nitro genous substance, but from the consumption of its contained hydro-carbons and oxy-hydro-carbons ; and inferring that a large amount of muscular action may be performed without a corresponding loss of nitrogenous substance ; we get a clue to the process of increase in a specially-exercised muscle. For if osmotic exchange and osmotic distension conspire to produce a more rapid passage of plasma out of the capillaries into this active tissue than into inactive tissues ; and if, of the substances in this larger supply of plasma, only the non-nitrogenous are consumed ; then there must be an accumulation of the nitrogenous substances. If the waste of the albuminous components of the tissue has not kept pace with the consumption of its carbonaceous con tents; then there will exist in the liquid permeating it more albuminous substance than is needed for its repair — there will be material for its growth. The growth thus resulting, however, will be limited both by the capacity of the channels of supply and by the competing absorption of other active tissues. So long as one muscle, or set of muscles, is specially exercised, while the rest discharge but small amounts of duty — so long, that is, as the quantity of 364 PHYSIOLOGICAL DEVELOPMENT. tissue- forming matters taken from the alimentary canal into the blood, is not largely draughted off elsewhere, this local growth may go on. But if many other sets of muscles are similarly active, the abstraction of tissue- forming matters at various places, will so far diminish their abundance in the blood, as to reduce the supply available at any one place for growth : eventually leaving sufficient for repair only. Though we lack data for thus interpreting specifically the repair and growth of other active tissues, yet we may see, in a general way, that a parallel interpretation holds. For if any tissue that consumes, transforms, excretes, or secretes matters that pass into it from the blood, is not formed of the same constituents as these matters it transforms or excretes ; or if it does not undergo waste proportionate to the quantity of matter it transforms or excretes ; then it seems fairly inferable that along with any unusual quantity of such matters to be transformed or excreted, the plasma passing into it must bring a surplus of the materials for its own repair in id. growth. CHAPTER IX. PHYSIOLOGICAL INTEGRATION IN ANIMALS. § 305. Physiological differentiation and physiological inte gration, are correlatives that vary together. We have but to recollect the familiar parallel between the division of labour in a society and the physiological division of la bour, to see that as fast as the kinds of work performed by the component parts of an organism become more numerous, and as fast as each part becomes more restricted to its own work, so fast must the parts have their actions combined in such ways that no one can go on without the rest and the rest cannot go on without each one. Here our inquiry must be,, how the relationship of these two processes is established — what causes the inte gration to advance pan passu with the differentiation. Though it is manifest, d priori, that the mutual dependence of functions must be proportionate to the specialization of functions ; yet it remains to find the mode in which the in creasing co-ordination is determined. Already, among the Inductions of Biology, this relation between differentiation and integration has been specified and illustrated (§ 59). Before dealing with it deductively, a few further examples, grouped so as to exhibit its several aspects, will be advantageous. § 30 G. If the lowly-organized Planar ia has its body broken uu and its gullet detached, this will, for a while, 366 PHYSIOLOGICAL DEVELOPMENT. continue to perform its function when called upon, just as though it were in its place : a fragment of the creature's own body placed in the gullet, will be propelled through it, or swallowed by it. But, as the seeming strangeness of this fact implies, we find no such independent actions of analogous parts in the higher animals. A piece cut out of the disc of a Medusa, continues with great persistence repeating those rhythmical contractions which we see in the disc as a whole; and thus proves to us that the contractile function in each portion of the disc, is in great measure independent But it is not so with the locomotive organs of more differen tiated types. When separated from the rest, these lose their powers of movement. The only member of a vertebrate animal winch continues to act after detachment, is the heart ; and the heart has a motor apparatus complete within itself. Where there is this small dependence of each part upon the whole, there is but small dependence of the whole upon each part. The longer time which it takes for the arrest of a function to produce death in a less differentiated animal than in a more differentiated animal, may be illus trated by the case of respiration. Suffocation in a man speedily causes resistance to the passage of the blood through the capillaries, followed by congestion and stoppage of the heart : great disturbance throughout the system results in a few seconds ; and in a minute or two all the functions cease. But in a frog, with its undeveloped respiratory organ, and a skin through which a considerable aeration of the blood is carried on, breathing may be suspended for a long time without injury. Doubtless this difference is proximately due to the greater functional activity in the one case than in the other, and the mort pressing need for discharging the pro duced carbonic acid ; but the greater functional activity being itself made possible by the higher specialization of functions, this remains the primary cause of the greater dependence of the other functions on respiration, where the respiratory apparatus has become highly specialized. Here, PHYSIOLOGICAL INTEGRATION IN ANIMALS. 367 indeed, we see the relation under another aspect. This more rapid rhythm of the functions which increased heterogeneity of structure makes possible, is itself a means of integrating the functions. Watch, when it is running down, a compli cated machine of which the parts are not accurately adjusted, or are so worn as to be somewhat loose. There will be observed certain irregularities of movement just before it comes to rest — certain of the parts which stop first, are again made to move a little by the continued movement of the rest, and then become themselves, in turn, the causes of renewed motion in other parts which have ceased to move. That is to say, while the connected rhythmical changes of the machine are quick, their actions and re actions on one another are regular — all the motions are well integrated ; but as the velocity diminishes, irregularities arise — the motions become somewhat disintegrated. Similarly with organic functions : increase of their rapidity involves increase of a joint momentum which controls each and co ordinates all. Thus, if we compare a Snake with a Mammal, we see that its functions are not tied together so closely. The Mammal, and especially the superior Mammal, requires food with considerable regularity ; keeps up a respiration that varies within but moderate limits ; and has periods of activity and rest that alternate evenly and frequently. But the Snake, taking food at long intervals, may have these intervals greatly extended without fatal results ; its dormant and its active states recur less uniformly ; and its rate of respiration varies within much wider limits — now being scarcely perceptible, and now, as you may prove by exciting it, becoming conspicuous. So that here, where the rhythms are very slow, they are individually less regular, and am united into a less regular compound rhythm — are less in tegrated. Perhaps the clearest general idea of the co-ordination of functions that accompanies their specialization, is obtained by observing the slowness with which a little-differentiated animal 50 368 PHYSIOLOGICAL DEVELOPMENT. responds to a stimulus applied to one of its parts, and the rapidity with which such a local stimulus is responded to by a more-differentiated animal. A Polype and a Polyzoon, two creatures somewhat similar in their outward appearances but very unlike in their internal structures, will serve for the comparison. A tentacle of a Polype, when touched, slowly contracts ; and if the touch has been rude, the contraction presently extends to the other tentacles and eventually to the entire body : the stimulus to movement is gradually diffused throughout the organism. But if you touch a tentacle of a Polyzoon, or slightly disturb the water near it, the whole cluster of tentacles is instantly withdrawn, along with the protruded part of the creature's body, into its sheath. Whence arises this contrast ? The one creature has no specialized contractile organs, or fibres for conveying impressions. The other has definite muscles and nerves. The p:irts of the little-differentiated Polype have their functions so feebly co ordinated, that one may be strongly affected for a long time before any effect is felt by another at a distance from it ; but in the more-differentiated Polyzoon, various remote parts instantly have changes propagated to them from the affected part, and by their united actions thus set up, the whole organism adjusts itself so as to avoid the danger. These few added illustrations will make the nature of this general relation sufficiently clear. Let us now pass to the interpretation of it. § 307. If a Hydra is cut in two, the nutritive liquids diffused through its substance cannot escape rapidly, since there are no open channels for them ; and hence the condi tion of the parts at a distance from the cut is but little affected. But where, as in the more-differentiated animals, the nutritive liquid is contained in vessels that have con tinuous communications, cutting the body in two, or cutting off any considerable portion of it, is followed by escape of the liquid from these vessels to a large extent ; and thia PHYSIOLOGICAL INTEGRATION IN ANIMALS. 369 ntfects the nutrition and efficiency of organs remote from the place of injury. Then where, as in further-developed creatures, there exists an apparatus for propelling the blood through these ramifying channels, injury of a single one will cause a loss of blood that quickly prostrates the entire organism. Hence the rise of a completely-differentiated vas cular system, is the rise of a system which integrates all members of the body, by making each dependent on the in tegrity of the vascular system, and therefore on the integrity of each member through which it ramifies. In another mode, too, the establishment of a distributing apparatus produces a physiological union that is great in proportion as this distributing apparatus is efficient. As fast as it assumes a function unlike the rest, each part of an animal modifies the blood in a way more or less unlike the rest, both by the materials it abstracts and by the products it adds ; and hence the more differentiated the vascular system becomes, the more does it integrate all parts by making each of them feel the qualitative modification of the blood which every other has produced. This is simply and conspicuously exemplified by the lungs. In the absence of a vascular system, or in the absence of one that is well marked off from the imbedding tissues, the nutritive plasma or the crude blood, gets what small aeration it can, only by coming near the creature's outer surface, or those inner surfaces that are bathed by water ; and it is probably more by osmotic ex change than in any other way, that the oxygenated plasma slowly permeates the tissues. But where there have been formed definite channels branching throughout the body ; and particularly where there exist specialized organs for pumping the blood through these channels ; it manifestly becomes possible for the aeration to be carried on in one part peculiarly modified to further it, while all other parts have the aerated blood brought to them. And how greatly the differentiation of the vascular system thus becomes a means of integrating the various organs, is shown by the fata] 370 PHYSIOLOGICAL DEVELOPMENT. result that follows when the current of aerated blood is interrupted. Here, indeed, it becomes obvious both that certain physio logical differentiations make possible certain physiological integrations ; and that, conversely, these integrations make possible other differentiations. Besides the waste products that escape through the lungs, there are waste products that escape through the skin, the kidneys, the liver. The blood has separated from it in each of these structures, the par ticular product which this structure has become adapted to separate; leaving the other products to be separated by the other adapted structures. How have these special adaptations been made possible ? By union of the organs as recipients of one circulating mass of blood. While there is no efficient apparatus for transfer of materials through the body, the waste products of each part have to make their escape locally; and the local channels of escape must be competent to take off indifferently all the waste products. But it becomes prac ticable and advantageous for the differently- localized ex creting structures, to become fitted to separate different waste products, as soon as the common circulation through them grows so efficient that the product left unexcreted by one is quickly carried to another better fitted to excrete it. So that the integration of them through a common vasculnr system, is the condition under which only they can become differen tiated. How the specialization of each is rendered possible only by its connexion with others that have become similarly specialized, we indirectly see in such a fact as that in chronic jaundice secondary disease of the kidneys is apt to arise in consequence of the biliverdine accumulated in the system being partly excreted through them : the implication being that a structure peculiarly fitted to excrete urea can exist only when it is functionally united with another structure peculiarly fitted to excrete biliverdine. Perhaps the clearest idea of the way in which differentiation leads to integration, and how, again, increased integration makes PHYSIOLOGICAL INTEGRATION IN ANIMALS. 371 possible still further differentiation, will be obtained by con* templating the analogous dependence in the social organism. While it has no roads, a country cannot have its industries much specialized : each locality must produce, as best it can, the various commodities it consumes, so long as it has no facilities for barter with other localities. But the localities being unlike in their natural fitnesses for the various indus tries, there tends ever to arise some exchange of the commo dities they can respectively produce with least labour. This exchange leads to the formation of channels of communica tion. The currents of commodities once set up, make their foot-paths and horse-tracks more permeable ; and as fast as the resistance to exchange becomes less, the currents of commodities become greater. Each locality takes more of the products of adjacent ones, and each locality devotes itself more to the particular industry for which it is naturally best fitted : the functional integration makes possible a further functional differentiation. This further functional differen tiation reacts. The greater demand for the special product of each locality, excites improvements in production — leads to the use of methods which both cheapen and perfect the com modity. Hence results a still more active exchange ; a still clearer opening of the channels of communication ; a still closer mutual dependence. Yet another influence comes into play. As fast as the intercourse, at first only between neigh bouring localities, makes for itself better roads — as fast as rivers are bridged and marshes made easily passable, the resistance to distribution becomes so far diminished, that the things grown or made in each district can be profitably carried to a greater distance ; and as the economical integration is thus extended over a wider area, the economical differentia tion is again increased ; since each district, having a larger market for its commodity, is led to devote itself more exclu sively to producing this commodity. These actions and re actions continue until the various localities, becoming greatly developed and highly specialized in their industries, are at 372 PHYSIOLOGICAL DEVELOPMENT. the same time functionally integrated by a network of roads, and finally railways, along which rapidly circulate the cur rents severally sent out and received by the localities. And it will be manifest that in individual organisms a like corre lative progress must have been caused in an analogous way. fc 308. Another and higher form of physiological integra tion in animals, is that which the nervous system effects. Each part as it becomes specialized, begins to act upon the rest not only indirectly through the matters it takes from and adds to the blood, but also directly through the molecular disturbances it sets up and diffuses. Whether nerves them selves are differentiated by the molecular disturbances thus propagated in certain directions, or whether they are other wise differentiated, it must equally happen that as fast as they become channels along which molecular disturbances travel, the parts they connect become physiologically in tegrated, in so far that a change in one initiates a change in the other. We may dimly perceive that if portions of what was originally a uniform mass having a common function, undertake sub-divisions of the function, the molecular changes going on in them will be in some way complemen tary to one another : that peculiar form of molecular motion which the one has lost in becoming specialized, the other has gained in becoming specialized. And if the molecular motion that was common to the two portions while they were undiffer- entiated, becomes divided into two complementary kinds of molecular motion ; then between these portions there will be a contrast of molecular motions such that whatever is plus in the one will be minus in the other ; and hence there will be a special tendency towards a restoration of the molecular equili brium between the two : the molecular motion continually propagated away from either will have its line of least resist ance in the direction of the other. If, as argued in the last chapter, repeated restorations of molecular equili brium, always following the line of least resistance, tend ever PHYSIOLOGICAL INTEGRATION TN ANIMALS. 373 to make it a line of diminished resistance ; then, in propor tion as any parts become more physiologically integrated by the establishment of this channel for the easy transmission of molecular motion between them, they may become more physiologically differentiated. The contrast between their molecular motions leads to the line of discharge ; the line of discharge, once formed, permits a greater contrast of their molecular motions to arise ; thereupon the quantities of molecular motion transferred to restore equilibrium, being increased, the channel of transfer is made more permeable ; and its further permeability, so caused, renders possible a still more marked unlikeness of action between the parts. Thus the differentiation and the integration progress hand in hand as before. How the same principle holds through out the higher stages of nervous development, can be seen only still more vaguely. Nevertheless, it is comprehensible that as functions become further divided, there will arise the need for sub-connexions along which there may take place secondary equilibrations subordinate to the main ones. It is manifest, too, that whereas the differentiation of functions proceeds, not necessarily by division into two, but often by division into several, and usually in such ways as not to leave any two functions that are just complementary to one another, the restorations of equilibrium cannot be so simple as above supposed. And especially when we bear in mind that many differentiated functions, as those of the senses, cannot be held complementary to any other functions in particular ; it becomes manifest that the equilibrations that have to be made in an organism of much heterogeneity, are extremely complex, and do not take place between each organ and some other, but between each organ and all the others. The pecu liarity ot the molecular motion propagated from each organ, has to be neutralized by some counter-peculiarity in the average of the molecular motions with which it is brought into relation. All the variously-modified molecular motions from the various parts, must have their pluses and minuses 374 PHYSIOLOGICAL DEVELOPMENT. mutually cancelled : if not locally, then at some centre to which each unbalanced motion travels until it meets with some opposite unbalanced motion to destroy it. Still, involved as these actions must become, it is possible to see how the general principle illustrated by the simple case above sup posed, will continue to hold. For always the molecular motion proceeding from any one differentiated part, will travel most readily towards that place where & molecular motion most complementary to it in kind exists — no matter whether this complementary molecular motion be that proceeding from any one other organ, or the resultant of the molecular motions proceeding from many other organs. So that the tendency will be for each channel of communication or nerve, to unite itself with some centre or ganglion, where it comes into relation with other nerves. And if there be any parts of its peculiar molecular motion uncancelled by the mole cular motions it meets at this centre ; or if, as will pro bably happen, the average molecular motion which it there unites to produce, differs from the average molecular motion elsewhere ; then, as before, there will arise a discharge along another channel or nerve to another centre or ganglion, where the residuary difference may be cancelled by the differences it meets ; or from whence it may be still further propagated till it is so cancelled. Thus there will be a tendency to a general nervous integration keeping pace with the differen tiation. Of course this must be taken as nothing more than the indication of initial tendencies — not as an hypothesis suffi cient to account for all the facts. It leaves out of sight, the origin and functions of ganglia, considered as something more than nerve-junctions. Were there only these lines of easy transmission of molecular disturbance, a change set up in one organ could never do more than produce its equivalent of change in some other or others ; and there could be none of that large amount of motion initiated by a small sensation, */hich we habitually see. The facts show, unmistakably, that PH1SIOJ.OGICAL INTEGRATION IN ANIMALS. 373 the slight disturbance communicated to a ganglion, causes an overthrow of that highly-unstable nervous matter contained in it, and a discharge from it of the greatly- increased quantity of molecular motion so generated. This, however, is beyond our immediate topic. All we have here to note is the inter dependence and unification of functions that naturally follow the differentiation of them. § 309. Something might be added concerning the further class of integrations by which organisms are con stituted mechanically-coherent wholes. Carrying furthei certain of the arguments contained in the last chapter, it might be not unreasonably inferred that the binding together of parts by bones, muscles, and ligaments, is a secondary result of those same actions by which bones, muscles, and ligaments are specialized. But adequate treatment of this division of the subject is at present scarcely possible. What little of fact and inference has been above set down, will, however, serve to make comprehensible the general truths respecting which, in their main outlines, there can be no question. Beginning with the feebly-differentiated sponge, of which the integration is also so feeble that cutting oft' a piece interferes in no appreciable degree with the activity and growth of the rest, it is undeniable that the advance is through stages in which the multiplication of unlike parts having unlike actions, is accompanied by an increasing inter dependence of the parts and their actions ; until we come to structures like our own, in which a slight change initiated in one part will instantly and powerfully affect all other parts — will convulse an immense number of muscles, send a wave of contraction through all the blood-vessels, awaken a crowd of ideas with an accompanying gush of emotions, affect the action of the lungs, of the stomach, and of all the secreting organs And while it is a manifest necessity that along with this subdivision of functions which the higher organisms show us, there must be this close co-ordination of them, the fore- 376 PHYSIOLOGICAL DEVELOPMENT. going paragraphs suggest how this necessary correlation is brought about. For a great part of the physiological union that accompanies the physiological specialization, there appears to ba a sufficient cause in the process of direct equili bration ; and indirect equilibration may be fairly presumed a sufficient ctMise for that which, remains. CHAPTER X. SUMMARY OF PHYSIOLOGICAL DEVELOPMENT. § 310. Intercourse between each part and the particular conditions to which it is exposed, either habitually in the individual or occasionally in the race, thus appears to be the origin of physiological development ; as we found it to be the origin of morphological development. The unlikenesses of form that arise among members of an aggregate that were originally alike, we traced to unlikenesses in the incident forces. And in the foregoing chapters we have traced to unlikenesses in the incident forces, those unlikenesses of minute structure and chemical composition that simultaneously arise among the parts. In summing up the special truths illustrative of this general truth, it will be proper here to contemplate more especially their dependence on first principles. Dealing with biological phenomena as phenomena of evolution, we have to interpret not only the increasing morphological heterogeneity of organisms, but also their increasing physiological hetero geneity, in terms of the re-distribution of matter and motion. While we make our rapid re-survey of the facts, let us then more particularly observe how they are subordinate to the universal course of this le-distribution. § 311. The instability of the homogeneous, or, strictly speaking, the inevitable lapse of the more homogeneous into the less homogeneous, which we before saw endlessly exem- 378 PHYSIOLOGICAL DEVELOPMENT. plified by the morphological differentiations of the parts of organisms, we have here seen afresh exemplified in ways also countless, by the physiological differentiations of their parts. And in the one case as in the other, this change from uni formity into multiformity in organic aggregates, is caused, as it is in all inorganic aggregates, by the necessary exposure of their component parts to actions unlike in kind or quan tity or both. General proof of this is furnished by the order in which the differences appear. If parts are rendered physiologically heterogeneous by the heterogeneity of the incident forces ; then the earliest contrasts should be between r»arts that are the most strongly contrasted in their relations to incident forces ; the next earliest contrasts should occur where there are the next strongest contrasts in these relations ; and so on. It turns out that they do this. Everywhere the differentiation of outside from inside comes first. In the simplest plants the unlikeness of the cell-wall to the cell- contents is the conspicuous trait of structure. The contrasts seen in the simplest animals are of the same kind : the film that covers a Rhizopod and the more indurated coat of an Infusorium, are more unlike the contained sarcode than the other parts of this are from one another; and the tendency during the life of the animal is for the unlikeness to become greater. What is true of Protophyta and Protozoa, is true of the germs of all organ isms up to the highest : the differentiation of outer from inner is the first step. When the endochrome of an Alga-cell has broken up into the clusters of granules which are eventually to become spores, each of these quickly acquires a mem branous coating ; constituting an unlikeness between surface and centre. Similarly with the ovule of every higher plant : the mass of cells forming it, early exhibits an outside layer of cells distinguished from the cells within. With animal germs it is the same. Be it in a ciliated gemmule, be it in the pseud- ova of Aphides and of the Cecidomyia, or be it in true ova, the primary differentiation conforms to the relations SUMMARY OF PHYSIOLOGICAL DEVELOPMENT. of exterior and interior. If we turn to adult or ganisms, vegetal or animal, we see that whether they do or do not display other contrasts of parts, they always display this contrast. Though otherwise almost homogeneous, such Fungi as the Puff-ball, or, among Alyce, all which have a thallus of any thickness, present marked differences between those of their cells which are in immediate contact with the environment and those which are not. Such differences they present in common with every higher plant ; which, here in the shape of bark and there in the shape of cuticle, has an envelope inclosing it even up to its petals : the only parts not so inclosed, being those short-lived terminations of the fructifying organs, from which the dis integrated tissue is being cast off to form the germs of new individuals. In like manner among animals, there is always either a true skin or an outer coat analogous to one. Wher ever aggregates of the first order have united into ag gregates of the second and third orders — wherever they have become the morphological units of such higher aggre gates — the outermost of them have grown unlike those lying within. Even th > Sponge is not without a layer that may by analogy be called dermal. This lapse of the relatively homogeneous into the rela tively heterogeneous, first showing itself, as on the hypothesis of evolution it must do, by the rise of an unlikeness between outside and inside, goes on next to show itself, as we infer that it must do, by the establishment of secondary contrasts among the cuter parts answering to secondary contrasts among the forces falling on them. So long as the whole sur face of a plant remains similarly related to the environment, fts in a Protococcns or a Volwx, it remains uniform ; but when there come to be an attached surface and a free surface, these, being subject to unlike actions, are rendered unlike. This is visible even in a unicellular Alga when it becomes fixed ; it is shown in the distinction between the under und upper parts of ordinary Fungi; and we see it in 380 PHYSIOLOGICAL DEVELOPMKNT. the universal difference between the imbedded ends and the exposed ends of the higher plants. And then among the less marked contrasts of surface answering to the less marked contrasts in the incident forces, come those between the upper and under sides of leaves ; which, as we have seen, vary in decree as the contrasts of forces vary in degree, and disappear where these contrasts disappear. Equally clear proof is furnished by animals, that the original uni formity of surface lapses into multiformity, in proportion as the actions of the environment upon the surface become multiform. In a Worm, burrowing through damp soil that acts equally on all its sides, or in a T&nln, uniformly bathed by the contents of the intestine it inhabits, the parts of the integument do not appreciably differ from one anoiher ; but in creatures not surrounded by the same agencies, as those that crawl and those that have their bodies partially inclosed, there are unlikenesses of integument corresponding to unlike- nesses of the conditions. A Snail's foot has an under surface not uniform with the exposed surface of its body, and this again is not uniform with the protected surface. Among articulate animals there is usually a distinction between the ventral and the dorsal aspects ; and in those of the Articulata which subject their anterior and posterior ends to different environing agencies, as do the Ant-lion and the Hermit-crab, these become superficially differentiated. Ana logous general contrasts occur among the Verfebrata. Fish, though their outsides are uniformly bathed by water, have their backs more exposed to light than their bellies ; and the two are commonly distinct in colour. Where it is not the back and belly that are thus dissimilarly conditioned, but the sides, as in the Pleiironectidce, then it is trre sides that be come contrasted ; and there may be significance in the fact, that those abnormal individuals of this order which revert to the ancestral undistorted type, and swim vertically, have the two sides alike. In such higher vertebrates as Reptiles, we see repeated this differentiation of the upper and under BUT- SUMMARY OF PHYSIOLOGICAL DEVELOPMENT. 881 faces : especially in those of them which, like Snakes, ex pose these surfaces to the most diverse actions. Even in Birds and Mammals which usually, by raising the under surface considerably above the ground, greatly diminish the contrast between its conditions and the conditions to which the upper surface is subject, there still remains some unlike- ness of clothing answering to the remaining unlikeness be tween the conditions. Thus, without by any means saying that all such differentiations are directly caused by differences in the actions of incident forces, which, us before shown (§ 294), they cannot be, it is clear that many of them are so caused. It is clear that parts of the surface exposed to very unlike environing agencies, become very unlike ; and this is all that needs be shown. Complex as are the transformations jf the inner parts of organisms from the relatively homogeneous into the rela tively heterogeneous, we still see among them a conformity to the same general order. In both plants and animals the earlier internal differentiations answer to the stronger con trasts of conditions. Plants, absorbing all their nutriment through their outer surfaces, are internally modi fied mainly by the transfer of materials and by mechanical stress. Such of them as do not raise their fronds above the surface, have their inner tissues subject to no marked con trasts save those caused by currents of sap ; and the lines of lengthened and otherwise changed cells that are formed where these currents run, and are most conspicuous where these currents must obviously be the strongest, are the only decided differentiations of the interior. But where, as in the higher Cryptogams and in Phasnogams, the leaves are upheld, and the supporting stem is transversely bent by the wind, the inner tissues, subject to different amounts of mechanical strain, differentiate accordingly : the deposit of dense substance commences in that region where the sap- containing cells and canals suffer the greatest intermittent compressions. Animals, or at least such of them 382 PHYSIOLOGICAL DEVELOPMENT. as take food into their interiors, are subject to forces of another class tending to destroy their original homogeneity. Food is a foreign substance which acts on the interior as an environing object which touches it acts on the exterior — is literally a portion of the environment, which, when swal lowed, becomes a cause of internal differentiations as the rest of the environment continues a cause of external differentia tions. How essentially parallel are the two sets of actions and reactions, we have seen implied by the primordial identity of the endoderrn and ectoderm in simple animals, and of the skin and mucous membrane in complex animals (§§ 288, 289). Here we have further to observe that as food is the original source of internal differentiations, these may be expected to show themselves first where the influence of the food is greatest ; and to appt^r later in proportion as the parts are more removed from the Influence of the food. They do this. In animals of low type, the coats of the alimentary cavity or canal, are more differentiated than the tissue that lies between the alimentary canal and the wall of the body. This tissue in the higher Coelenterata, is a feebly-organized parenchyma traversed by lacunae — either simple channels, or canals lined with simple ciliated cells ; and in the lower Molltisca the structures bounding the perivisceral cavity and its ramifying sinuses, are similarly imperfect. Further, it is observable that the differentiation of this perivisceral sac and its sinuses into a vascular system, proceeds centrifugally from the region where the absorbed nutriment enters the mass of cir culating liquid, and where this liquid is qualitatively more unlike the tissues than it is at the remoter parts of the body. Physiological development, then, is initiated by that insta bility of the homogeneous which we have seen to be every where a cause of evolution (First Principles, §§ 109 — 115) . That the passage from comparative uniformity of composition and minute structure to comparative multiformity, is set up in organic aggregates, as in all other aggregates, by the neces sary unlikenesses of the actions to which the parts are sub- SUMMARY OF PHYSIOLOGICAL DEVELOPMENT. 383 ject, is shown by the universal rise of the primary differentia tion between the parts that are universally most contrasted in their circumstances, and by the rise of secondary differen tiations obviously related in their order to secondary contrasts of conditions. § 312. How physiological development has all along been aided by the multiplication of effects — how each differen tiation has ever tended to become the parent of new differen tiations, we have had, incidentally, various illustrations. Let, us here re view- the working of this cause. Among plants we see it in the production of progressively- multiplying heterogeneities of tissue by progressive increase of bulk. The integration of fronds into axes and of axes into groups of axes, sets up unlikenesses of action among the in tegrated units, followed by unlikenesses of minute structure. Each gust transversely strains the various parts of the stem in various degrees, and longitudinally strains in various degrees the roots ; and while there is inequality of stress at every place in stem and branch, so, at every place in stem and branch, the outer layers and the successively inner layers are severally extended and compressed to unequal amounts, and have un equal modifications wrought in them. Let the tree add to its periphery another generation of the units composing it, and immediately the mechanical strains on the supporting parts are all changed in different degrees, initiating new differences internally. Externally, too, new differences are initiated. Shaded by the leaf -bearing outer stratum of shoots, the inner structures cease to bear leaves, or to put out shoots that hear leaves ; and instead of that green covering which they originally had, become covered with bark of increasing thickness. Manifestly, then, the larger integration of units that are originally simple and uniform, entails physiological changes of various orders, varying in their degrees at all parts of the aggregate. Each branch which, favourably cir cumstanced, flourishes more than its neighbours, becomes u 381 PHYSIOLOGICAL DEVELOPMENT. cause of physiological differentiations, not only in its neigh bours from which it abstracts sap and presently turns from leaf-bearers into fruit-bearers, but also in the remoter parts. That among animals physiological development is fur thered by the multiplication of effects, we have lately seen proved by the many changes in o^her organs, which the growth or modification of each excreting and secreting organ initiates. By the abstracted as well as by the added materials, it alters the quality of the blood passing through nil members of the body ; or by the liquid it pours into the alimentary canal, it acts on the food, and through it on the blood, and through it on the system as a whole : an addi tional differentiation in one part thus setting up additional differentiations in many other parts; from each of which, a^ain, secondary differentiating forces reverberate through the organism. Or, to take an influence of another order, we have seen how the modified mechanical action of any member not only modifies that member, but becomes, by its reactions, a cause of secondary modifications — how, for example, the burrowing habits of the common Mole, leading to an almost exclusive use of the fore limbs, have entailed a dwindling of the hind limbs, and a concomitant dwindling of the pelvis, which, becoming too small for the passage of the young, has initiated still more anomalous modifications. So that throughout physiological development, as in evolution at large, the multiplication of effects has been a factor constantly at work, and working more actively as the development has advanced. The secondary changes wrought by each primary change, have necessarily become more numerous in proportion as organisms have become more complex. And every increased multiplication of effects, further differentiating the organism and, by consequence, further integrating it, has prepared the way for still higher differentiations and integrations similarly caused. § 313. The general truth next to be resumed, is that these 8TTMMAUY OF PHYSIOLOGICAL DEVELOPMENT. 385 processes have for their limit a state of equilibrium — proxi- mately a moving equilibrium and ultimately a complete equili brium. The changes we have contemplated are but the con comitants of a progressing equilibration. In every aggregate which we call living, as well as in all other aggregates, the instability of the homogeneous is but another name for tlie absence of balance between the incident forces and the forces which the aggregate opposes to them ; and the passage into heterogeneity is the passage towards a state of balance. And to say that in every aggregate, organic or other, there goes on a multiplication of effects, is but to say that one part which has a fresh force impressed on it, must go on changing and communicating secondary changes, until the whole of the impressed force has been used up in generating equivalent reactive forces. The principle that whatever new action an organism is subject to, must either overthrow the moving equilibrium of its functions and cause the sudden equilibration called death, or else must progressively alter the organic rhythms until, by the establishment of a new reaction balancing the new action, a new moving equilibrium is produced, applies as much to each member of an organism as to the organism in its totality. Any force falling on any part not adapted to bear it, must either cause local destruction of tissue, or must, without destroying the tissue, continue to change it until it can change it no further ; that is — until the modified reaction of the part has become equal to the modified action What ever the nature of the force, this must happen. If it is a, mechanical force, then the immediate effect is some distortion of the part — a distortion having for its limit that attitude in which the resistance of the structures to further change of position, balances the force tending to produce the further change ; and the ultimate effect, supposing the force to be con tinuous or recurrent, is such a permanent alteration of form, or alteration of structure, or both, as establishes a permanent balance. If the force is physico-chemical, or chemical, the 386 PHYSIOLOGICAL DEVELOPMENT. general result is still the same : the component molecules of the tissue must have their molecular arrangements changed, and the change in their molecular arrangements must go on until their molecular motions are so re-adjusted as to equili brate the molecular motions of the new physico-chemical or chemical agent. In other words, the organic matter com posing the part, if it continues to be organic matter at all, must assume that molecular composition which enables it to bear, or as we say adapts it to, the incident forces. Nor is it less certain that throughout the organism as a whole, equilibration is alike the proximate limit of the changes wrought by each action, as well as the ultimate limit of the changes wrought by any recurrent actions or continuous action. The ordinary movements every instant going on, are movements towards a new state of equilibrium. Raising a limb causes a simultaneous shifting of the centre of gravity, and such altered tensions and pressures throughout the body as re- adjust the disturbed balance. Passage of liquid into or out of a tissue, implies some excess of force in one direction there at work ; and ceases only when the force so diminishes or the counter-forces so increase that the excess disappears. A nervous discharge is reflected and re-reflected from part to part, until it has all been used up in the re-arrangements pro duced — equilibrated by the reactions called out. A.nd what is thus obviously true of every normal change, is equally true of every abnormal change — everv disturbance of the estab lished rhythm of the functions. If such disturbance is a single one, the perturbations set up by it, reverberating throughout the system, leave its moving equilibrium slightly altered. If the disturbance is repeated or persistent, its suc cessive effects accumulate until they have produced a new moving equilibrium adjusted to the new force. Each re-balancing of actions, having for its necessary con comitant a modification of tissues, it is an obvious corollary that organisms subjected to successive changes of conditions, must undergo successive differentiations and re- differentia- SUMMARY OF PHYSIOLOGICAL DEVELOPMENT. 387 fcions. Direct equilibration in organisms, with all its accom panying structural alterations, is as certain as is that uni versal progress towards equilibrium of which it forms parfc. And just as certain is that indirect equilibration in organisms to which the remaining large class of differentiations is due. The development of favourable variations by the killing of individuals in which they do not occur or are least marked, is, as before, a balancing between certain local structures and the forces they are exposed to ; and is no less inevitable than the other. § 314. In all which universal laws, we find ourselves again brought down to the persistence of force, as the deepest knowable cause of those modifications which constitute physiological development ; as it is the deepest knowable cause of all other evolution. Here, as elsewhere, the per petual lapse from less to greater heterogeneity, the perpetual begetting of secondary modifications by each primary modi fication, and the perpetual approach to a temporary balance on the way towards a final balance, are necessary implica tions of the ultimate fact that force cannot disappear, but can only change its form. It is an unquestionable deduction from the persistence of force, that in every individual organism each new incident force must work its equivalent of change ; and that where it is a constant or recurrent force, the limit of the change it works must be an adaptation of structure such as opposes to the new outer force an equal inner force. The only thing open to question is, whether such re -adjustment is inherit able ; and further consideration will, I think, show, that to say it is not inheritable is indirectly to say that force does not persist. If all parts of an organism have their func tions co ordinated into a moving equilibrium, such that every part perpetually influences all other parts, and cannot bo changed without initiating changes in all other parts — if the 'imit of change is the establishment of a complete harmony 388 PHYSIOLOGICAL DEVELOPMENT. among the movements, molecular and other, of all parts; then among other parts that are modified, molecularly or other wise, must be those which cast off the germs of new organisms. The molecules of their produced germs must tend ever to conform the motions of their components, and therefore the arrangements of their components, to the molecular forces of the organism as a whole ; and if this aggregate of molecular forces is modified in its distribution by a local change of structure, the molecules of the germs must be gradually changed in the motions and arrangements of their components, until they are re-adjusted to the aggre gate of molecular forces. For to hold that the moving equi librium of an organism may be altered without altering the movements going on in a particular part of it, is to hold that these movements will not be affected by the altered distribu tion of forces ; and to hold this Ls to deny the persistence of force, PART VI. LAWS OF MULTIPLICATION CHAPTER I. THE FACTORS.* § 315. If organisms have been evolved, their respective powers of multiplication must have been determined by natural causes. Grant that the countless specialities of structure and function in plants and animals, have arisen from the actions and reactions between them and their environments, continued from generation to generation ; and it follows that from these actions and reactions have also arisen those countless degrees of fertility which we see among them. As in all other respects an adaptation of each species to its conditions of existence is directly or indirectly brought about ; so must there be directly or indirectly brought about an adaptation of its reproductive activity to its conditions of existence. We may expect to find, too, that permanent and temporary differences of fertility have the same general interpretation. If the small variations of structure and function that arise within the limits of each species, are due to actions like those * An outline of the doctrine set forth in the following chapters, was originally published in the Westminster Review for April, 1852, under the title of, A Theory of Population deduced from the General Law of Animal Fertility ; and was shortly afterwards republished with a prefatory note, to the effect that it must be accepted as a sketch which 1 hoped at some future time to elaborate. In now revising and completing it, I have omitted a non- essential part of the argument, while I have expanded the remainder by adding to the number of facts put in evidence, by meeting objections which want of space before obliged me to pass over, and by drawing various secondary conclusions. 392 LAWS OF MULTIPLICATION. which, by their long-accumulating effects, have produced the immense contrasts between the various types ; we may con clude that, similarly, the actions to which changes in the rate of multiplication of each species are due, also produce, in great periods of time, the enormous differences between the rates of multiplication of different species. Before inquiring in what ways the rapidities of increase are adjusted to the requirements, both temporary and permanent, it will be needful to look at the factors. Let us set down first those which belong to the environment, and then those which belong to the organism. § 316. Every living aggregate being one of which the inner actions are adjusted to balance outer actions, it follows that the maintenance of its moving equilibrium depends on its exposure to the right amounts of these actions. Its moving equilibrium may be overturned if one of these actions is either too great or too small in amount ; and it may be so overturned either by excess or defect of some inorganic agency in its environment, or by excess or defect of some organic agency. Thus a plant, constitutionally fitted to a certain warmth and humidity, is killed by extremes of temperature, as well as by extremes of drought and moisture. It may dwindle away from want of soil, or die from the presence of too great 01- too small a quantity of some mineral substance which the soil supplies to it. In like manner, every animal can main tain the balance of its functions so long only as the environ ment adds to or deducts from its heat at rates not exceeding definite limits. Water, too, must be accessible in amount sufficient to compensate its loss : if the parched air is rapidly abstracting its liquid which there is no pool or river to restore, its functions cease ; and if it is an aquatic creature, drought may kill it either by drying up its medium or by giving it a medium inadequately aerated. Thus each organ ism, adjusted to a certain average in the actions of ita THE FACTORS. 393 inorganic environment, or rather, we should say, adjusted to certain moderate deviations from this average, is destroyed by extreme deviations. So, too. is it with the environing organic agencies. Among plants, only the para sitic kinds depend for their individual preservation on the presence of certain other organisms (though the presence of certain other organisms is needful to most plants for the preservation of the race by aiding fertilization). Here, for the continuance of individual life, particular organisms must be absent or not very numerous — beasts that browse, cater pillars that devour leaves, aphides that suck juices. Among animals, however, the maintenance of the functional balance is both positively and negatively dependent on the amounts of surrounding organic agents. There must be an accessible sufficiency of the plants or animals serving for food ; and of organisms that are predatory or parasitic or otherwise detri mental, the number must not pass a certain limit. This dependence of the moving equilibrium in every indi vidual organism on an adjustment of its forces to the forces of the environment, and the overthrow of this equilibrium by failure of the adjustment, is comprehensive of all cases. At first sight it does not seem to include what we call natural death ; but only death by violence, or starvation, or cold, or drought. But in reality natural death, no less than every other kind of death, is caused by the failure to meet some outer action by a proportionate inner action. The apparent difference is due to the fact that in old age, when the quantity of force evolved in the organism gradually dimi nishes, the momentum of the functions becomes step by step less, and the variations of the external forces relatively greater; until there finally comes an occasion when some quite moderate deviation from the average to which the feeble moving equilibrium is adjusted, produces in it a fatal perturbation. § 317. The individuals of every species being thus depend- 394 LAWS OF MULTIPLICATION. ent on certain environing actions ; and severally having theii moving equilibria sooner or later overthrown by one or other of these environing actions ; we have next to consider in what ways the environing actions are so met as to prevent extinction of the species. There are two essentially different ways. There may be in each individual a small or great ability to adjust itself to variations of the agencies around it and to a small or great number of such varying agencies — there may be little or much power of preserving the balance of the functions. And there may be much or little power of producing new individuals to replace those whose moving equilibria have been overthrown. A few facts must be set down to enforce these abstract statements. There are both active and passive adaptations by which organisms are enabled to survive adverse influences. Plants show us but few active adaptations : that of the Pitcher-plant and those of the reproductive parts of some flowers (which do not, however, conduce to self-preservation) are exceptional instances. But plants have various passive adaptations ; as thorns, stinging hairs, poisonous and acrid juices, repugnant odours, and the woolliness or toughness that makes their leaves uneatable. Animals exhibit far more numerous adjustments, both passive and active. In some cases they survive desiccation, they hybernate, they acquire thicker clothing, and so are fitted to bear unfavourable inorganic actions ; and they are in many cases fitted passively to meet the adverse actions of other organisms, by bearing spines or armour or shells, by simulating neighbouring objects in colour or form or both, by emitting disagreeable odours, or by having disgusting tastes. In still more numerous ways they actively contend with unfavourable conditions. Against the seasons they guard by storing up food, by secreting themselves in crevices, or by forming burrows and nests. They save them selves from enemies by developed powers of locomotion, taking the shape of swiftness or agility or aptitude for changing their media ; by their strength either alone or aided by wea- THE FACTORS. 395 pons ; lastly by their intelligence, without which, indeed, their other superiorities would avail them little. And then these various active powers serving for defence, become, in other cases, the powers that enable animals to aggress, and to preserve their lives by the success of their aggressions. The second process by which extinction is prevented — the formation of new individuals to replace the individuals destroyed — is carried on, as described in the chapter on " Genesis," by two methods, the sexual and the asexual. Plants multiply by spontaneous fission, by gemmation, by proliferation, and by the evolution of young ones from de tached cells and scales and leaves ; and they also multiply by the casting off of spores and sporangia and seeds. In like manner among animals, there are varied kinds of agamo- genesis, from spontaneous fission up to parthenogenesis, all of them conducing to rapid increase of numbers ; and we have the more familiar process of gamogenesis, also carried on in a great variety of ways. This formation of new individuals to replace the old, is, however, inadequately conceived if we conten, plate only the number born or detached on each occasion. There are four factors, all variable, on which the rate of multiplication depends. The first is the age at which reproduction commences ; the second is the frequency with which broods are produced ; the third is the number contained in each brood ; and the fourth is the length of time during which the bringing forth of broods con tinues. Ihere must be taken into account a further element — the amount of aid given by the parent to each germ in the ahape of stored-up nutriment, continuous feeding, warmth, protection, &c. : on which amount of aid, varying between immensely wide limits, depends the number of the new indi viduals that survive long enough to replace the old, and perform the same reproductive process. Thus, regarding every living organism as having a moving equilibrium dependent on environing forces, but ever liable to be overthrown by irregularities in those forces, and always LAWS OF MULTIPLICATION. so overthrown sooner or later ; we see that each kind of organism can be maintained only by generation of new indi viduals with a certain rapidity, and by helping them more or less fully to establish their moving equilibria. § 318. Such are the factors with which we are here con cerned. I have presented them in abstract shapes, for the purpose of showing how they are expressible in general terms of force — how they stand related to the ultimate laws of re distribution of matter and motion. For the purposes of the argument now to follow, we may, however, conveniently deal with these factors under a more familiar guise. Ignoring their other aspects, we may class the actions which affect each race of organisms as forming two conflicting sets. On the one hand, by what we call natural death, by enemies, by lack of food, by atmospheric changes, &c., the race is constantly being destroyed. On the other hand, partly by the endurance, the strength, the swift ness, and the sagacity of its members, and partly by their fertility, it is constantl}7 being maintained. These conflicting sets of actions may be generalized as — the forces destructive of race and the forces preservative of race. So generalizing them, let us ask what are the necessary implications. CHAPTER II. A PRIORI PRINCIPLE. p 319. The number of a species must at any time be either decreasing or stationary or increasing. If, generation after generation, its members die faster than others are born, the species must dwindle and finally disappear. If its rate of multiplication is equal to its rate of mortality, there can be no numerical change in it. And if the deductions by death are fewer than the additions by birth, the species must be come more abundant. These we may safely set down as necessities. The forces destructive of race must be either greater than the forces preservative of race, or equal to them, or less than them ; and there cannot but result these effects on number. We are here concerned only with races that continue to exist ; and may therefore leave out of consideration those cases in which the destructive forces, remaining permanently in excess of the preservative forces, cause extinction. Prac tically, too, we may exclude the stationary condition of a species ; for the chances are infinity to one against the main tenance of a permanent equality between the births and the deaths. Hence, our inquiry resolves itself into this : — In races that continue to exist, what laws of numerical variation result from these variable conflicting forces, that are respec tively destructive of race and preservative of race ? § 320. Clearly if the forces destructive of race, when once 398 LAWS OF MULTIPLICATION. in excess, had nothing to prevent them from remaining in excess, the race would disappear ; and clearly if the forces preservative of race, when once in excess, had nothing to prevent them from remaining in excess, the race would go on increasing to infinity. In the absence of any compensating actions, the only possible avoidance of these opposite extreme* would be an unstable equilibrium between the conflicting forces, resulting in a perfectly constant number of the species: a state which we know does not exist, and against the existence of which the probabilities are, as already said, infinite. It follows, then, that as in every continuously- existing species, neither of the two conflicting sets of forces remains permanently in excess ; there must be some way of stopping that excess of the one or the other which is ever occurring. How is this done ? Should any one allege, in conformity with the .old method of interpretation, that there is in each cnse a providential interposition to rectify the disturbed balance, he commits himself to the supposition that of the millions of species inhabiting the Earth, each one is yearly regulated in its degree of fertility by a miracle ; since in no two years do the forces which foster, or the forces which check, each species, remain the same ; and therefore, in no two years is there required the same fertility to balance the mortality. Few if any will say that God continually alters the reproductive activity of every parasitic fungus and every Tape- worm or Trichina, so as to prevent its extinction or undue multiplication ; which they must say if they adopt the hypothesis of a supernatural adjustment. And in the absence of this hypothesis there remains only one other. The alternative possibility is, that the balance of the pre servative and destructive forces is self-sustaining — is of the kind distinguished as a stable equilibrium: an equilibrium such that any excess of one of the forces at work, itself generates, by the deviation it produces, certain counter- forces that eventually out-balance it, and initiate an opposite devuv A PRIORI PRINCIPLE. 390 oion. Let us consider how, in the case before us, such a stable equilibrium must be constituted. § 321. When a season favourable to it, or a diminution of creatures detrimental to it, causes any species to become more numerous than usual ; an immediate increase of certain destructive influences takes place. If it is a plant, the supposed greater abundance itself implies occupation of the available places for growth — an occupation which, leaving fewer such places as the multiplication goes on, itself becomes a check on further multiplication — itself causes a greater mortality of seeds that fail to root themselves. And after wards, in addition to this passive resistance to continued increase, there comes an active resistance : the creatures that thrive at the expense of the species — the larvae, the birds, the herbivores — increase too. If it be an animal that has grown more numerous, then, unless by some exceptional coincidence a simultaneous and proportionate addition to the animals or plants serving for food has occurred, there must result a relative scarcity of food. Enemies, too, be they beasts of prey or be they parasites, must quickly begin to multiply. Hence, each kind of organism, previously existing in some thing like its normal number, cannot have its number raised without a rise of the destructive forces, negative and positive, quickly commencing. Both negative and posi tive destructive forces must augment until this increase of the specios is arrested. The competition for places on which to grow, if the species be vegetal, or for food if it be animal, must become more intense as the over-peopling of the habitat progresses ; until there is reached the limit at which the mortality equals the reproduction. And as, at the same time, enemies will multiply with a rapidity which soon brings them abreast of the augmented supply of prey, the positive restraint they exert will help to bring about an earlier arrest of the expansion than pressure of population alone would cause. One more inference may be 58 400 LAWS OF MULTIPLICATION. drawn. Had the species to meet no repressing influence save that negative one of relatively-diminished space or relatively-diminished food-supply, the cause leading to its increase might carry it up to the limit set by this, and there leave it : its enlarged number might be permanent. But the positive repressing influence that has been called into existence, will prevent this. For the increase of enemies, commencing, as it must, after the increase of the species, and advancing in geometrical progression until it is itself checked in like manner, will end in an excess of enemies. Whereupon must result a mortality of the species greater than its multiplication — a decrease which will continue until its habitat is underpeopled, its unduly-numerous enemies decimated by starvation, and the destroying agencies so reduced to a minimum. Whence will follow another in crease. Thus, as before indicated (First Prin. § § 96, 133), there is here, as wherever antagonistic forces are in action, an alter nate predominance of each, causing a rhythmical movement — a rhythmical movement which constitutes a moving equili brium in those cases where the forces are not dissipated with appreciable rapidity, or are re-supplied as fast as they are dissipated. While, therefore, on the one hand, we see that the continued existence of a species necessarily implies some action by which the destructive and preservative forces are self- adjusted ; we see, on the other hand, that such an action is un inevitable consequence of the universal process of equilibration. § 322. Is this the sole equilibration that must exist ? Clearly not. The temporary compensating adjustments of multiplication to mortality in each species, are but intro ductory to the permanent compensating adjustments of mul tiplication to mortality among species in general. The above reasoning would hold just as it now does, were all species equally prolific and all equally short-lived. It yields no A PRIORI PRINCIPLE. 401 answer to the inquiries — why do their fertilities differ so enormously, or why do their mortalities differ so enormously P and how is the general fertility adapted to the general mor tality in each ? The balancing process we have contemplated, can go on only within moderate limits — must fail entirely in the absence of a due proportion between the ordinary birth rate and the ordinary death-rate. If the reproduction oi mice proceeded as slowly as the reproduction of men, mice would be extinct before a new generation could arise : even did their natural lives extend to fifteen or sixteen years, it would still be extremely improbable that any would for so long survive all the dangers they are exposed to. Con versely, did oxen propagate as fast as infusoria, the race would die of starvation in a week. Plence, the minor adjust ment of varying multiplication to varying mortality in each species, implies some major adjustment of average multipli cation to average mortality. What must this adj ustment be ? We have already seen that the forces preservative of race are two — ability in each member of the race to preserve itself, and ability to produce other members — power to main tain individual life, and power to generate the species. These must vary inversely. When, from lowness of organi zation, the ability to contend with external dangers is small, there must be great fertility to compensate for the conse quent mortality ; otherwise the race must die out. When, on the contrary, high endowments give much capacity of self-preservation, a correspondingly-low degree of fertility is requisite. Given the dangers to be met as a constant quan tity; then, as the ability of any species to meet them must be a constant quantity too, and as this is made up of the two factors — power to maintain individual life and power to mul tiply — these cannot do other than vary inversely : one must decrease as the other increases. It needs but to conceive the results of nonconformity to this law, to see that every species must either conform to it or cease to exist. Suppose, first, a species whose individuals 402 LAWS OF MULTIPLICATION. having but small self-preservative powers are rapidly de stroyed, to be at the same time without reproductive powers proportionately great. The defect of fertility, if extreme, will result in the death of one generation before another has grown up. If less extreme, it will entail a scarcity such that in the next generation sexual congress will be too infre quent to maintain even the small number that remains ; and the race will dwindle with increasing rapidity. If still less extreme, the consequent degree of rareness, while not so great as to prevent an adequate number of procreative unions, will be so great as to render special food very abundant and special enemies very few — will thus diminish the destruc tive forces so much that the self preservative forces will be come relatively great : so great, relatively, that when com bined with the small ability to propagate the species, they will suffice to balance the small destructive forces. Suppose, next, a species whose individuals have great powers of self-preservation, while they have powers of multiplication much beyond what is needful. The excess of fertility, if extreme, will cause sudden extinction of the species by starvation. If less extreme, it must produce a permanent increase in the number of the species ; and this, followed by intenser competition for food and augmented number of enemies, will involve such an increase of the dangers to individual life, that the great self-preserving powers of the individuals will not be more than sufficient to cope with them. That is to say, if the fertility is relatively too great, then the ability to maintain individual life inevitably becomes smaller, relatively to the requirements; and the inverse pro portion is thus established. So that when, from comparing the different states of the same species, we go on to compare the states of different species, we see that there is an analogous adjustment — analogous in the sense that great mortality is associated with great multiplication, and small mortality with small multiplication. And we see that the unlikeness of the cases consists merely A PRIORI PRINCIPLE. 403 in this, that what is a temporary relation in the ono is a per manent relation in the other. § 323. For the moment it does not concern us to inquire what is the origin of this permanent relation. That which we have now to note, is simply that in some way or other ihere must be established an inverse proportion between the power to sustain individual life and the power to produce new individuals. Here it is enough for us to recognize this as a necessary truth. Whether or not the permanent rela tion is self-adjusting in long periods of time, as the tempo rary relation is self-adjusting in short periods of time, is a separate question. The purpose of this chapter is fulfilled by showing that such a permanent relation must exist. But having recognized the a priori principle that in races which continuously survive, the forces destructive of race must be equilibrated by the forces preservative of race ; and that supposing these are constant, there must be an inverse proportion between self-preservation and race- preservation ; we may go on to inquire how this relation, necessary in theory, arises in fact. Leaving out the untenable hypothesis of a supernatural pre-adjustment, we have to ask in what way an adjustment comes about as a result of Evolution. Is it due to the survival of varieties in which the proportion of fertility to mortality happens to be the best ? Or is the fertility adapted to the mortality in a more direct way P To these questions let us now address oursel ves. CHAPTER III. OBVERSE A PRIORI PRINCIPLE. § 324. When dealing with its phenomena inductively, we saw that however it may be carried on, Genesis " is a process of negative or positive disintegration ; and is thus essentially opposed to that process of integration, which is one element of individual evolution." (§ 76.) Each new individual, whe ther separated as a germ or in some more-developed form, is a deduction from the mass of a pre-existing individual or of two pre-existing individuals. Whatever nutritive matter is stored up along with the germ, if it be deposited in the shape of an egg, is so much nutritive matter lost to the parent. No drop of blood can be absorbed by the foetus, and no draught of milk sucked by the young when born, without taking from the mother tissue- forming and force-evolving materials to an equivalent amount. And all subsequent supplies given to progeny, if they are nurtured, involve, to a parent or parents, so much waste in exertion that does not bring its return in assimilated food. Conversely, the continued aggregation of materials into one organism, renders impossible the formation of other organ- isms out of those materials. As much assimilated food as ia united into a single whole, is so much assimilated food with held from a plurality of wholes that might else have been produced. Given the absorbed nutriment as a constant quantity, and the longer the building of it up into a con* OBVERSE A PRIORI PRINCIPLE. 405 crete shape goes on, the longer must be postponed any build ing of it up into discrete shapes. And similarly, the larger the proportion of matter consumed in the functional actions of parents, the smaller must be the proportion of matter that can remain to establish and support the functional actions of offspring. Though the necessity of these universal relations is toler ably obvious as thus generally stated, it will be useful to dwell for a brief space on their leading aspects. § 325. That disintegration which constitutes genesis, may be such as to disperse entirely the aggregate which integra tion has previously produced — the parent may dissolve wholly into progeny. This dissolution of each aggregate into two or many aggregates, may occur at very short intervals, in which case the bulk attained can be but extremely small ; or it may occur at longer intervals, in which case a larger bulk may be attained. Instead of quickly losing its own individuality in the individualities of its offspring, each member of the race may, after growing for a time, have portions of its substance begin to develop into the parental shape and presently detach themselves ; and the parent, maintaining its own identity, may continue indefinitely so to produce young ones. But clearly, the earlier it commences doing this, and the more rapidly it does it, the sooner must the increase of its own bulk be stopped. Or again, growth and development continuing for a long period without anv deduction of materials, an individual of considerable size and organization may result ; and then the abstraction of substance for the formation of new individuals, or rather the eggs of them, may be so great that as soon aa the eggs are laid the parent dies of exhaustion — dies, that is, from an excessive loss of the nutritive matters needed for its own activities. Once more, the deduction of materials for the propagation 406 LAWS OF MULTIPLICATION. of the species may be postponed long enough to allow of great bulk and complex structure being attained. The procreative subtraction then setting in, while it checks and presently stops growth, may be so moderate as to leave vital capital sufficient to carry on the activities of the parent ; may go on as long as parental vigour suffices to furnish, without fatal result, the materials needed to produce young ones ; and may cease when such a surplus cannot be supplied, leaving the parental life to continue. § 326. The opposite side of this antagonism has also several aspects. Progress of organic evolution may be shown in increased bulk, in increased structure, in increased amount or variety of action, or in combinations of these ; and under any of its forms this carrying higher of each individuality, implies a correlative retardation in the establishment of new individualities. Other things equal, every addition to the bulk of an organism is an augmentation of its life. Besides being an advance in integration, it implies a greater total of acti vities gone through in the assimilation of materials ; and it implies, thereafter, a greater total of the vital changes taking place from moment to moment in all parts of the enlarged mass. Moreover, while increased size is thus, in so far, the expression of increased life, it is also, where the organism is active, the expression of increased ability to maintain life — increased strength. Aggregation of sub stance is almost the only mode in which self- preserving power is shown among the lowest types; and even among the highest, sustaining the body in its integrity is that in which self-preservation fundamentally consists — is the end which the widest intelligence is indirectly made to subserve. While, on the one hand, the increase of tissue constituting growth is conservative both in essence and in result ; on the other hand, decrease of tissue, either from injury, disease, or old age, is in both essence and result the OBVERSE A PRIORI PRINCIPLE. 407 reverse. And if so, every addition to individual life thu8 implied, necessarily delays or diminishes the casting off of matter to form new individuals. Other things equal, too, a greater degree of organization involves a smaller degree of that disorganization shown by the separation of reproductive gemmae and germs. Detach ment of a living portion or portions from what was previously a living whole, is a ceasing of co-ordination ; and is therefore essentially at variance with that establishment of greater co ordination which is achieved by structural development. In the extreme cases where a living mass is continually dividing and subdividing, it is manifest that there cannot arise much physiological division of labour ; since progress towards mutual dependence of parts is prevented by the parts becoming independent. Contrariwise, it is equally clear that in proportion as the physiological division of labour is carried far, the separative process must be localized in some comparatively small portion of the organism, where it may go on without affecting the general structure — must become relatively subordinate. The advance that is shown by greater heterogeneity, must be a hindrance to multiplication in another way. For organization entails cost. That transfer and transformation of materials implied by differentiation, can be effected only by expenditure of force; and this sup poses consumption of digested and absorbed food, which might otherwise have gone to make new organisms, or the germs of them. Hence, that individual evolution which consists in progressive differentiation, as well as that which consists in progressive integration, necessarily diminishes that species of dissolution, general or local, which propagation of the race exhibits. In active organisms we have yet a further opposition between self-maintenance and maintenance of the race. All motion, sensible and insensible, generated by an animal for the preservation of its life, is motion liberated from decomposed nutriment — nutriment which, if not thus decom- 408 LAWS OF MULTIPLICATION. posed, would have been available for reproduction ,* or rather — might have been replaced by nutriment fitted for repro ductive purposes, absorbed from other kinds of food. Hence, in proportion as the activities increase — in proportion as, by its more varied, complex, rapid, and vigorous actions, an animal gains power to support itself and to cope with sur rounding dangers, it must lose power to propagate. § 327. How may this antagonism be best expressed in a brief way ? If self-preservation displayed itself in the highest organisms, as it does in the lowest, in little else but continuous growth ; and if race-preservation consisted always, as it does often, of nothing beyond detachment of portions from the parental mass ; then the antagonism would be, throughout, the obviously-necessary one of integration and disintegration. Maintenance of the individual and propaga tion of the species, being respectively aggregative and separa tive, it would be as self-evident that they vary inversely, as it is self-evident that addition and subtraction undo one another. But though the simplest types show us the opposi tion of self- maintenance and race-maintenance almost wholly under this form ; and though higher types, up to the most complex, exhibit it to a great extent under this form ; yet, as we have just seen, this is not its only form. The total material monopolized by the individual and withheld from the race, must be stated as the quantity united to form its fabric, plus the quantity expended in differentiating its fabric, plus the quantity expended in its self- conserving actions. Similarly, the total material devoted to the race at (he expense of the individual, includes that which is directly subtracted from the parent in the shape of egg or foetus, plus that which is directly subtracted in the shape of milk, phis that which is indirectly subtracted in the shape of matter consumed in the exertions of fostering the young. Hence this inverse variation is not expressible in simple terms of aggregation and separation. As we advance to more highly- OBVERSE A PRIORI PRINCIPLE. 409 evolved organisms, the total cost of an individual becomes very much greater than is implied by the amount of tissue composing it. So, too, the total cost of producing each new individual becomes very much greater than that of its mere substance. And it is between these two total costs that the antagonism exists. We may, indeed, reduce the antagonism to a form compre hensive of all cases, if we consider it as existing between tho sums of the forces, latent and active, used for the two pur poses. The molecules which make up a plant or animal, have been formed by the absorption of forces directly or indirectly derived from the sun ; and hence the quantity of matter raised to the form called organic, which a plant or animal presents, is equivalent to a certain amount of force. Another amount of force is expressed by the totality of its differentiations. A further amount of force is that dissipated in its actions. And in these three amounts added together, we have the whole expense of the individual life. So, too, the whole expense of establishing each new individual includes — first the forces latent in the substance composing it when born or hatched ; second the forces latent in the prepared nutriment afterwards supplied ; and third the forces expended in feeding and protecting it. These two sets of forces being taken from a common fund, it is manifest that either set can increase only by decrease of the other. If, of the force which the parent obtains from the environ ment, much is consumed in its own life, little remains to be consumed in producing other lives ; and, conversely, if there is a great consumption in producing other lives, it can only be where comparatively little is reserved for parental life. Hence, then, Individuation and Genesis are necessarily antagonistic. Grouping under the word Individuation all processes by which individual life is completed and main tained; and enlarging the meaning of the word Genesis so as to include all processes aiding the formation and per fecting of new individuals ; we see that the two are funda- 110 LAWS OF MULTIPLICATION. mentally opposed. Assuming other things to remain the same — assuming that environing conditions as to climate, food, enemies, &c., continue constant ; then, inevitably, every higher degree of individual evolution is followed by a lower degree of race multiplication, and vice versa. Progress in bulk, complexity, or activity, involves retrogress in fertility ; and progress in fertility involves retrogress in bulk, com plexity, or activity. This statement needs a slight qualification. For reasons to be hereafter assigned, the relation described is never com pletely maintained ; and in the small departure from it, we shall find an admirable self-acting tendency to further the supremacy of the most-developed types. Here, however, this hint must suffice : explanation would carry us too far out of our line of argument. For the present it will not lead ns astray if we regard this inverse variation of Individuation and Genesis as exact. § 328. Thus, then, the condition which each race must fulfil if it is to survive, is a condition which, in the nature of things, it ever tends to fulfil. In the last chapter we saw that a species cannot be maintained unless the power to preserve individual life and the power to propagate other individuals vary inversely. And here we have seen that, irrespective of an end to be subserved, these powers cannot do other than vary inversely. On the one hand, given a certain totality of destroying forces with which the species has to contend ; and in proportion as its members have severally but small ability to resist these forces, it is requisite that they should have great ability to form new individuals, and vice versa. On the other hand, given the quantity of force, absorbed as food or otherwise, which the species can une to counterbalance these destroying forces ; and in propor tion as much of it is expended in preserving the individual; little of it can be reserved for producing new individuals and vice versa. There is thus complete accordance between OBVKRSE A PRIORI PRINCIPLE. 411 the requirements considered under each aspect. The twc necessities correspond. We might rest on these deductions and their several corol laries. Without going further we might with safety assert the general truths that, other things equal, advancing evolu tion must be accompanied by declining fertility ; and that, in the highest types, fertility must still further decrease if evolution still further increases. We might be sure that if, other things equal, the relations between an organism and its environment become so changed as permanently to diminish the difficulties of self-preservation, there will be a permanent increase in the rate of multiplication ; and, conversely, that a decrease of fertility will result where altered circumstances make self-preservation more laborious. But we need not content ourselves with these d priori inferences. If they are true, there must be an agreement between them and the observed facts. Let us see how far such an agreement is traceable. CHAPTER IV. DIFFICULTIES OF INDUCTIVE VERIFICATION. § 329. Were all species subject to tlie same kinds and amounts of destructive forces, it would be easy, by comparing different species, to test the inverse variation of Individuation and Genesis. Or if either the power of self-preservation or the power of multiplication were constant, there would be little difficulty in seeing how the other changed as the destroying forces changed. But comparisons are nearly always partially vitiated by some want of parity. Each factor, besides being variable as a whole, is compounded of factors that are severally variable. Not simply is the sum of the force? destructive of race different in every case ; and not simply are both sets of forces preservative of race unlike in their totalities in every case ; but each is made up of actions that bear such changing proportions to one another as to prevent any positive estimation of its amount. Before dealing with the facts as well as we can, it will be best to glance at the chief difficulties ; so that we may see the kind of verification which is alone possible. § 330 Either absolutely, or relatively to any species, every environment differs more or less from every other. There are the unlikenesses of media — air, water, earth, organic matter ; severally involving special resistances to movement, and special losses of heat. There are the con- DIFFICULTIES OF INDUCTIVE VERIFICATION. 413 trasts of climate : here great expenditure for the maintenance of temperature is needed, and there very little ; in one zone an organism is supplied with abundant light all the year round, and in another only for a few months ; this region yields an almost unfailing supply of water, while that entails the exertion of travelling many miles every night for a draught. Permanent differences in the natures and distributions of aliment greatly interfere with the comparisons. The Swal low goes through more exertion than the Sparrow in securing a given weight of food ; but then their foods are dissimilar in nutritive qualities. There is a want of parallelism between the circumstances of those herbivores that live where the plains are annually covered for a time with rich herbage, but afterwards become parched up, and of those inhabiting more temperate regions. Insects whose larvae feed on an abundant plant, as those of the genus Vanessa on the Nettle, have practically an environment very unlike that of insects such as Deilephila Euphorbia*, whose larvae feed on a com paratively rare plant — the Sea- Spurge. Again, comparisons between creatures otherwise akin in their constitutions and circumstances, are hindered by ine qualities in their relations to enemies. Tuo animals, of which one is predatory and has no foes but parasites, while the other is much pursued, cannot properly be contrasted with a view to determining the influence of size or com plexity. Without multiplying instances, it will be clear enough then that the aggregate of destructive actions, positive and negative, which each species has to contend with, is so indefinable in the amounts and kinds of its components, that nothing beyond a vague idea of its relative total can be formed. § 331. Besides these immense variations in the outer actions to be counter- balanced, thero are immense variations 414 LAWS OF MULTIPLICATION. in the inner actions required to counter-balance them. E\vn if species were similarly conditioned, self-preservation would require of them extremely unlike expenditures of force. The cost of locomotion increases in a greater ratio than the size In virtue of the law that the weights of animals increase as the cubes of their dimensions, while their strengths increase only as the squares of their dimensions (§ 46), a given speed requires a large animal to consume more substance in propor tion to its weight, than it requires a small animal to consume ; and this law holding of all the mechanical actions, there results, other things equal, a difficulty of self- maintenance that augments in a more rapid ratio than the bulk Nor must we overlook the further complication, that among aquatic creatures the variation of resistance of the medium partially neutralizes this effect. Again, the heat-consumption is a changing element in the total expense of self-preservation. Creatures that have tem peratures scarcely above that of the air or water, may, other things equal, accumulate more surplus nutriment than creatures that have to keep their bodies warm spite of the continual loss by radiation and conduction. This difference of cost is modified by the presence or absence of natural clothing ; and it is also modified by unlikenesses of size. Here the bulky animals have the advantage : small masses cool ing more rapidly than large ones. Dissimilarities of attack and defence are also causes of variation in the outlay for self -maintenance. A creature that has to hunt, as compared with another that gets a sufficiency of prey by lying in wait, or a creature that escapes by speed as compared with another that escapes by concealment, obviously leads a life that is physiologically more costly. Animals that protect themselves passively, as the Hedge- hog by its spines or as the Skunk and the Musk-rat by their intolerable odours, are relatively econo mical ; and have the more vital capital for other purposes. Amplification is needless. These instances will show that DIFFICULTIES OF INDUCTIVE VERIFICATION. 415 anything beyond very general conceptions of the individual expenditures in different cases, cannot be reached. § 332. Still more entangled are we among qualifying con siderations when we contrast species in their powers of multi plication. The total cost of Genesis admits of even less definite estimation than does the total cost of Individua- tion. I do not refer merely to the truth that the degree of fertility depends on four factors — the age of commencing reproduction, the number in each brood, the frequency of the broods, and the time during which broods continue to be repeated. There are many further obstacles in the way of comparisons. Were all multiplication carried on sexually, the problem would be less involved ; but there are many kinds of asexual multiplication alternating with the sexual. This asexual multiplication is in some cases perpetual instead of occa sional ; and often has more forms than one in the same species. The result is that we have to compare what is here a periodic process with what is elsewhere a cyclical process partly continuous and partly periodic — the calculation of fer tility in this last case being next to impossible. We have to avoid being misled by the assumption that the cost of Genesis is measured by the number of young produced, instead of being measured, as it is, by the weight of nutri ment abstracted to form the young, plus the weight con sumed in caring for them. This total weight may be very diversely apportioned. In contrast to the Cod with its million of small ova spawned without protection, we may put the Hippocampus or the Pipe-fish, with its few relatively- large ova carried about by the male in a caudal pouch, or seated in hemispherical pits in its skin ; or we may put the still more remarkable genus Arias, and especially Arius Boakeii — a fish some six or seven inches long, which produces ten or a dozen eggs as large as marbles, that are carried by the male in his mouth till they are hatched. Here though 59 LAWS OF MULTIPLICATION. the degrees of fertility, if measured by the numbers of fertilized germs deposited, are extremely unlike, they aro less unlike if measured by the numbers of young that are hatched and survive long enough to take care of themselves ; nor will the tax on the parent- Cod seem so immensely dif ferent from that on the parent-^nws, if the masses of the ova, instead of their numbers, are compared. Again, while sometimes the parental loss is little else but the matter deducted to form eggs, &c. ; at other times it takes the shape of a small direct deduction joined with a large indirect outlay. The Mason-wasp furnishes a typical instance. In journeyings hither and thither to fetch bit by bit the materials for building a cell ; in putting together these materials, as well as in secreting glutinous matter to act as cement ; and then, afterwards, in the labour of seeking for, and carrying, the small caterpillars with which it fills up the cell to serve its larva with food when it emerges from the egg ; the Mason-wasp probably expends more substance than is contained in the egg itself. And this supplementary ex penditure is manifestly so great, that but few eggs can be housed and provisioned. Estimates of the cost of Genesis are further complicated by variations in the ratio borne by the two sexes. Among Fishes the mass of milt approaches in size the mass of spawn ; but amorg higher Vertebrata the substance lost by the one sex in the shape of sperm- cells is small compared with that lost by the other sex in the shape of albumen stored-up in the eggs, or blood supplied to the foetus, or milk given to the young. Then there come the differences of indirect tax on males and females. While, frequently, the fostering of the young devolves entirely on the female, occasionally, the male undertakes it wholly or in part. After building a nest, the male Stickleback guards the eggs till they are hatched ; as does also the great Sllurus glanis for some forty days, during which he takes no food. And then, among most birds, we have the male occupied in feeding the female during DIFFICULTIES OF INDUCTIVE VERIFICATION. 417 incubation, and the young afterwards. Evidently all these differences affect the proportion between the total cost of re production and the total cost of individuation. Whether the species is monogamous or polygamous, and whether there are marked differences of size or of structure between males and females, are also questions not to be over looked. If there are many females to cue male, the total quantity of assimilated matter devoted by each generation to the production of a new generation, is greater than if there is a male to each female. Similarly, where the requirements are such that small males will suffice, the larger quantity of food left for the females, makes possible a greater surplus available for reproduction. And where, as in some of the Cirrhipedia, or such a parasite as Sphcerularia Bombi, the female is a thousand or many thousand times the size of the male, the reproductive capacity is almost doubled : the effect on the rate of multiplication being something like that which would result if any ordinary race could have all its males replaced by fertile females. Conversely, where the habits of the race render it needless that both sexes should have developed powers of locomotion — where, as in. the Glow worm and sundry Lepidoptera, the female is wingless while the male has wings — the cost of Individuation not being so great for the species as a whole, there arises a greater reserve ior Genesis : the matter which would otherwise have gone to i.he production of wings and the using of them, may go to the production of ova. Other complications, as those which we see in Bees and Ants, might be dwelt on; but the foregoing will amply serve the intended purpose. § 333. To ascertain by comparison of cases whether Indi vid nation and Genesis vary inversely, is thus an under taking so beset with difficulties, that we might despair of any sati*factory results, were not the relation too marked a one to be hidden even by all these complexities. Species are 418 LAWS OF MULTIPLICATION. so extremely contrasted in their degrees of evolution, and so extremely contrasted in their rates of multiplication, that the law of relation between these characters becomes unmis takable when the evidence is looked at in its ensemble. This we shall soon find on ranging in order a number of typical cases. In doing this it will be convenient to neglect, temporarily, all unlikenesses among the circumstances in which organ isms are placed. At the outset, we will turn our attention wholly to the antagonism displayed between the integrative process which results in individual evolution and the disinte- grative process which results in multiplication of individuals ; and this we will consider first as we see it under the several forms of agamogenesis, and then as we see it under the seve ral forms of gamogenesis. We will next look at the anta gonism between propagation and that evolution which is shown by increased complexity. And then we will consider the remaining phase of the antagonism, as it exists between the degree of fertility and the degree of evolution expressed by activity. Afterwards, passing to the varying relations between organisms and their environments, we will note how relative increase in the supply of food, or relative decrease in the quantity of force expended by the individual, entails relative increase in the quantity of force devoted to multiplication, iind vice versa. Certain minor qualifications, together with sundry impor tant corollaries, rnay then be entered upon. CHAPTER V. ANTAGONISM BETWEEN GROWTH AND ASEXUAL GENESIS. § 334. When illustrating, in Part IV., the morphological composition of plants and animals, there were set clown in groups, numerous facts which we have here to look at from another point of view. Then we saw how, by union of small simple aggregates, there are produced large compound aggre gates. Now we have to observe the reactive effect of this process on the relative numbers of the aggregates. Our present subject is the antagonism of Individuation and Genesis as seen under its simplest form, in the self-evident truth that the same quantity of matter may be divided into many small wholes or few large wholes ; but that number negatives largeness and largeness negatives number. In setting down some examples, we may conveniently adopt the same arrangement as before. We will look at the facts as they are presented by vegetal aggregates of the first order, of the second order, and of the third order ; and then as they are presented by animal aggregates of the same three orders. § 335. The ordinary unicellular plants are at once micro scopic and enormously prolific. The often cited Protococciis nivalis, which shows its immense powers of multiplication by reddening wide tracts of snow in a single night, does this by developing in its cavity a brood of young cells, which, being 420 LAWS OF MULTIPLICATION. presently set free by the bursting of the parent-cell, severally grow and quickly repeat the process. The like occurs among sundry of those kindred forms of minute Alyce which, by their enormous numbers, sometimes suddenly change pools to an opaque green. So, too, the Dcsmidiaccce often multiply so greatly as to colour the water; and among the Diatomacece the rate of genesis by self-division, " is something really extra ordinary. So soon as a frustule is divided into two, each ;f the latter at once proceeds with the act of self-division ; so that, to use Professor Smith's approximative calculation of the possible rapidity of multiplication, supposing the process to occupy, in any single instance, twenty-four hours, ' we should have, as the progeny of a single frustule, the amazing number of one thousand millions in a single month/ ': In these cases the multiplication is so carried on that the parent is lost in the offspring — the old individuality disappears either in the swarms of zoospores it dissolves into, or in the two or four new individualities simultaneously produced by fission. Vegetal aggregates of the first order, have, however, a form of agamogenesis in which the parent individuality is not lost : the young cells arise from the old cells by external gemmation. This process, too, repeated as it is at short inteivals, results in immense fertility. The Yeast-fungus, which in a few hours thus propagates itself throughout a large mass of wort, offers a familiar example. In certain compound forms that must be classed as plants of the second order of aggregation, though very minute ones, self-division similarly increases the numbers at high rates. The Sarcina ventriculi, a parasitic plant that infests the stomach and swarms afresh as fast as previous swarms are vomited, shows us a spontaneous fission of clusters of cells. An allied mode of increase occurs in Oonium pectorale : each cell of the cluster resolving itself into a secondary cluster, and the secondary clusters then separating. " Supposing1, which is very probable, that a young Gouium after twenty - four hours is capable of development by fission, it follows GROWTH AND ASEXUAL GENESIS. 421 that, under favourable conditions a single colony may on the second day develop 16, on the third 256, on the fourth 4,096, and at the end of a week 268,435,456 other organisms like itself." In the Vohodnce this continual dissolution of a primary compound individual into secondary compound individuals, is carried on endogenously — the parent bursting to liberate tho young ; and the numbers arising by this method, also are some times so great as to tint large bodies of water. More fully established and organized aggregates of the second ord T, such as the higher Thallogens and the lower Acrogens, do not sacrifice their individualities by fission ; but never theless, by the kindred process of gemmation, are continually hindered in the increase of their individualities. The gemma) called tetraspores are cast off in great numbers by the marine Alyce. Among those simple Jungennanniacece which consist of single fronds, the young ones that bud out grow for a time in connexion with their parents, send rootlets from their under sides into the soil, and presently separate themselves — - a habit which augments the number of individuals in propor tion as it checks their growths. Plants of the third order of composition, arising by arrest of this separation, exhibit a further corresponding decrease in the abundance of the aggregates formed. Acrogens of inferior types, in which the axes produced by integration of fronds are but small and feeble, are characterized by the habit of throwing off bulbils — bud-shaped axes which, falling and taking root, add to the number of distinct individuals. This agamic multiplication, very general among the Mosses and their kindred, and not uncommon under a modified form in such higher types as the Ferns, many of which produce young ones from the surfaces of their fronds, becomes very unusual among Phaenogams. The detachment of bulbils, though not unknown among them, is exceptional. And while it is true that some flowering plants, as the Strawberry, multiply by a process allied to gemmation, yet this is anything but characteristic of the class. A leading trait of 422 LAWS OP MULTIPLICATION. these highest groups, to which the largest members of the vegetal kingdom belong, is that agamogcnesis has so far ceased that it does not originate independent plants. Though the axes which, budding one out of another, compose a tree, are the equivalents of asexually-produced individuals ; yet the asexual production of them stops short of separation. These vast integrations arise where spontaneous disintegra tion, and the multiplication effected by it, have come to an end. Thus, not forgetting that certain Phsenogams, as Begonia phyllomaniaca, revert to quite primitive modes of increase, we may hold it as beyond question that while among the most minute plants asexual multiplication is universal, and pro duces enormous numbers in short periods, it becomes step by step more restricted in range and frequency as we advance to large and compound plants ; and disappears so generally from the largest, that its occurrence is regarded as anomalous. § 336. Parallel examples showing the inverse variation of growth and asexual genesis among animals, make clear the purely quantitative nature of this relation under its original form. Of the Amoeba it is said that " when a large variable o process has been shot out far from the chief mass and become enlarged at the extremity, the expanded end retains its posi tion, whilst the portion connecting it with the body becomes finer and finer by being withdrawn into the parent mass, until it at last breaks across, leaving a detached piece, which immediately on its own account shoots out processes, and manifests an independent existence. This phenomenon is therefore one of simple detachment, and cannot rightly be called a process of fission." But it shows us, nevertheless, how the primordial form of multiplication is nothing more than a separation, instead of a continued union, of the grow ing mass. Among the Protozoa, as among the Protophyta, there occurs that process by which the in dividuality of the parent is wholly lost in producing offspring GROWTH AND ASEXUAL GENESIS. 423 —the breaking up of the parental mass into a number of germs. An example is supplied by one of the lowest of the class — the Gregarina. This creature, which is nothing more than a minute spheroidal nucleated mass of protoplasm, having a structureless outer layer denser than the rest, but being without mouth or any organ, resolves itself into a multitude of still more minute masses, which when set free by bursting of the envelope, shortly become Amoeba-form, and severally assuming the structure of the parent, go through the same course. Some of the Infusoria, as for in stance those of the genus Kolpoda, similarly become encysted and subsequently break up into young ones. The more familiar mode of increase among these animal-aggre gates of the first order, by fission, though it sacrifices the parent individuality by merging it in the individualities of the two produced, sacrifices it less completely than does the dissolution into a great number of germs. Occurring, how ever, as this fission does, very frequently, and being com pleted, in some cases that have been observed, in the course of half-an-hour, it results in immensely-rapid multiplication. If all its offspring survive, and continue dividing them selves, a single Partunecium is said to be capable of thus originating 268 millions in the course of a month. Nor is this the greatest known rate of increase. Another animalcule, visible only under a high magnifying power, " is calculated to generate 170 billions in four days." And these enormous powers of propagation are accompanied by a minuteness so extreme, that of some species one drop of water would contain as many individuals as there are human beings on the Earth I Making allowance for exag- eration in those estimates, it is beyond question that among these smallest of animals the rate of asexual multiplication is by far the greatest ; and this suffices for the purposes of the argument.* * That these estimated rates are not greater than is probable, may bo inferred from such observations as that of Mr. Brightvvell on the buda of Zxothamnium. "At nine in the morning, one of these buds, or jva, was 424 LAWS OF MULTIPLICATION. Of animal aggregates belonging to the second order, that multiply asexually with rapidity, the familiar Polypes furnish conspicuous examples. By gemmation in most cases, in other cases by fission, and in some cases by both, the agamogenesis is carried on among these tribes. As showrn in Fig. 148, the budding of young ones from the parent Hydra is carried on so actively, that before the oldest of them is cast off half-a-dozen or more others have reached various stages of growth ; and even while still attached, the first-formed of the group have commenced budding out from their sides a second generation of young ones In the Hydra tuba this gemmiparous multiplication is from time to time interrupted by a transverse splitting- up of the body into segments, which successively separate and swim away : the result of the two processes being, that in the course of a season there are produced from a single germ, great numbers of young Msdusw, which are the adult or sexual forms of the species. Respecting Ccelenterate animals of this degree of composition, it may be added that when we ascend to the larger kinds we find asexual genesis far less active. Though comparisons are interfered with by differences of structure and mode of life, yet the contrasts are too striking to have their meanings much obscured. If, for instance, we take a solitary Actinozoon and a solitary Hydrozoon, we see that the relatively-great bulk of the first, goes along with a relatively-slow agamogenesis. The common Sea-anemones are but occasionally observed to undergo self-division : their numbers are not rapidly increased by this process. A higher class of secondary aggregates exemplifies the same observed fixed to the glass by a sheathed pedicle ; a ciliary motion became perceptible at the top of the bulb ; and at ten it had divided longitudinally into two buds, each supported by a short stalk. The ciliary motion continued m the centre of each of these two buds, which by degrees expanded longitudi nally, and at twelve had become four buds. By four in the afternoon, these four buds had divided in like manner and increased to nine, with an elongated footstalk, and interior contractile muscle." GROWTH AND ASEXUAL GENESIS. 425 general truth with a difference. In the smaller members the ao-amogenesis is incomplete, and in the larger it disappears. Each sub-section of iheMolluscoida shows us this. The gemina tion of the minute Polyzoa, though it does not end in the sepa ration of the young individuals, habitually goes to the extent of producing families of partially-independent individuals ; but their near allies the Biachiopoda, which immensely exceed them in size, are solitary and not gemmiparous. So, too, is it with the Ascidioida. And then among the true Mollu&ca, including all the largest forms belonging to this sub- kingdom, no such thing is known as fission or gemmation. Take next the Annulosa, including under this' title the Annuloida. When treating of morphological composition, reasons were given for the belief that the annulose animal is an aggregate of the third order, the segments of which, produced one from another by gemmation, originally became separate, as they still become in the cestoid Entozoa ; but that by progressive integration, or arrested disintegration, there resulted a type in which many such segments were permanently united (§§ 205-7). Part of the evidence there assigned, is evidence to be here repeated in illustration of the direct antagonism of Growth and Asexual- Genesis. We saw how, among the lower Annelids, the string of segments produced by gemmation presently divides trans versely into two strings ; and how, in some cases, this resolu tion of the elongating string of segments into groups that are to form separate individuals, goes on so actively that as many as six groups are found in different stages of progress to ultimate independence — a fact implying a high rate of iissiparous multiplication. Then we saw that, in the superior annulose types, distinguished in the mass by including the larger species, fission does not occur. The higher Annelids do not propagate in this way ; there is no known case of new individuals being so formed among the Muriapoda ; nor do the Crustaceans afford us a single instance of tins primordial mode of increase. It is, indeed, true that while 426 LAWS OF MULTIPLICATION. articulate animals never multiply asexually after this simplest; method, and while they are characterized in the mass by the cessation of agamogenesis of every kind, there nevertheless occur in a few of their small species, those higher forms of dgamogenesis known as parthenogenesis, pseudo-partheno genesis and internal metagenesis ; and that by these some of them multiply very rapidly. Hereafter we shall find, in the interpretation of these anomalies, further support for the general doctrine. To the above evidence has to be added that which the Vertebrata present. This may be very briefly summed up. On the one hand, this class, whether looked at in the aggre gate or in its particular species, immensely exceeds all other classes in the sizes of its individuals ; and on the other hand, agamogenesis under any form is absolutely unknown in it, § 337. Such are a few leading facts serving to show how deduction is inductively verified, in so far as the anta gonism between Growth and Asexual Genesis is con cerned. In whatever way we explain this opposition of the integrative and disintegrative processes, the facts and their implications remain the same. Indeed we need not commit ourselves to any hypothesis respecting the physical causation : it suffices to recognize the results under their most general aspects. We cannot help admitting there are at work these two antagonist tendencies to aggregation and separation ; and we cannot help admitting that the propor tion between the aggregative and separative tendencies, must in each case determine the relation between the increase in bulk of the individual and the increase of the race in number. The antithesis is as manifest a posteriori as it is neces- saiy d priori. While the minutest organisms multiply asexually in their millions ; while the small compound types next above them thus multiply in their thousands ; while larger and more compound types thus multiply in their hundreds and their tens ; the largest types do not thus GROWTH AND ASEXUAL GENESIS. 427 multiply at all. Conversely, those which do not multiply asexually at all, are a billion or a million times the size of those which thus multiply with greatest rapidity ; and are a thousand times, or a hundred times, or ten times the size of those which thus multiply with less and less rapidity. With out saying that this inverse proportion is regular, which, as \ve shall hereafter see, it cannot be, we may unhesitatingly assert its average truth. That the smallest organisms habitually reproduce asexually with immense rapidity ; that the largest organisms never reproduce at all in this manner ; and that between these extremes there is a general decrease of asexual reproduction along with an increase of bulk; are proposi tions that admit of no dispute. CHAPTER VI. ANTAGONISM BETWEEN GROWTH AND SEXUAL GENESIS. § 338. Iii so far as it is a process of separation, sexual genesis is like asexual genesis ; and is therefore, equally with asexual genesis, opposed to that aggregation which results in growth. Whether a deduction is made from one parent or from two, whether it is made from any part of the body indifferently or from a specialized part, or whether it is made directly or indirectly, it remains in any case a deduction ; and in proportion as it is great, or frequent, or both, it must restrain the increase of the individual. Here we have to group together the leading illustrations of this truth. We will take them in the same order as before. § 339. The lowest vegetal forms, or rather, we may say, those forms which we cannot class as either distinctly vegetal or distinctly animal, show us a process of sexual multiplica tion that differs much less from the asexual process than in the higher forms. The common character which distinguishes sexual from asexual genesis, is that the mass of protoplasm whence a new generation is to arise, has been produced by the union of two portions of matter that were before more widely separated. I use this general expression, because among the simplest A.lgwt this is not invariably matter supplied by different individuals : certain DiatamacecB exhibit within a feingle cell, the formation of a sporangium by a drawing GROWTH AND SEXUAL GENESIS. 429 together of the opposite halves of the endochrome info a ball. Mostly, however, sporangia are products of conjuga tion. The endochromes of two cells unite to form the germ- mass ; and these conjugating cells may be either entirely independent, as in many Desinidiaccce and in the Palme/ice; or they may be two of the adjacent cells forming a thread, as in some Coiy'iHjfftixs ; or they may be colls belonging to adjacent threads, as in Zygnetna. But whether it is originated by a single parent-cell, or by two parent-cells, the sporangium, after remaining quiescent until there recur the fit conditions for growth, breaks up into a multitude of spores, each of which produces an individual that multiplies asexually ; and the fact here to be noted is, that as the entire contents of the parent- cells unite to form the sporangium, their individualities are lost in the germs of a new generation. In these minute simple types, sexual propagation just as completely sacrifices the life of the parent or parents, as does that form of asexual propa gation in which the endochrome resolves itself directly into zoospores. And in the one case as in the other, this sacrifice is the concomitant of a prodigious fertility. Slightly in advance of this, but still showing us an almost equal loss of parental life in the lives of offspring, is the process seen in such unicellular Algae as Hydrogastrum, and in minute Fungi of the same degree of composition. These exhibit a relatively- enormous development of the spore-producing part, and an almost entire absorption of the parental substance into it. As evidence of the resulting powers of multiplication, we have but to remember that the spread of mould over stale food, the rapid destruction of crops by mildew, and other kindred occurrences, are made possible by the incalculably numerous spores thus generated and universally dispersed. Plants a degree higher in composition, supply a parallel series of illustrations. We have among the larger Fungi, in which the reproductive apparatus is relatively so enormous as to constitute the ostensible plant, a similar subordination of the individual to the race, and a similarly- immense fertility. 4:30 LAWS OF MULTIPLICATION. Thus, as quoted by Dr. Carpenter, Fries says — " in a single individual of Reiicularia maxima, I have counted (calculated?) 10,000,000 sporules." It needs but to note the clouds of particles, so minute as to look like smoke, which ripe puff- balls give off when they are burst, and then to remember that each particle is a potential fungus, to be impressed with the almost inconceivable powers T)f propagation which these plants possess. The Lichens, too, furnish examples. Though they are nothing like so prolific as the Fungi (the difference yielding, as we shall hereafter see, further support to the general argument), yet there is a great production of germs, and a proportionate sacrifice of the parental indi viduality. Considerable areas of the frond here and there develop into apothecia and spermagonia, which resolve them selves into sperm-cells and germ-cells. Some con trasts presented by the higher AlgoB may also be named as exemplifying the inverse proportion between the size of the individual and the extent of the generative structures. While in the smaller kinds relatively large portions of the fronds are transformed into reproductive elements, in the larger kinds these portions are relatively small : instance the Macrocystia pyrifem, a gigantic sea-weed, which sometimes attains a length of 1,500 feet, of which Dr. Carpenter remarks — " This development of the nutritive surface takes place at the expense of the fructifying apparatus, which is here quite subordinate." When we turn to vegetal aggregates of the third order of composition, facts having the same meaning are conspicuous. On the average these higher plants are far larger than plants of a lowrer degree of composition ; and on the average their rates of sexual reproduction are far less. Similarly if, among Acrogens, Endogens, and Exogens, we compare the smaller types with the larger, we find them proportionately more prolific. This is not manifest if we simply calculate the number of seeds ripened by an individual in a single season ; but it becomes manifest if we take into account tho GROWTH ASD SEXUAL GENESIS. further factor which here complicates the result — the age at which sexual genesis commences. The smaller Phaenogarnei lire mostly either annuals, or perennials that die down annually ; and seeding as they do annually before their deaths, or the deaths of their reproductive parts, it results that in the course of a year, each gives origin to a multitude of potential plants, of which every one may the next year, if preserved, give origin to an equal multitude. Supposing but a hundred offspring to be produced the first year, ten thousand may be produced in the second year, a million in the third, a hundred millions in the fourth. Meanwhile, what has been the possible multiplication of a large Phae- nogam? While its small congener has been seeding and dying, and leaving multitudinous progeny to seed and die, it has simply been growing; and may so continue to grow for ten or a dozen years without bearing fruit. Before a Cocoa- nut tree has ripened its first cluster of nuts, the descendants of a wheat plant, supposing them all to survive and multiply, will have become numerous enough to occupy the whole surface of the Earth. So that though, when it begins to bear, a tree may annually shed as many seeds as a herb, yet in consequence of this delay in bearing, its fertility is incom parably less; and its relatively-small fertility becomes still further reduced where, as in Lodoicea Seche/lantm, the seeds take two years from the date of fertilization to the date of germination. § 340. Some observers state that in certain Protozoa there occurs a process of conjugation akin to that which the ProtopJiyta exhibit — a coalescence of the substance of two individuals to form a germ- mass. This has been alleged more especially of Actinophrys. The statement is question able ; but if proved true, then of the minute forms that appear to be more animal than vegetal in their characters, some have a mode of sexual multiplication by which the parents are sacrificed bodily in the production of a new GO LAWS OF MULTIPLICATION. generation. A modified mode, apparently not fatal to the parents, has heen observed in certain of the more developed Infusoria. Our knowledge of these microscopic types is, however, so rudimentary that evidence derived from them must be taken with a qualification. Among small animal aggregates of the second order, the first to be considered are of course the Ccelenterata. A Hydra occasionally devotes a large part of its substance to sexual genesis. In the walls of its body groups of ova, or sperma tozoa, or both, take their rise ; and develop into masses greatly distorting the creature's form, and leaving it greatly diminished when they escape. Here, however, gamogenesis is obviously supplementary to agamogenesis — the immensely rapid multiplication by budding continues as long as food is abundant and warmth sufficient, and is replaced by gamo genesis only at the close of the season. A better example of the relation between small size and active gamo genesis is supplied by the Planaria, which does not multiply asexually with so much rapidity. The generative system is here enormous. Ova are developed all through the body, occupying everywhere the interspaces of the assimilative system ; so that the animal may be said to consist of a part that absorbs nutriment and a part that transforms that nutri ment into sperm-cells and germ-cells. Even saying nothing of the probably-early maturity of these animals, and there fore frequent repetition of sexual multiplication, it is clear that their fertility must be very great. The Annulosa, including among them the inferior kindred types, have habits and conditions of life so various that only the broadest contrasts can be instanced in support of the pro position before us. Of the microscopic forms belonging to this sub-kingdom, the Rotifera may be named as having, along with small bulk, a great rate of sexual increase. Hyda- Una senta " is capable of a four- fold propagation every twenty- four or thirty-hours, bringing forth in this time four ova, which grow from the embryo to maturity, and exclude their GROWTH AND SEXUAL GENESIS. fertile ova in the same period. The same individual, pro ducing in ten days forty eggs, developed with the rapidity above cited, this rate, raised to the tenth power, gives one million of individuals from one parent, on the eleventh day four millions, and on the twelfth day sixteen millions, and so on." Ascending from this extreme, the differences of organization and activity greatly complicate the inverse variation of fertility and bulk. Bearing in mind, how ever, that the rate of multiplication depends much less on the number of each brood than on the quickness with which maturity is reached and a new generation commenced, it will be obvious that though Annelids produce great numbers of ova, yet as they do this at comparatively long intervals, their rates of increase fall immensely below that just instanced in the Rotifers. And when at the other extreme we come to the large articulate animals, such as the Crab and the Lobster, the further diminution of fertility is seen in the still longer delay that occurs before each new generation begins to re produce. Perhaps the best examples are supplied by vertebrate animals, and especially those that are most familiar to us. Comparisons between Fishes are unsatisfactory, because of our ignorance of their histories. In some cases Fishes equal in bulk produce widely different numbers of eggs ; as the Cod which spawns a million at once, and the Salmon by which nothing like so great a number is spawned. But then the eggs are very unlike in size ; and if the ovaria of the two fishes be compared, the difference between their masses is comparatively moderate. There are, indeed, contrasts whick seem at variance with the alleged relation ; as that between the Cod and the Stickleback, which, though so much smaller, produces fewer ova. The Stickleback's ova, however, are relatively large ; and their total bulk bears as great a ratio to the bulk of the Stickleback as does the bulk of the Cod's ova to that of the Cod. Moreover, if, as is not improbable, the reproductive age is arrived at earlier by the Stickleback thau 434 LAWS OF MULTIPLICATION, by the Cod, the fertility of the species may oe greater not withstanding the smaller number produced by each indi vidual. Evidence that admits of being tolerably well disentangled is furnished by Birds. They differ but little in their grades of organization; and the habits of life throughout extensive groups of them are so similar, that comparisons may be fairly made. It is true that, as hereafter to be shown, the differences of expenditure which differences of bulk entail, have doubtless much to do with the differences of fertility. But we may set down under the present head some of those cases in which the activity, being relatively slight, does not greatly interfere with the relation we are considering ; and may note that among such birds having similarly slight activities, the small produce more eggs than the large, and eggs that bear in their total mass a greater ratio to the mass of the parent. Consider, for example, the gallinaceous birds ; which are like one another and unlike birds of most other groups in flying comparatively little. Taking first the wild members of this order, which rarely breed more than once in a season, we find that the Pheasant has from 6 to 10 eggs, the Black-cock from 5 to 10, the Grouse 8 to 12, the Partridge 10 to 15, the Quail still more, some times reaching 20. Here the only exception to the relation between decreasing bulk and increasing number of eggs, occurs in the cases of the Pheasant and the Black-cock ; and it is to be remembered, in explanation, that the Pheasant inhabits a warmer region and is better fed — often artificially. If we pass to domesticated genera of the same order, we meet with parallel differences. From the numbers of eggs laid, little can be inferred ; for under the favourable con ditions artificially maintained, the laying is carried on inde finitely. But though in the sizes of their broods the Turkey and the Fowl do not greatly differ, the Fowl begins breeding at a much earlier age than the Turkey, and produces broods more frequently : a considerably higher rate of multiplication being the result. Now these contrasts GROWTH ASD SEXUAL GENESIS. 436 among domestic creatures that are similarly conditioned, and closely -allied by constitution, may be held to show, more clearly than most other contrasts, the inverse varia tion between bulk and sexual genesis ; since here the cost of activity is diminished to a comparatively small amount. There is little expenditure in flight — sometimes almost none ; and the expenditure in walking about ia not great : there is more of standing than of actual movement. It is true that young Turkeys commence their existences as larger masses than chickens ; but it is tolerably manifest that the total weight of the eggs produced by a Turkey during each season, bears a less ratio to the Turkey's weight, than the total weight of the eggs which a Hen produces during each season, bears to the Hen's weight ; and this is the fairest way of making the comparison. The comparison so made shows a greater difference than appears likely to be due to the different costs of locomotion ; con sidering the inertness of the creatures. Remembering that the assimilating surface increases only as the squares of the dimensions, while the mass of the fabric to be built up by the absorbed nutriment increases as the cubes of the dimensions, it will be seen that the expense of growth becomes relatively greater with each increment of size ; and that hence, of two similar creatures commencing life with different sizes, the larger one in reaching its superior adult bulk, will do this at a more than proportionate expense ; and so will either be delayed in commencing its reproduction, or will have a diminished reserve for reproduction, or both. Other orders of Birds, active in their habits, show more markedly the con nexion between augmenting mass and declining fertility. But in them the increasing cost of locomotion becomes an important, and probably the most important, factor. The evidence they furnish will therefore come better under another head. Contrasts among Mammals, like those which Birds present, have their meanings obscured by inequalities of the expenditure for motion. The smaller 4:36 LAWS OF MULTIPLICATION. fertility which habitually accompanies greater bulk, must in all cases be partly ascribed to this. Still, it may be well if we briefly note, for as much as they are worth, the broader contrasts. While a large Mammal bears but a single young one at a time, is several years before it commences doing this, and then repeats the reproduction at long intervals ; we find, as we descend to the smaller mem bers of the class, a very early commencement of breeding, an increasing number at a birth, reaching in small Rodents to 10 or even more, and a much more frequent recurrence of broods : the combined result being a relatively prodigious fertility. If a specific comparison be desired between Mammals that are similar in constitution, in food, in con ditions of life, and all other things but size, the Deer-tribe supplies it. While the large Red-deer has but one at a birth, the small Roe-deer has two at a birth. § 341. The antagonism between growth and sexual genesis, visible in these general contrasts, may also be traced in the history of each plant and animal. So familiar is the fact that sexual genesis does not occur early in life, and in all organisms which expend much begins only when the limit of size is nearly reached, that we do not sufficiently note its significance. It is a general physiological truth, however, that while the building-up of the individual is going on rapidly, the reproductive organs remain imperfectly developed and inactive ; and that the commencement of reproduction at once indicates a declining rate of growth, and becomes a cause of arresting growth. As was shown in § 78, the ex ceptions to this rule are found where the limit of growth is indefinite ; either because the organism expends little or nothing in action, or expends in action so moderate an amount that the supply of nutriment is never equilibrated by its expenditure. We will pass ever the inferior plants, and limiting our selves to Phaenogams, will not dwell on the less conspicu' GROWTH AND SEXUAL GENESIS. 437 ous evidence which the smaller types present. A few cases such as gardens supply will serve. All know that a Pear- tree continues to increase in size for years before it begins to bear ; and that, producing but few pears at first, it is long before it fruits abundantly. A young Mulberry, branch ing out luxuriantly season after season, but covered v* ith nothing but leaves, at length blossoms sparingly, and sets some small and imperfect berries, which it drops while they are green ; and it makes these futile attempts time after time before it succeeds in ripening any seeds. But these multi-axial plants, or aggregates of individuals some of which continue to grow while others become arrested and transformed into seed-bearers, show us the relation less de finitely than certain plants that are substantially, if not literally, uni-axial. Of these the Cocoa-nut may be in stanced. For some years it goes on shooting up without making any sign of becoming fertile. About the sixth year it flowers ; but the flowrers wither without result. In the seventh year it flowers and produces a few nuts ; but these prove abortive and drop. In the eighth year it ripens a moderate number of nuts ; and afterwards increases the number until, in the tenth year, it comes into full bearing. Meanwhile, from the time of its first flowering its growth begins to diminish, and goes on diminishing till the tenth year, when it ceases. Here we see the antagonism between growth and sexual genesis under both its aspects — see a struggle between self-evolution and race- evolution, in which the first for a time overcomes the last, and the last ultimately overcomes the first. The continued aggrandisement of the parent-individual makes abortive for two seasons the tendency to produce new individuals ; and the tendency to produce new individuals, becoming more decided, stops any further aggrandisement of the parent-individual. Parallel illustrations occur in the animal kingdom. The eggs laid by a pullet are relatively small and few. Similarly, it is alleged that, as a general rule, " a bitch has fewer 438 LAWS OF MULTIPLICATION. puppies at first, than afterwards/' According to Burdach, as quoted by Dr. Duncan, " the elk, the bear, &c , have at first only a single young one, then they come to have most frequently two, and at last again only one. The young hamster produces only from three to six young ones, whilst that of a more advanced age produces from eight to sixteen. The same is true of the pig." It is remarked by Buffon that when a sow of less than a year old has young, the number of the litter is small, and its members are feeble and even im perfect. Here we have evidence that in animals growth checks sexual genesis. And then, conversely, we have evidence that sexual genesis checks growth. It is well known to breeders that if a filly is allowed to bear a foal, she is thereby prevented from reaching her proper size. And a like loss of perfection as an individual, is suffered by a cow that breeds too early. § 342. Notwithstanding the way in which the inverse variation of growth and sexual genesis is complicated with other relations, its existence is thus, I think, sufficiently mani fest. Individually, many of the foregoing instances are open to criticism, and have to be taken with qualifications ; but when looked at in the mass, their meaning is beyond doubt. Comparisons between the largest with the smallest types, whether vegetal or animal, yield results that are unmis- takeable. On the one hand, remembering the fact that during its centuries of life an Oak does not produce as many acorns as a Fungus does spores in a single night, we see that the Fungus has a fertility exceeding that of the Oak in a de gree literally beyond our powers of calculation or imagina tion. When, on the other hand, taking a microscopic protophyte which has millions of descendants in a few days, we ask how many such would be required to build up the forest tree that is years before it drops a seed, we are met \yy a parallel difficulty in conceiving the number, if not in setting it down. Similarly, if we turn from the minute and GROWTH AND SEXUAL GENESIS. 439 p/rodigiously-fertile Rotifer, to the Elephant, which approaches thirty years before it bears a solitary young one, we find the connexions between small size and great fertility and between great size and small fertility, too intensely marked to be much disguised by the perturbing relations that have been indicated. Finally, as this induction, reached by a survey of organisms in general, is verified by observations on the rela tion between decreasing growth and commencing reproduc tion in individual organisms, we may, I think, consider the alleged antagonism as proved.* * When, after having held for some years the general doctrine elaborated in these chapters, I agreed, early in 1852, to prepare an outline of it for the West minster Review, I consulted, among other works, the just-issued third edition of Dr. Carpenter's Principles of Physiology, General and Comparative — seeking in it for facts illustrating the different degrees of fertility of different organisms. 1 met with a passage, quoted above in § 339, which seemed tacitly to assert that individual aggrandizement is at variance with the propagation of the race ; but nowhere found a distinct enunciation of this truth. I did not then read the Chapter entitled "General View of the Functions," which held out no promise of such evidence as I was looking for. But on since referring to this chapter, I discovered in it the definite statement that — "there is a certain degree of antagonism between the Nutritive and Reproductive functions, the one being executed at the expense of the other. The reproductive apparatus derives the materials of its operations through the nutritive system, and is entirely dependent upon it for the continuance of its function If, there fore, it be in a state of excessive activity, it will necessarily draw off from the individual fabric some portion of the aliment destined for its maintenance. It may be universally observed that, when the nutritive functions are particularly active in supporting the individual, the reproductive system is in a corresponding degree undeveloped, — and vice versd." -Principles of Fhy~ tiofogy, General and Compar.iiiiu, Third E-litiui , 1351, p. 592. CHAPTER VII. THE ANTAGONISM BETWEEN DEVELOPMENT AND GENESIS, ASEXUAL AND SEXUAL. § 343. By Development, as here to be dealt with apart from Growth, is meant increase of structure as distinguished from increase of mass. As was pointed out in § 50, this is the biological definition of the word. In the following sections, then, we have to note how complexity of organiza tion is hindered by reproductive activity, and conversely. This relation partially coincides with that which we have just contemplated ; for, as was shown in § 44, degree of growth is to a considerable extent dependent on degree of organization. But while the antagonism to be illustrated in this chapter, is much entangled with that illustrated in the last chapter, it may be so far separated as to be identified as an additional antagonism. Besides the direct opposition between that continual dis integration which rapid genesis implies, and the fulfilment of that pre-requisite to extensive organization — the formation of an extensive aggregate, there is an indirect opposition which we may recognize under several aspects. The change from homogeneity to heterogeneity takes time ; and time taken in transforming a relatively-structureless mass into a de veloped individual, delays the period of reproduction. Usually this time is merged in that taken for growth ; but certain cases of metamorphosis show us the one separate from the DEVELOPMENT AND GENESIS. 44i other. An insect, passing from its lowly- organized cater pillar-stage into that of chrysalis, is afterwards a week, a fort night, or a longer period in completing its structure : the re commencement of genesis being by so much postponed, and the rate of multiplication therefore diminished. Further, that re- arrangement of substance which development implies, en tails expenditure. The chrysalis loses weight in the course of its transformation ; and that its loss is not loss of water only, may be inferred from the fact that it respires, and that respiration indicates consumption. Clearly the matter con sumed, is, other things equal, a deduction from the surplus that may go to reproduction. Yet again, the more widely and completely an organic mass becomes diffe rentiated, the smaller the portion of it which retains the re- latively-undifl'erentiated state that admits of being moulded into new individuals, or the germs of them. Protoplasm which has become specialized tissue, cannot be again generalized, and afterwards transformed into something else ; and hence the progress of structure in an organism, by diminishing the unstructured part, diminishes the amount available for making offspring. It is true that higher structure, like greater growth, may insure to a species advantages that eventually further its mul tiplication — may give it access to larger supplies of food, or enable it to obtain food more economically ; and we shall hereafter see how the inverse variation we are considering is thus qualified. But here we are concerned only with the necessary and direct effects; not with those that are con tingent and remote. These necessary and direct effects wo will now look at as exemplified. § 344. Speaking generally, the simpler plants propagate both sexually and asexually ; and, speaking comparatively, the complex plants propagate only sexually : their asexual propagation is usually incomplete — produces a united aggre gate of individuals instead of numerous distinct individuals. 442 LAWS OF MULTIPLICATION. The Protophytes that perpetually subdivide, the merely- cellular Algce that shed their tetraspores, the Acrogens that spontaneously separate their fronds and drop their gemmae, show us an extra mode of multiplication which, among flower ing plants, is exceptional. This extra mode of multiplication among these simpler plants, is made easy by their low de velopment. Tetraspores arise only where the frond consists of untransformed cells ; gemmae bud out and drop off only where the tissue is comparatively homogeneous. Should it be said that this is but another aspect of the antagonism already set forth, since these undeveloped forms are also the smaller forms ; the reply is that though in part true, this is not wholly true. Various marine Algce which propagate asexually, are larger than some Phaenogams which do not thus propagate. The objection that difference of medium vitiates this comparison, is met by the fact that it is the same among land -plants themselves. Sundry of the lowly- organized Liverworts that are habitually gemmiparous, exceed in size many flowering plants. And the Ferris show us agamic multiplication occurring in plants which, while they are inferior in complexity of structure, are superior in bulk to a great proportion of annual Endogens and Exogens. § 345. In the ability of the lowly-organized, or almost unorganized, sarcode of a Sponge, to transform itself into multitudes of gemmules, we have an instance of this same direct relation in the animal kingdom. Moreover, the instance yields very distinct proof of an antagonism between development and genesis, independent of the antagonism between growth and genesis ; for the Sponge which thus multiplies itself asexually, as well as sexually, is far larger than hosts of more complex animals which do not multiply asexually. Once again may be cited the creature so often brought in evidence, the Hydra, as showing us how rapidity of agamic- propagation is associated with inferiority of structure. Its DEVELOPMENT AND GENESIS. 413 power to produce young ones from nearly all parts of its body, is due to the comparative homogeneity of its body. In kindred but more-organized types, the gemmiparity is greatly restricted, or disappears. Among the free-swimming Ilydrazoa, multiplication by budding, when it occurs at all, occurs only at special places. That increase of structure apart from increase of size, is here a cause of dec-lining agamo- genesis, we may see in the contrast between the simple and the compound Hydroida ; which last, along with more- differentiated tissues, show us a gemmation which does not go on all over the body of each polype, and much of which does not end in separation. It is, however, among the Annuhsa that progressing organization is most conspicuously operative in diminishing agamogenesis. The segments or " somites" that compose an animal belonging to this class, are primordially alike ; and, as before argued (§§ 205-7), are probably the hornologues of what were originally independent individuals. The progress from the lower to the higher types of the class, is at once a progress towards types in which the strings of segments cease •to undergo subdivision, and towards types in which the seg ments, no longer alike in their structures and functions, have become physiologically integrated or mutually dependent. Already this group of cases has been named as illustrating the antagonism between growth and asexual genesis ; but it is proper also to name it here ; since, on the one hand, the greater size due to the ceasing of fission, is made possible only by the specialization of parts and the development of a co ordinating apparatus to combine their actions, and since, on the other hand, specialization and co-ordination can advance only in proportion as fission ceases. § 346. The inverse variation of development and sexual genesis is by no means easy to follow. One or two facts indi cative of it may, however, be named. Phacnogams that have but little supporting tissue may 444 LAWS OF MULTIPLICATION. Fairly be classed as structurally inferior to those provided with stems formed of woody fibres ; for these imply additional dif ferentiations, and constitute wider departures from the primi tive type of vegetal tissue That the concomitant of this higher organization is a slower gamogenesis, scarcely needs pointing out. While the herbaceous annual is blossoming and ripening seed, the young tree is transforming its ori ginally-succulent axis into dense fibrous substance ; and year by year the young tree expends in doing the like, nutriment which successive generations of the annual expend in fruit. Here the inverse relation is between sexual reproduction and complexity, and not between sexual reproduction and bulk seeing that besides seeding, the annual often grows to a size greater than that reached by the young infertile tree in several years. Proof of the antagonism between complexity and gamo genesis in animals, is still more difficult to disentangle. Per haps the evidence most to the point is furnished by the contrast between Man and certain other Mammals approaching to him in mass. To compare him with the domestic Sheep, which, though not very unlike in size, is relatively prolific, is objec- jectionable because of the relative inactivity of Sheep ; and this, too, may be alleged as a reason why the Ox, though far more bulky, is also far more fertile, than Man. Further, against a comparison with the Horse, which, while both larger and more prolific, is tolerably active, it may be urged that, in his case, and the cases of herbivorous creatures generally, the small exertion required to procure food, joined with the great ratio borne by the assimilative organs to the organs they have to build up and repair, vitiates the result. We may, however, fairly draw a parallel between Man and a large-carnivore. The Lion, superior in size, and perhaps equal in activity, has a digestive system not proportionately greater ; and yet has a higher rate of multiplication than Man. Here the only de cided want of parity, besides that of organization, is that of food. Possibly a carnivore gains an advantage in having a DEVELOPMENT AND GENESIS. 445 surplus nutriment consisting almost wholly of those nitro genous materials from which the bodies of young ones are mainly formed. But, allowing for all other differences, it appears not improbable that the smallness of human fertility compared with the fertility of large feline animals, is due to the greater complexity of the human organization — more especially the organization of the nervous system. Taking degree of nervous organization as the chief correlative of mental capacity ; and remembering the physiological cost of that discipline whereby high mental capacity is reached ; we may suspect that nervous organization is very expensive : the inference being that bringing it up to the level it reaches in Man, whose digestive system, by no means large, has at the same time to supply materials for general growth and daily waste, involves a great retardation of maturity and sexual genesis. CHAPTER VIII. ANTAGONISM BETWEEN EXPENDITURE AND GENESIS. § 347. Under this head we have to set down no evidence derived from the vegetal kingdom. Plants are not expenders of force in such degrees as to affect the general relations with which we are dealing. They have not to maintain a heat above that of their environment ; nor have they to generate motion ; and hence consumption for these two purposes does not diminish the stock of material that serves on the one hand for growth and on the other hand for propagation. It will be well, too, if we pass over the lower animals : especially those aquatic ones which, being nearly of the same temperature as the water, and nearly of the same specific gravity, lose but little in evolving motion, sensible and insensible. A further reason for excluding from con sideration these inferior types, is, that we do not know enough of their rates of genesis to permit of our making, with any satisfaction, those involved comparisons here to be entered upon. The facts on which we must mainly depend are those to be gathered from terrestrial animals ; and chiefly from those higher classes of them which are at the same time great expenders and have rates of multiplication about which our knowledge is tolerably definite. We will restrict ourselves, then, to the evidence which Birds and Mammals supply § 348. Satisfactory proof that loss of substance in the EXPEND1TTJHE AND GENESIS. 44 T maintenance of heat diminishes the rapidity of propagation, is difficult to obtain. It is, indeed, obvious that the warm blooded Vertelrata are less prolific than the cold-blooded ; but then they are at the same time more vivacious. Similarly, between Mammals and Birds (which are the warmer-blooded of the two) there is, other things equal, a parallel, though much smaller, difference ; but here, too, the unlikenesses of muscular action complicate the evidence. Again, the annual return of generative activity has an average correspondence with the annual return of a warmer season, which, did it stand alone, might be taken as evidence that a diminished cost of heat-maintenance leads to such a surplus as makes reproduction possible. But thenx this periodic rise of tem perature is habitually accompanied by an increase in the quantity of food — a factor of equal or greater importance. We must be content, therefore, with such few special facts as admit of being disentangled. Certain of these we are introduced to by the general rela tion last named — the habitual recurrence of genesis with the recurrence of spring. For in some cases a domesticated crea ture has its supplies of food almost equalized ; and hence the effect of varying nutrition may be in great part eliminated from the comparison. The common Fowl yields an illustra tion. It is fed through the cold months, but nevertheless, in mid- winter, it either wholly leaves off laying or lays very sparingly. And then we have the further evidence that if it lays sparingly, it does so only on condition that the heat, as well as the food, is artificially maintained. Hens lay in cold weather only when they are kept warm. To which fact may be added the kindred one that " when pigeons receive arti ficial heat, they not only continue to hatch longer in autumn, \>ut will recommence in spring sooner than they would other wise do." An analogous piece of evidence is that, in winter, inadequately-sheltered Cows either cease to give milk or give it in diminished quantity. For though giving milk is not the same thing as bearing a young one, yet, ay milk 61 LAWS OF MULT1PLICATIOX. is part of the material from which a young one ia built up, it is part of the outlay for reproductive purposes, and diminu tion of it is a loss of reproductive power. Indeed the case aptly illustrates, under another aspect, the struggle between self-preservation and race-preservation. Maintenance of the cow's life depends on maintenance of its heat; and main tenance of its heat may entail such reduction in the supply of milk as to cause the death of the calf. Evidence derived from the habits of the same or allied genera in different clitnates, may naturall}r be looked for ; but it is difficult to get, and it can scarcely be expected that the remaining conditions of existence will be so far similar as to allow of a fair comparison being made. The only illustrative facts I have met with which seem noteworthy, are some named by Mr. Gould in his work on The Birds of Australia. He says: — " I must not omit to mention, too, the extraordinary fecundity which prevails in Australia, many of its smaller birds breeding three or four times in a season ; but laying fewer eggs in the early spring when insect life is less developed, and a greater number later in the season, when the supply of insect food has become more abundant. I have also some reason to believe that the young of many species breed during the first season, for among others, I frequently found one section of the Honey-eaters (the Melithrepti) sitting upon eggs while still clothed in the brown dress of immaturity ; and we know that such is the case with the introduced Gallinacece (or poultry) three or four generations of which have been often produced in the course of a year. " Though here Mr. Gould refers only to variation in the quantity of food as a cause of variation in the rate of multiplication, may we not suspect that the warmth is r part- cause of the high rate which he describes as general? § 349. Of the inverse variation between activity and genesis, we get clear proof. Let us begin with that which Birds furnish. EXPENDITURE AND GENESIS. 446 First we have the average contrast, already hinted, between the fertility of Birds and the fertility of Mammals. Compar ing the large with the large and the small with the small, we see that creatures which continually go through the muscular exertion of sustaining themselves in the air and propelling themselves rapidly through it, are less prolific than creatun g of equal weights which go though the smaller exertion oi moving about over solid surfaces. Predatory Birds have fewer .young ones than predator}7 Mammals of approximately the same sizes. If we compare Rooks with Rats, or Finches with Mice, we find like differences. And these differences are greater than at first appears. For whereas among Mammals a mother is able, unaided, to bear and suckle and rear half way to maturity, a brood that probably weighs more in pro portion than does the brood of a Bird ; a Bird, or at least a Bird that flies much, is unable to do this. Both parents have to help ; and this indicates that the margin for reproduction in each adult individual is smaller. Among Birds themselves occur contrasts which may be next considered. In the Raptorial class, various species of which, differing in their sizes, are similarly active in their habits, we see that the small are more prolific than the large. The Golden Eagle has usually 2 eggs: sometimes only 1. As we descend to the Kites and Falcons, the number is 2 or or 3, and 3 or 4. And when wre come to the Sparrow-Hawk, 3 to 5 is the specified number. Similarly among the Owls : while the Great Eagle-Owl has 2 or 3 eggs, the comparatively small Common Owl has 4 or 5. As before hinted, it is im possible to say what proportions of these differences are due to unlikenesses of bulk merely, and what proportions are duo to unlikenesses in the costs of locomotion. But we may fairly assume that the unlikenesses in the costs of locomotion are here the more important factors. Weights varying as the cubes of the dimensions, while muscular powers vary as the squares, the expense of flight increases more rapidly than the size increases ; and as motion through the air requires more i50 LAWS OF MULTIPLICATION. effort than motion on the ground, this geometric.il progression tells more rapidly on Birds than on Mammals, lie this as it may, however, these contrasts support the argument ; as do various others that may be set down. The Finch family, for example, have broods averaging about 5 in number, and havo eommonly 2 broods in the season ; while in the Crow farnilv the number of the brood is on the average less, and there is but one brood in a season. And then on descending to such small birds as the Wrens and the Tits, we have 8, 10, 12 to 35 eggs, and often two broods in the year. One of the best illustrations is furnished by the Swallow-tribe, throughout which there is little or no difference in mode of life or in food. The Sand-Martin, much the least of them, has usually G eggs ; the Swallow, somewhat larger, has 4 or 5 ; and the Swift, larger still, has but 2. Here we see a lower fertility associated in part with greater size, but associated still more con spicuously with greater expenditure. For the difference of fertility is more than proportionate to the difference of bulk, as shown in other cases ; and for this greater difference then is the reason, that the Swift has to support not only the cosi of propelling its larger mass through the air, but also the cost of propelling it at a higher velocity. Omitting much evidence of like nature, let us note that disclosed by comparisons of certain groups of birds with other groups. "Skulkers "is the descriptive title applied to thy Water-Rail, the Corn-Crake, and their allies, which evade enemies by concealment — consequently expending but little in locomotion. These birds have relatively large broods — 6 to 11, 8 to 12, &c. Not less instructive are the contrasts be tween the Gallinaceous Birds and other Birds of like sizes but more active habits. The Partridge and the Wood-Pigeon are about equal in bulk, and have much the same food. Yet while the one has from 10 to 15 young ones, the other has but 2 young ones twice a-year: its annual reproduction is but one- third. It may be said that the ability of the Partridge to bring up so large a brood, is due to that habit of its tribe EXPENDITURE AND GENESIS. 451 which one of its names, " Scrapers," describes ; and to the accompanying habit of the young, which begin to get their own living as soon as they are hatched : so saving the parents' labour. Conversely, it may be sail that the inability of the Pigeon to rear more than 2 at a time, is caused by the necessity of fetching everything they eat. But the alleged relation holds nevertheless. On the one hand, a great part of the food which the Partridge chicks pick dp, is food which, in their absence, the mother would have picked up : though each chick costs her far less than a young Pigeon costs its parents, yet the whole of her chicks cost her a great deal in the shape of abstinence — an abstinence she can bear because she has to fly but little. On the other hand, the Pigeon's habit of laying and hatching but two eggs, must not be referred to any fore seen necessity of going through so much labour in supporting the young, but to a constitutional tendency established by such labour. This is proved by the curious fact that when do mesticated, and saved from such labour by artificial feeding, Pigeons, says Macgillivray, " are frequently seen sitting on eggs long before the former brood is able to leave the nest, so that the parent bird has at the same time young birds and eggs to take care of." oe> § 350. Made to illustrate the effect of activity on fertility, most comparisons among Mammals are objectionable: other cir cumstances are not equal. A few, however, escape this criticism. One is that bet \veen the Hare and the Rabbit. These are closely-allied species of the same genus, similar in their diet but unlike in their expenditures for locomotion. The rela tively-inert Rabbit has 5 to 8 young ones in a litter, and several litters a-year ; while the relatively-active Hare has but 2 to 5 in a litter. This is not all. The Rabbit begins to breed at six months old; but a year elapses before the Hare begins to breed. These two factors compounded, result in a difference of fertility far greater than can be ascribed to unlikeness of the two creatures in size. 452 LAWS OF MULTIPLICATION. Perhaps the most striking piece of evidence which Mam mals furnish, is the extreme infertility of our common Bat. The Cheiroptera and the Rodentia are very similar in their internal structures. Diversity of constitution, therefore, cannot vitiate the comparison between Bats and Mice, which are about the same in size. Though their diets differ, the difference is in favour of the Bat : its food being exclusively animal while that of the Mouse is mainly vegetal. What now are their respective rates of genesis ? The Mouse pro duces many young at a time, reaching even 10 or 12 ; while the Bat produces only one at a time. Whether the Bat repeats its one more frequently than the Mouse repeats its ten is not stated ; but it is quite certain that even if it does so, the more frequent repetition cannot be such as to raise its fertility to anything like that of the Mouse. And this relatively- low rate of multiplication we may fairly ascribe to its relatively- high rate of expenditure. Here let us note, in passing, an interesting example of the way in which a species that has no specially- great power of self-preservation, while its power of multiplication is extremely small, nevertheless avoids extinction because it has to meet an unusually-small total of race-destroying forces. Leaving out parasites, the only enemy of the Bat is the Owl ; and tho Owl is sparingly distributed. § 351. These general evidences may be enforced by some special evidences. We have few opportunities of observing how, within the same species, variations of expenditure are related to variations of fertility. But a fact or two showing the connexion may be named. Doctor Duncan quotes a statement to the point respecting the breeding of dogs. Already in § 341 I have extracted a part of this statement, to the effect that before her growth is com plete, a bitch bears at a birth fewer puppies than when she becomes full-grown. An accompanying allegation is, that her declining vigour is shown bv a decrease in the number of c5 O *f EXPENDITURE AND GENESIS. 453 puppies contained in a litter, " ending in one or two." And then it is further alleged that, " as regards the amount of work a dog has to perform, so will the decline be rapid or gradual ; and hence, if a bitch is worked hard year after year, she will fail rapidly, and the diminution of her puppies will be accordingly ; but if worked moderately and well kept, she \riil fail gradually, and the diminution will be less rapid." Tn this place, more fitly than elsewhere, may be added a fact of like implication, though of a different order. Of course whether excessive expenditure be in the continual repairs oi' nervo-muscular tissues or in replacing other tissues, the re active effects, if not quite the same, will be similar — there will be a decrease of the surplus available for genesis. If, then, in any animals there from time to time occur unusual outlays for self- maintenance, we may expect the periods of such outlays to be periods of diminished or arrested repro duction. That they are so the moulting of birds shows us. When hens begin to moult they cease to lay. While they are expending so much in producing new clothing, they viuve nothing to expend for producing eggs. CHAPTER IX. COINCIDENCE BETWEEN HIGH NUTRITION AND GENESIS. § 352. Under this head may be grouped various facts which, in another way, tell the same tale as those contained in the last chapter. The evidence there put together went to show that increased cost of self-maintenance entailed de creased power of propagation. The evidence to be set down here, will go to show that power of propagation is augmented by making self- maintenance unusually easy. For into this may be translated the effect of abundant food. To put the proposition more specifically — we have seen that after individual growth, development, and daily con sumption have been provided for, the surplus nutriment measures the rate of multiplication. This surplus may be raised in amount by such changes in the environment as bring a larger supply of the materials or forces on which both parental life and the lives of offspring depend. Be there, or be there not, any expenditure, a higher nutrition will make possible a greater propagation. We may expect this to hold both of agamogenesis and of gamogenesis ; and we shall find that it does so. § 353. On multi-axial plants, the primary effect of surplus nutriment is a production of large and numerous leaf-shoots. How this asexual multiplication results from excessive nutri tion, is well shown when the leading axis, or a chief branch, is broken off towards its extremity. The axillary buds below NUTRITION AND GENESIS. the breakage quickly swell and burst into lateral shoots, which often put forth secondary shoots : two generations of agamic individuals arise where there probably would have been none but for the local abundance of sap, no longer drawn off. In like manner the abnormal againogenesis which we have in proliferous flowers, is habitually accompanied by a general luxuriance, implying an unusual plethora. JNTo less conclusive is the evidence iurnished by agamo- genesis in animals. Sir John Dalyell, speaking of Hydra tula, whose peculiar metagenesis he was the first to point out, says — " It is singular how much propagation is promoted by abundant sustenance." This Polype goes on budding-out young polypes from its sides, with a rapidity proportionate to the supply oi materials. iSo, too, is it with the agamic reproduction of the Apliis. As cited by Professor Huxley, Kyber " states that he raised viviparous broods of both this species (Aphis DianthiJ and A. llosce for four consecutive years, without any intervention of males or oviparous females, and that the energy of the power of agamic reproduction was at the end of that period undiminished. 'I he rapidity of the agamic prolification throughout the whole period was directly proportional to the amount of warmth and food supplied/' In these cases the relation is not appreciably complicated by expenditure. The parent having reached its limit of growth, the absorbed food goes to asexual multiplication : scarcely any being deducted for the maintenance of parental life. § 354. The sexual multiplication of organisms under changed conditions, undergoes variations conforming to a parallel law. Cultivated plants and domesticated animals yield us proof of this. Facts showing that in cultivated plants, sexual genesis in creases with nutrition, are obscured by facts showing that a less rapid asexual genesis, and an incipient sexual genesis, ac company the fall from a high to a moderate nutrition. The confounding of these two relations has led to mistaken infer- 456 LAWS OF MULTIPLICATION. ences. When treating of Genesis inductively, we reached the generalization that " the products of a fertilized germ go on accumulating by simple growth, so long as the forces whence growth results are greatly in excess of the antagonist f>rces; but that when diminution of the one set of forces, or increase of the other, causes a considerable decline in this ex cess, and an approach towards equilibrium, fertilized germs are again produced/' (§ 78.) It was pointed out that this holds of organisms which multiply by heterogenesis, as well as those which multiply by homogenesis. And plants were referred to as illustrating, both generally and locally, the decline of agamic multiplication and commencement of gamic multiplication, along with a lessening rate of nutrition. JVow the many cases that are given of fruitfulness caused in trees by depletion, are really cases of this change from agamogenesis to gamogenesis ; and simply go to prove that what would naturally arise when decreased peripheral growth had followed increased size, may be brought about artificially by diminishing the supply of materials for growth. Cramp ing its roots in a pot, or cutting them, or ringing its branches, will make a tree bear very early : bringing about a pre mature establishment of that relative innutrition which would have spontaneously arisen in course of time. Such facts by no means show that in plants, sexual genesis in creases as nutrition diminishes. When it has once set in, sexual genesis is scanty or imperfect unless nutrition is good. Though the starved plant may blossom, yet many of its blossoms will fail ; and such seeds as it produces will be ill- furnished with those enveloping structures and that store of albumen, &c., needed to give good chances of successful germi nation — the number of surviving offspring will be diminished. Were it otherwise, the manuring of fields that are to bear seed-crops, would be not simply useless but injurious. Were it otherwise, dunging the roots of a fruit-tree would in all cases be impolitic ; instead of being impolitic only where the growth of sexless axes is still luxuriant. Were it otherwise, NUTRITION AND GENESIS. 457 a tree which has borne a heavy crop, should, by the con sequent depletion, be led to bear a still heavier crop next year ; whereas it is apt to be wholly or partially barren next year — -has to recover a state of tolerably -high nutrition before its sexual genesis again becomes large. But the best illustrations are those yielded by animals, in which we have, besides an increased supply of nutriment, a diminished expenditure. Two classes of comparisons, alike in their implications, may be made — comparisons between tame and wild animals of the same species or genus, and com parisons between tame animals of the same species differently treated. To begin with Birds, let us first contrast the farm-yard Gallinacece with their kindred of the fields and woods. Not withstanding their greater size, which, other things equal, should be accompanied by smaller fertility, the domesticated kinds have more numerous offspring than the wild kinds. A Turkey has a dozen in a brood, while a Pheasant has from 6 to 10. Twice or thrice in a season, a Hen rears as many chickens as a Partridge rears once in a season. Anserine birds show us parallel differences. The Tame Goose sits on 12 or more eggs, but the Wild Goose sits on 5, 6, or 7 ; and these are noted as considerably smaller. It is the same with Ducks : the domesticated variety lays and hatches twice as many eggs as the wild variety. And the like holds of Pigeons. After remarking of the Columba lioia that "in spring when they have plenty of corn to pick from the newly-sown fields, they begin to get fat and pair ; and again, in harvest, when the corn is cut down, " Macgillivray goes on to say, that " the same pair when tamed generally breed four times" in the year. That between different poultry-yards, in equalities of fertility are caused by inequalities in the supplies of food, is a familiar truth. High feeding shows its effects not unly in the continuous laying, but also in the sizes of the eggs. Among directions given for obtaining eggs from pullets late in the year, it is especially insisted on that they 458 LAWS OF MULTIPLICATION. shall have a generous diet. Respecting Pigeons Macgillivrav writes : — " that their breeding depends much on their having plenty of food to fatten them, seems, I think, evident from the circumstance that, when tamed, which they easily are, they are observed to breed in every month of the year. I do not mean that the same pair will breed every month; but some in the flock, if well fed, will breed at any season." There may be added a fact of like meaning which partially-domesticated birds yield. The Sparrow is one of the Finch tribe that has taken to the neighbourhood of houses ; and by its boldness secures food not available to its conveners. The result is that it has several broods in a sea- o son, while its field-haunting kindred have none of them more than two broods, and some have only one. Equallv clear proof that abundant nutriment raises the rate of multiplication, occurs among Mammals. Compare the litters of the Dog with the litters of the Wolf and the Fox. Whereas those of the one range in number from 6 to 14, the others contain respectively 5 or 6 or occasionally 7, and 4 or 5 or rarely 6 Again, the wild Cat has 4 or 5 kittens ; but the tame Cat has 5 or 6 kittens 2 or 3 times a-yt>ar. So, too, is it with the Weasel tribe. The Stoat has 5 young ones once a-year. The Ferret has 2 litters yearly, each containing from 6 to 9 ; and this notwithstanding that it is the larger of the two. Perhaps the most striking contrast is that between the wild and tame varieties of the Pig. While the one produces, according to its age, from 4 to 8 or 10 young ones, once a year, the other produces sometimes as many as 17 in a litter ; or, in other cases, will bring up 5 litters of 10 each in two years — a fate of reproduc tion that is unparalleled in animals of as large a size. And let us not omit to note that this excessive fertility occurs where there is the greatest inactivity — where there is plenty lo eat and nothing to do. There is no less distinct evidence that among domesticated Mammals themselves, the well-fed individuals are more prolific than NUTRITION AND GENESIS. 459 the ill-fed individuals. On the high and comparatively- infertile Cotswolds, it is unusual for Ewes to Lave twins ; but they very commonly have twins in the adjacent rich valley of the Severn. Similarly, among the barren hills of the west of Scotland, two lambs will be borne by about one Ewe in twenty ; whereas in England, something like one Ewe in three will bear two lambs. Nay, in rich pastures, twins are more frequent than single births ; and it occasionally happens that, after a genial autumn and consequent good grazing, a flock of Ewes will next spring yield double their number of jambs — the triplets balancing the uniparae. So direct is this relation, that I have heard a farmer assert his ability to fore tell, from the high, medium, or low, condition of an Ewe in the autumn, whether she will next spring bear two, or one, or none. § 355. An objection must here be met. Many facts may be brought to prove that fatness is not accompanied by ferti lity but by barrenness ; and the inference drawn is that high feeding is unfavourable to genesis. The premiss may be admitted while the conclusion is denied. There is a distinction between what may be called normal plethora, and an abnormal plethora, liable to be confounded with it. The one is a mark of constitutional wealth ; but the other is a mark of constitutional poverty. Normal plethora is a superfluity of materials both for the building up of tissue and the evolution of force ; and this is the plethora which we have found to be associated with unusual fecundity. Abnormal plethora, which, as truly alleged, is accompanied by infecundity, is a superfluity of force-evolving materials joined with either a positive or a relative deficiency of tissue- forming materials : the increased bulk indicating this state, being really the bulk of so much inert or dead matter. Note, first, a few of the facts which show us that obesity implies physiological impoverishment. Neither in brutes nor men does it ordinarily occur either 460 LAWS OF MULTIPLICATION. in youth or in that early maturity during which the vigour is the greatest and the digestion the best : it does not habitually accompany the highest power of taking up nutri tive materials. When fatness arises in the prime of life, whether from peculiarity of food or other circumstance, it is not the sign of an increased total vitality. On the contrary, if great muscular action has to be gone through, the fat must be got rid of — either, as in a man, by training, or as in a horse that has grown bulky while out at grass, by putting him on such more nutritive diet as oats. The frequency of senile fatness, both in domesticated creatures and in ourselves, has a similar implication. Whether we consider the smaller ability of those who display it to with stand large demands on their powers, or whether we consider the comparatively-inferior digestion common among them, we see that the increased size indicates, not an abundance of materials which the organism requires, but an abundance of materials which it does not require. Of like mean ing is the fact that women who have had several children, and animals after they have gone on bearing young for some time, frequently become fat ; and lose their fecundity as they do this. In such cases, the fatness is not to be taken as the cause of the infecundity ; but the constitutional ex haustion which the previous production of offspring has left, shows itself at once in the failing fecundity and the com mencing fatness. There is yet another kind of evidence. Obesity not uncommonly sets in after the system has been subject to debilitating influences. Often a serious illness is followed by a corpulence to which there was previously no tendency. And the prolonged administration of mercury, con • stitutionally injurious as it is, sometimes produces a like effect. Closer inquiry verifies the conclusion to which these facts point. The microscope shows that along with the increase of bulk common in advanced life, there goes on what is called " fatty degeneration :" oil- globules are deposited where there should be particles of flesh — or rather, we may say, the hydro- NUTRITION AMD GENESIS. 461 carbonaceous molecules locally produced by decomposition of the nitrogenous molecules, have not been replaced by other nitrogenous molecules, as they should have been. This fatty degeneration is, indeed, a kind of local death. For so regard ing it we have not simply the reason that an active substance has its place occupied by an inert substance ; but we have the reason that the flesh of dead bodies, under certain conditions, is transformed into a fatty matter called adipocere. The infertility that accompanies fatness in domestic animals, has, however, othsr causes than that declining constitutional vigour which the fatness indicates. Being artificially fed, these animals cannot always obtain what their systems need. That which is given to them is often given expressly because of its fattening quality. And since the capacity of the digestive apparatus remains the same, the absorption of fat- producing materials in excess, implies defect in the absorption of ma terials from which the tissues are formed, and out of which young ones are built up. Moreover, this special feeding with a view to rapid and early fattening, continued as it is through generations, and accompanied as it is by a selection of individuals and varieties which fatten most readily, tends to establish a modified constitution, more fitted for producing fat and correspondingly-less fitted for producing flesh — a constitution which, from this relatively-deficient ab sorption of nitrogenous matters, is likely to become infertile ; as, indeed, these varieties generally become. Hence, no conclusions respecting the effects of high nutrition, pro perly so called, can be d/awn from cases of this kind. The cases are, in truth, of a kind that could not exist but for human agency. Under natural conditions no animal would diet itself in the way required to produce such results. And if it did, its race would quickly disappear.* * It is worth while inquiring whether unfitness of the food given to them, ir not the chief cause of that sterility which, as Mr. Darwin says, " is the great bar to the domestication of animals." He remarks that "when animals and i'lants are removed from their natural conditions, they are extremely liable to 4.62 LAWS OF MULTIPLICATION. There is yet another mode in which accumulation of fat diminishes fertility. Even supposing it unaccompanied by a smaller absorption of nitrogenous materials, it is still a cause of lessening the surplus of nitrogenous materials. For the repair of the motor tissues becomes more costly. Fat stored-up is weight to be carried. A creature loaded with inert matter must, other things equal, consume a greater amount of tissue-forming substances for keeping its loco motive apparatus in ord'^r ; and thus expending more for self- maintenance can expend less for race-maintenance. Abnormal plethora is thus antagonistic to reproduction in a double way. It ordinarily implies a smaller absorption of tissue-forming matters, and an increased demand on the diminished supply. Hence fertility decreases in a geometrical progression. The counter-conclusion drawn from facts of this class, is, then, due to a misconception of their nature — a misconception arising partly from the circumstance that the increase of bulk produced by fat is somewhat like the increase of bulk which growth of tissues causes, and partly from the circumstance that abundance of good food normally produces a certain quantity of fat, which, within narrow limits, is a valuable store of force -evolving material. When, however, we limit the phrase high nutrition to its proper meaning — an abun dance of, and due proportion among, all the substances which the organism needs — we find that, other things equal, fertility always increases as nutrition increases. And we see that these apparently-exceptional cases, are cases that really show us the same thing ; since they are cases of relative innutrition. h*,ve their reproductive systems seriously affected." Possibly the relative or absolute arrest of genesis, is less due to a direct effect on the reproductive sys tem, than to a changed nutrition of which the reproductive system most clearly shows the results. The matters required for forming an embryo are in a greater proportion nitrogenous than are the matters required for maintain ing an adult. Hence, an animal forced to live on insuffieiently-nitrogenized food, may have its surplus for reproduction cut off, but still have a sufficiency to keep its own tissues in repair, and appear to be in good health — meanwhile increasing in bulk from excess of the non-nitrogencis matters it eats. CHAPTER X. SPECIALITIES OF THESE RELATIONS. S 356. Tests of the general doctrines set fortli in preceding chapters, are afforded by organisms having modes of life that diverge widely from ordinary modes. Here, as elsewhere, aberrant cases yield crucial proofs. If certain organisms are so circumstanced that highly- nutritive matter is supplied to them without stint, and they have nothing to do but absorb it, we may infer that their powers of propagation will be enormous. If there are classes of creatures that expend very little for self-support in comparison with allied creatures, a relatively extreme prolificness may be expected of them. Or if, again, we find species presenting the peculiarity that while some of their individuals have much to do and little to eat, others of their individuals have much to eat and little to do, we may look for great fertility in these last and comparative infertility or barrenness in the first. These several anticipations we shall find completely verified. § 357. Plants which, like the Rafflosiacece, carry their para- Bitism to the extent of living on the juices they absorb from other plants, exhibit one of these relations in the vegetal kingdom. In them the organs for self-support being need less, are rudimentary ; and the parts directly or indirectly 461 LAWS OF MULTIPLICATION concerned in the production and distribution of germs, con stitute the mass of the organism. That small ratio which the race-preserving structures bear to the self-preserving structures in ordinary Phaenogams, is, in these Phrenogams, inverted. A like relation occurs in the common Dodder. There may be added a kindred piece of evidence which the Fungi present. Those of them which grow on living plants, repeat the above connection completely ; and those of them which, though not parasitic, nevertheless subsist on organized materials previously elaborated by other plants, substantially repeat it. The spore-producing part is relatively enormous : and the fertility is far greater than that of Cryptogams of like sizes, which have to form for themselves the organic com pounds of which they and their germs consist. § 358. The same lesson is taught us by animal-parasites. Along with the decreased cost of Individuation, they similarly show us an increased expenditure for Genesis ; and they show us this in the most striking manner where the deviation from ordinary conditions of life is the greatest. Take, among the Epizoa, such an instance as the Nicot/ice. Belonging to the Entomostraca, both males and females of this species are, in their early days, similar to their allies ; and the males continue so throughout life. Each female, however, presently fixes herself on the skin of an aquatic animal, where she sits and sucks its juices, enlarges rapidly, and undergoes an extreme distortion from the growth of the ovaries. These, bulging out from her sides, become lateral sacs, each of which attains something like three times her size ; and then a further distortion is produced by two vast egg-bags, severally larger than herself, which also are formed and become pendant. So that the germ- producing organs and their contents, eventually acquire a total bulk some eight or ten times that of the rest of the body. Numerous species of this type and habit, repeat this relation between a life of in action with high feeding, and an enormous rate of genesis. SPECIALITIES OF THESE RELATIONS. 405 Entozoa yield us many examples of this causal relation raised to a still higher degree. The Gordius, or Hair-worm, is a creature which, finding its way when young into the body of an insect, there grows rapidly, and afterwards emerg ing to breed, lays as many as 8,000,000 eggs in les^ than a day. Similarly with the larger types that infest the higher animals. It has been calculated by Dr. Eschricht, as quoted by Professor Owen, that there are " 64,000,000 of ova in tho mature female Ascaris Lumbricoides. " Even a still greater fertility occurs among the cestoid Entozoa. Immersed as a Tape- worm is in nutritive liquid, which it absorbs through its integument, it requires no digestive apparatus. The room which one would occupy, and the materials it would use up, are therefore available for germ-producing organs, which nearly fill each segment: each segment, sexually complete in itself, is little else than an enormous reproductive system, with just enough of other structures to bind it together. Remembering that the Tape-worm, retaining its hold, con tinues to bud-out such segments as fast as the fully-developed ones are cast off, and goes on doing this as long as the infested individual lives; we see that here, where there is no ex penditure, where the cost of individuation is reduced to tho greatest extent while the nutrition is the highest possible, the degree of fertility reaches its extreme. These Etitozoa yield us further interesting evidence. Of their various species, most if not all undergo passive migration from animal to animal before they become nature. Usually, the form assumed in the body of the first host, is devoid of all that part in which the reproductive structures take their rise ; and this purt grows and develops reproductive structures, only in some predatory animal to which its first host falls a sacrif ce. Occasionally, however, the egg gives origin to the sexua. form in the animal that originally swallowed it, but the development remains incomplete — there is no sexual genesis, no formation of eggs in the rudimentary segments. That these may become fertile, it is needful, as before, for the 466 LAWS OF MULTIPLICATION. containing an 1m ill to be devoured; so that the imperfect Tape worm may find its way into the intestine of a higher animal. -Thus the Bothriocephalus solidtts, found in the abdominal cavity of the Stickleback, is barren while it remains there ; but if the Stickleback is eaten by a Water-fowl, the reproductive system of the transferred Bof/iriocephaliis becomes developed and active. So, too, a kind of Tape- worm which remains infertile while in the intestine of a Mouse, becomes fertile in the in testine of a Cat that devours the mouse. May we not regard these facts as again showing the dependence of fertility on nutrition ? Barrenness here accompanies conditions unfavour able to the absorption of nutriment ; and it gives way to fecundity where nutriment is large in quantity and superior in quality. § 359. Extremely significant are those cases of partial reversion to primitive forms of genesis, that occur under special conditions in some of the higher Annulosri. I refer to the pseudo-parthenogenesis and metagenesis in Insects. Under what conditions do the Aphides exhibit this strange deviation from the habits of their order ? Why among them should imperfect females produce, again ically, others like themselves, generation after generation, with great rapidity? There is the obvious explanation that they get plenty of easilv-assimilated food without exertion. Piercing the tender coats of young shoots, they sit and suck — appropriating the nitrogenous elements of the sap and ejecting its saccharine matter as "honey dew." Along with a sluggishness strongly contrasted with the activity of their allies — along with a very low rate of consumption and a correlative degra dation of structure ; we have hera a retrogression to asexual genesis, and a greatly-increased rate of multiplication. The recentlv- discovered instance of internal metagenesis o in the maggots of certain Flies has a like meaning. In credible as it at first seemed to naturalists, it is now proved that the Cecydomia-larvft develops in its interior a brood of larvoj BPECIALIT1 liS OF THESE RELATIONS. 467 of like structure with itself. In this case, as in the last, abun dant food is combined with low expenditure. These larva) are found in such habitats as the refuse of beet-root-stigar fac tories — masses of nitrogenous debris remaining after the extraction of the saccharine matter. Each larva has a- practically-unlimited supply of sustenance imbedding it on all sides. It is true that some other maggots, as those of the Flesh-fly, are similarly, or still better, circumstanced ; and, it may be said, ought therefore to have the same habit. But this does not necessarily follow. Survival of the fittest will determine whether such specially-favourable conditions result in the aggrandisement of the individual or in the multiplication of the race. And in the case of the Flesh-fly, there is a reason why greater individuation rather than more rapid genesis will occur. For a decomposing animal body lasts so short a time, that were Flesh-fly larvne to multiply agamicaliy, the second generation would die from the disappearance of their food. Hence, individuals in which the excessive nutrition led to internal metagenesis, would leave no posterity ; and natural selection would establish the variety in which greater growth resulted. All which the argument requires is, that when such reversion to agamogenesis does take place, it shall be where the food is unusually abundant and the expenditure unusually small ; and this the cases instanced go to show. § 360. The physiological lesson taught us by Bees and Ants, not quite harmonizing with the moral lesson they are supposed to teach, is that highly-fed idleness is favourable to fertility, and that excessive industry has barrenness for its concomitant. The egg of a Bee develops into a small barren female or into a large fertile female, according to the supply of food given to the larva hatched from it. We here see that the germ-producing action is an overflow of the surplus remain ing after completion of the individual ; and that the lower 468 LAWS OF MULTIPLICATION. feeding which the larva of a working Bee has, results in a dwarfing of the adult and an arrested development of the generative organs. Further, we have the fact that the con dition under which the perfect female, or mother-Bee, goes on, unlike insects in general, laying eggs continuously, is that she has plenty of food brought to her, is kept warm, and goes through no considerable exertion. While, contrariwise, it is to be noted that the infertility of the workers, is asso ciated with the ceaseless labour of bringing materials for the combs and building them, as well as the labour of feeding the queen, the larvae, and themselves. Ants, and especially some of the tropical kinds, show us these relations in an exaggerated form. The differ ence of bulk between the fecund and infecund females is immensely greater. The mother- Ant has the reproductive system so enormously developed, that the remainder of her body is relatively insignificant. Entirely incapable of loco motion, she is unable to deposit her eggs in the places where they are to be hatched ; so that they have to be carried away by the workers as fast as they are extruded. Her life is thus reduced substantially to that of a parasite — an absorption of abundant food supplied gratis, a total absence of expendi ture, and a consequent excessive rate of genesis. " The queen-ant of the African Termites lays 80,000 eggs in twenty- four hours." § 361. It may be needful to say that these exceptional relations cannot be ascribed to the assigned causes acting alone. The extreme fertility which, among parasites and social insects, accompanies extremely high feeding, and an expenditure reduced nearly to zero, presupposes typical struc tures and tendencies of suitable kinds ; and these are not directly accounted for. On creatures otherwise organized, unlimited supplies of food and total inactivity are not fol lowed by such results. There of course requires a consti tution fitted to the special conditions ; and the evolution of SPECIALITIES OF THESE RELATIONS. 469 this cannot be due simply to plethora joined with rest. These cases are given as illustrating the conditions under which extreme exaltations of fertility become possible. Their mean ings, thus limited, are clear, and completely to the point. We see in them that the devotion of nutriment to race-preserva tion, is carried furthest where the cost of self-preservation is reduced to a minimum; and, conversely, that nothing is devoted directly to race- preservation by individuals OIL which falls an excessive expenditure for self-preservation find preservation of other's offspring. CHAPTER XL INTERPRETATION AXD QUALIFICATION. § 36'2. Considering' the difficulties of inductive verification, we have, I think, as clear a correspondence between the d priori and d posteriori conclusions, as can be expected. The manjr factors co-operating to bring about the result in every case, are so variable in their absolute and relative amounts, that we can rarely disentangle the effect of each one; and have usually to be content with qualified inferences. Though in the mass, organisms show us an unmistakable relation between great size and small fertility ; yet special comparisons among them are nearly always partially vitiated by differ ences of structure, differences of nutrition, differences of expenditure. Though it is beyond question that the more complex organisms are the less prolific ; yet as complexity has a certain general connexion with bulk, and in animals with expenditure, we cannot often identify its results as inde pendent of these. And, similarly, though the creatures that waste much matter in producing motion, sensible and insen sible, have lower rates of multiplication than those which waste less ; yet, as the creatures which waste much are generally larger and more complex, we are again met by an obstacle which limits our comparisons, and compels us to accept conclusions less definite than are desirable. Such difficulties arise, however, only when we endeavour, as in foregoing chapters, to prove the inverse variation INTERPRETATION AND QUALIFICATION. 47i between Genesis and each separate element of Individuation — growth development, activity. We are scarcely at all hampered by qualifications when, from contemplating these special relations, we return to the general relation. The antagonism between Individuation and Genesis, is shown by all the facts that have been grouped under each head. "W e have seen that in ascending from the lowest to the highest types, there is a decrease of fertility so great as to be abso lutely inconceivable, and even inexpressible by figures ; and whether the superiority of type consists in relative largeness, in greater complexity, in higher activity, or in some or all of these combined, matters not to the ultimate inference. The broad fact, enough for us here, is that organisms in which the integration and differentiation of matter and motion have been carried furthest, are those in which the rate of multipli cation has fallen lowest. How much of the decline of repro ductive power is due to the greater integration of matter, how much to its greater differentiation, how much to the larger amounts of integrated and differentiated motions gene rated, it may be impossible to say ; and it is not needful to say. These are all elements of a higher degree of life, an augmented ability to maintain the organic equilibrium amid environing actions — an increased power of self-preservation ; and we find their invariable accompaniment to be, a dimi nished expenditure of matter, or motion, or both, in race- preservation. In brief, then, examination of the evidence shows that there does exist that relation which we inferred mmt exist. Arguing from general data, we saw that for the maintenance of a species, the ability to produce offspring must be great, in proportion as the ability of the individuals to contend with destroying forces is small ; and conversely. Arguing from other general data, we saw that, derived as the self-sustain ing and race-sustaining forces are from a common stock of force, it necessarily happens that, other things equal, increase of one involves decrease of the other. And then, turning 472 LAWS OF MULTIPLICATION. to special facts, we have found that this inverse variation is clearly traceable throughout both the animal and vegetal kingdoms. We may therefore set it down as a law, that every higher degree of organic evolution, has for its con comitant a lower degree of that peculiar organic dissolution which is seen in the production of new organisms. § 3G3. Something remains to be said in reply to the in quiry — how is the ratio between Individuation and Genesis established in each case? This inquiry has been but partially answered in the course of the foregoing argument. All specialities of the reproductive process are due to the natural selection of favourable variations. Whether a creature lays a few large eggs or many small ones equal in weight to the few large, is not determined by any physiological neces sity : here the only assignable cause is the survival of varieties in which the matter devoted to reproduction, happens to be divided into portions of such size and number as most to favour multiplication. Whether in any case there are frequent small broods or larger broods at longer intervals, depends wholly on the constitutional peculiarity that has arisen from the dying out of families in which the sizes and intervals of the broods were least suited to the conditions of life. Whether a species of animal produces many offspring of which it takes no care or a few of which it takes much care — that is, whether its reproductive surplus is laid out wholly in germs or partly in germs and partly in labour on their behalf — must have been decided by that moulding of constitution to conditions, slowly effected through the more frequent preservation of descendants from those whose re productive habits were best adapted to the circum stances of the species. Given a certain surplus available for race-preservation, and it is clear that by indirect equilibration only, can there be established the more or less peculiar distribution of this surplus which we see in eacli case. Obviously, too, survival of the fittest INTERPRETATION AND QUALIFICATION. 473 has a share in determining the proportion between the amount of matter that goes to Individuation and the amount that goes to Genesis. Whether the interests of the species are most subserved by a higher evolution of the individual joined with a diminished fertility, or by a lower evolution of the individual joined with an increased fertility, are ques tions ever being experimentally answered. If the more- developed and less-prolific variety has a greater number of survivors, it becomes established and predominant. If, con trariwise, the conlitions of life being simple, the larger or more-organized individuals gain nothing by their greater size or better organization ; then the greater fertility of the less evolved ones, will insure to their descendants an increasing predominance. But direct equilibration all along maintains the limits within which indirect equilibration thus works. The necessary antagonism we have traced, rigidly restricts the changes that natural selection can produce, under given con ditions, in either direction. A greater demand for Individua tion, be it a demand caused by some spontaneous variation or by an adaptive increase of structure and function, inevitably diminishes the supply for Genesis ; and natural selection cannot, other things remaining the same, restore the rate of Genesis while the higher Individuation is maintained. Con versely, survival of the fittest, acting on a species that has, by spontaneous variation or otherwise, become more prolific, cannot again raise its lowered Individuation, so long as every- thing else continues constant. § 364. Here, however, a qualification must be made. It was parenthetically remarked in § 327 that the inverse varia tion between Individuation and Genesis is not exact ; and it was hinted that a slight modification of statement would be requisite at a more advanced stage of the argument. We have now reached the proper place for specifying this modification. 474 LAWS OF MULTIPLICATION. Each increment of evolution entails a decrement of r^ production that is not accurately proportionate, but somewhat less than proportionate. The gain in the one direction is not wholly canceled by a loss in the other direction, but only partially canceled : leaving a margin of profit to the species. Though augmented power of self-maintenance habitually necessitates diminished power of race- propagation, yet the product of the two factors is greater than before ; so that the forces preservative of race become, thereafter, in excess of the forces destructive of race, and the race spreads. We shall soon see why this happens. Each advance in evolution implies an economy. That any increase in bulk, or structure, or activity, may become esta blished, the life of the organism must be to some extent facilitated by the change — the cost of self-support must be, on the average, reduced. If the greater complexity, or the larger size, or the more agile movement, entails on the in dividual an outlay that is not repaid in food more-easily obtained, or danger more-easily escaped ; then the individual will be at a relative disadvantage, and its diminished posterity will disappear. If the extra outlay is but just made good by the extra advantage, the modified individual will not sur vive longer, or leave more descendants, than the unmodified individuals. Consequently, it is only when the expense of greater individuation is out-balanced by a subsequent saving, that it can tend to subserve the preservation of the indi vidual ; or, by implication, the preservation of the race The vital capital invested in the alteration must bring a more than equivalent return. A few instances will show that, whether the change results from direct equilibration or from indirect equilibration, this must happen. Suppose a creature takes to performing some act in an un usual way — leaps where ordinarily its kindred crawl, elude? pursuit by diving instead of, like others of its kind, by swim ming along the surface, escapes by doubling instead of by sheet speed. Clearly, perseverance in the modified habit will, other INTERPRETATION AND QUALIFICATION. 470 things equal, imply that it takes less effort. The creature's sensations will ever prompt desistance from the more laborious course ; and hence a congenital habit is not likely to be diverged from unless an economy of force is achieved by the divergence. Assuming, then, that the new method has no advantage over the old in directly diminishing the chances of death, the establishment of it, and of the structural complications involved, nevertheless implies a physiological gain. Suppose, again, that an animal takes to some abundant food previously refused by its kind. It is likely to persist only if that the comparative ease in obtaining this food, more than compensates for any want of adaptation to its digestive organs ; so that superposed modifications of the digestive organs are likely to arise only when an average economy results. What now must be the influence on the creature's system as a whole? Diminished expenditure in any direction, or increased nutrition however effected, will leave a greater surplus of materials. The animal will be physiologically richer. Part of its augmented wealth will go towards its own greater individuation — its size, or its strength, or both, will increase ; while another part will go towards more active genesis. Just as a state of plethora directly produced enhances fertility ; so will such a state indirectly produced. In another way, the same thing must result from those additions to bulk or complexity or activity that are due to survival of the fittest. Any change which prolongs individual life, will, other things remaining the same, further the pro duction of offspring. Even when it is not, like the foregoing, a means of economizing the forces of the individual, still, if it increases the chances of escaping destruction, it increases the chances of leaving posterity. Any further degree of evolution, therefore, will be so established only where the cost of it is more than repaid ; part of the gain being shown in the lengthened life of the individual, and part in the greater production of other individuals. 476 LAWS OF MULTIPLICATION. We have here the solution of various minor anomalies by which the inverse variation of Individuation and Genesis is obscured. Take as an instance the fertility of the Blackbird as compared with that of the Linnet. Both birds lay five eggs, and both usually have two broods. Yet the Blackbird is far the larger of the two ; and ought, according to the general law, to be much less prolific. What causes this noncon formity ? We shall find an answer in their respective foods and habits. Except during the time that it is rearing its young, the Linnet collects only vegetal food — lives during the winter on the seeds it finds in the fields, or, when hard pressed, picks up around farms; and to obtain this spare diet is continually flying about. The result, if it survives the frost and snow, is a considerable depletion ; and it recovers its condition only after some length of spring weather. The Blackbird, on the other hand, is omnivorous : while it eats grain and fruit when they come in its way, it depends largely on animal food. It cuts to pieces and devours the dew- worms which, morning and evening, it finds on the surface of a lawn, and, even discovering where they are, unearths them ; it swallows slugs, and breaking snail-shells, either with its beak or by hammering them against stones, tears out their tenants; and it eats beetles and larvae. Thus the strength of the Blackbird opens to it a store of good food, much of which is inaccessible to so small and weak a bird as a Linnet — a store especially helpful to it during the cold months, when the hybernating Snails in hedge-bottoms yield it abundant pro vision. The result is that the Blackbird is ready to breed very early in spring ; and is able during the summer to rear a second, and sometimes even a third, brood. Here, then, a higher degree of Individuation secures advantages so great, as to much more than compensate its cost : it is not that the decline of Genesis is less than proportionate to the increase of Individuation, but there is no decline at all. Com parison of the Rat with the Mouse yields a parallel result. Though they differ greatly in size, yet the one is as prolific IN TEKPRETATION AND QUALIFICATION. 477 as the other. This absence of difference cannot be ascribed to their unlike degrees of activity. We must seek its cause in some facility of living secured to the Rat by its greater intel ligence, greater power and courage, greater ability to utilize what it finds. The Rat is notoriously cunning ; and its cunning gives success to its foraging expeditions. It is not, like the Mouse, limited mainly to vegetal food ; but while it eats grain and beans like the Mouse, it also eats flesh and carrion, devours young poultry and eggs. The result is that, without a proportionate increase of expenditure, it gets a far larger supply of nourishment than tho Mouse ; and this rela tive excess of nourishment makes possible a large size without a smaller rate of multiplication. How clearly this is the cause, we see in the contrast between the common Rat and the Water-Rat. While the common Rat has habitually several broods a year of from 10 to 12 each, the Water-Rat, though somewhat smaller, has but 5 or 6 in a brood, and but one brood, or sometimes two broods, a-year. But the Water- Rat lives on vegetal food — lacks all that its bold, sagacious, omnivorous congener, gains from the warmth as well as the abundance which men's habitations yield. The inverse variation of Individuation and Genesis is, therefore, but approximate. Recognizing the truth that every increment of evolution which is appropriate to the circumstances of an organism, brings an advantage somewhat in excess of its cost ; we see the general law, as more strictly stated, to be that Genesis decreases not quite so fast as In dividuation increases. Whether the greater Individuation takes the form of a larger bulk and accompanying access of strength ; whether it be shown in higher speed or agility ; whether it consists in a modification of structure that facilitates some habitual movement, or in a visceral change that helps to utilize better the absorbed aliment ; the ultimate effect is identical. There is either a more economical per formance of the same actions, internal or external, or there is a securing of greater advantages by modified actions, which 478 LAWS OF MULTIPLICATION. cost no .more, or have an increased cost less than the in creased gain. In any case, the result is a greater surplus of vital capital ; part of which goes to the aggrandisement of the individual, ancl part to the formation of new in dividuals. While the higher tide of nutritive matters, everywhere filling the parent-organism, adds to its power of self-maintenance, it also causes a reproductive overflow larger than before. Hence every type that is best adapted to its conditions, which on the average means every higher type, has a rate of multiplication that insures a tendency to predominate. Survival of the fittest, acting alone, is ever replacing in ferior species by superior species. But beyond the longer urvivalj and therefore greater chance of leaving offspring, which superiority gives, we see here another way in which the spread of the superior is insured. Though the more- evol\'3d organism is the less fertile absolutely, it is the more fertile relatively. CHAPTER XII. MULTIPLICATION OF THE HUMAN RACE. § 365. The relative fertility of Man considered as a species, and those changes in Man's fertility which occur under changed conditions, must conform to the laws which we have traced thus far. As a matter of course, the inverse variation between Individuation and Genesis, holds of him as of all other organized beings. His extremely low rate of multipli cation — far below that of all terrestrial Mammals except the Elephant, (which though otherwise less evolved, is, in extent of integration, more evolved) — we shall recognize as the necessary concomitant of his much higher evolution. And the causes of increase or decrease in his fertility, special or general, temporary or permanent, we shall expect to find in those changes of bulk, of structure, or of expenditure, which we have in all other cases seen associated with sucli effects. In the absence of detailed proof that these parallelisms exist, it might suffice to contemplate the several communities between the reproductive function in human beings and other beings. I do not refer simply to the fact that genesis pro ceeds in a similar manner ; but I refer to the similarity of the relation between the generative function and the func tions that have for their joint end the preservation of the individual. In Man, as in other creatures that expend much, genesis commences only when growth and development are declining in rapidity and approaching their termination. Among the higher organisms in general, the reproductive G3 480 LAWS OF MULTIPI 1CATION. activity, continuing during the prime of life, ceases when the vigour declines, leaving a closing period of infertility; and in like manner among ourselves, barrenness supervenes when middle age brings the surplus vitality to an end. So, too, it is found that in Man, as in beings of lower orders, there is a period at which fecundity culminates. In § 341, facts were cited showing that at the commencement of the reproductive period, animals bear fewer offspring than afterwards ; and that towards the close of the reproductive period, there is a decrease in the number produced. In like manner it is shown by the tables of Dr. Duncan's recent work, that the fecundity of women increases up to the age of about 25 years ; and continuing high with but slight diminution till after 30, then gradually wanes. It is the same with the sizes and weights of offspring. Infants born of women from 25 to 29 years of age, are both longer and heavier than infants born of younger or older women ; and this difference has the same implication as the greater total weight of the offspring pro duced at a birth, during the most fecund age of a pluriparous animal. Once more, there is the fact that a too-early bearing of young produces on a woman the same injurious effects as on an inferior creature— an arrest of growth and an enfeeble- ment of constitution. Considering these general and special parallelisms, we might safely infer that variations of human fertility conform to the same laws as do variations of fertility in general. But it is not needful to content ourselves with an implication. Evidence is assignable that what causes increase or decrease of genesis in other creatures, causes increase or decrease of genesis in Man. It is true that, even more than hitherto, our reasonings are beset by difficulties. So numerous are the inequalities in the conditions, that but few unobjectionable comparisons can be made. The human races differ consider ably in their sizes, and notably in their degrees of cerebral development. The climates they inhabit entail on them widely different consumptions of matter for maintenance of MULTIPLICATION OF THE HUMAN RACE. 481 temperature. Both in their qualities and quantities, the foods the}7 live on are unlike ; and the supply is here regular and there very irregular. Their expenditures in bodily action are extremely unequal ; and even still more unequal are their expenditures in mental action. Hence the factors, varying so much in their amounts and combinations, can scarcely ever have their respective effects identified. Never theless there are a few comparisons, the results of which may withstand criticism. § 366. The increase of fertility caused by a nutrition that is greatly in excess of the expenditure, is to be detected by contrasting populations of the same race, or allied races, one of which obtains good and abundant sustenance much more easily than the other. Three cases may here be set down. The traveller Barrow, describing the Cape-Boors, says: — " Unwilling to work and unable to think," * * * " indulging to excess in the gratification of every sensual appetite, the African peasant grows to an unwieldy size ; " and respecting the other sex, he adds — " the women of the African peasantry lead a life of the most listless inactivity." Then, after illus trating these statements, he goes on to note " the prolific tendency of all the African peasantry. Six or seven children in a family are considered as very few ; from a dozen to twenty are not uncommon." The native races of this region yield evidenca to the same effect. Speaking of the cruelly- used Hottentots (he is writing sixty years ago), who, while they are poor and ill- fed, have to do all the work for the idle Boors, Barrow says that they " s>eldom have more than two or three children ; and many of the women are barren." This unusual infertility stands in remarkable con trast with the unusual fertility of the Kaffirs, of whom he afterwards gives an account. Rich in cattle, leading easy lives, and living almost exclusively on animal food (chiefly milk with occasional flesh), these people were then reputed 482 LAWS OF MULTIPLICATION. to have a very high rate of multiplication. Barrow writes : — " They are said to be exceedingly prolific ; that twins are almost as frequent as single births, and that it is no un common thing for a woman to have three at a time." Pro bably both these statements are in excess of the truth ; but there is room for large discounts without destroying the extreme difference. A third instance is that of the French Canadians. "Nous sommes terribles pour les enfants /" observed one of them to Prof. Johnston ; who tells us that the man who said this " was one of fourteen children — was himself the father of fourteen, and assured me that from eight to sixteen was the usual number of the farmers' families. He even named one or two women who had brought their husbands fi ve-and-twenty, and threatened ' le vingt-sixibme pour le pretrc' ' From these large families, joined with the early marriages and low rate of mortality, it results that, by natural increase, " there are added to the French- Canadian population of Lower Canada four persons for every one that is added to the population of England." Now these French- Canadians are described by Prof. Johnston as home-loving, contented, unenterprising; and as living in a region where " land and subsistence are easily obtained." Very moderate industry brings to them liberal supplies of necessaries ; and they pass a considerable portion of the year in idleness. Hence the cost of Individ uation being much reduced, the rate of Genesis is much increased. That this uncommon fertility is not due to any direct influence of the locality, is implied by the fact that along with the " restless, discontented, striving, burning energy of their Saxon neigh bours " no such rate of multiplication is observed ; while further south, where the physical circumstances are more favourable if anything, the Anglo-Saxons, leading lives of excessive activity, have a fertility below the average. And that the peculiarity is not a direct effect of race, is proved by the fact that in Europe, the rural French are certainly not more prolific than the rural English. MULTIPLICATION OF THE HUMAN RACE. 483 To every reader there will probably occur the seemingly adverse evidence furnished by the Irish ; who, though not well fed, multiply fast. Part of this more rapid increase is due to the earlier marriages common among them, arid con sequent quicker succession of generations — a factor which, as we have seen, has a larger effect than any other on the rate of multiplication. Part of it is due to the greater generality of marriage — to the comparative smallness of the number who die without having had the opportunity of pro ducing offspring. The effects of these causes having been deducted, we may doubt whether the Irish, individually con sidered, would be found more prolific than the English. Perhaps, however, it will be said that, considering their diet, they ought to be less prolific. This is by no means obvious. It is not simply a question of nutriment absorbed : it is a question of how much remains after the expenditure in self- maintenance. Now a notorious peculiarity in the life of the Irish peasant, is, that he obtains a return of food that is large in proportion to his outlay in labour. The cultivation of his potatoe-ground occupies each cottager but a small part of the year ; and the domestic economy of his wife is not of a kind to entail on her much daily exertion. Consequently, the crop, tolerably abundant in quantity though innutritive in quality, very possibly suffices to meet the comparatively4ow expendi ture, and to leave a good surplus for genesis — perhaps a greater surplus than remains to the males and females of the English peasantry, who, though fed on better food, are harder worked. We conclude, then, that in the human race, as in all other races, such absolute or relative abundance of nutriment as leaves a large excess after defraying the cost of carrying on parental life, is accompanied by a high rate of genesis.* * This is exactly the reverse of Mr. Doubleday's doctrine ; which is that throughout both the animal and vegetal kingdoms, " over-feeding checks in crease ; whilst, on the other hand, a limited or deficient nutriment stimu lates and adds to it." Or, as he elsewhere says— " Be the range of th« . 184 LAWS OF MULTIPLICATION. § 367. Evidence of the converse truth, that relative in crease of expenditure, leaving a diminished surplus, reduces the degree of fertility, is not wanting. Some of it has been set down for the sake of antithesis in the foregoing section. Here may be grouped a few facts of a more special kind having the same implication. To prove that much bodily labour renders women less prolific, requires more evidence than is obtainable. Some evi dence, however, may be set down. De Boismont in France and Dr. Szukits in Austria, have shown by extensive statistical comparisons, that the reproductive age is reached a year later by women of the labouring class than by middle-class women; and while ascribing this delay in part to inferior natural power to increase in any species what it may, the plethoric state invariably checks it, and the deplethoric state invariably develops it ; and this happens in the exact ratio of the intensity and completeness of each state, until each state be carried so far as to bring about the actual death of the animal or plant itself." I have space here only to indicate the misinterpretations on which Mr. Doubleday has based his argument. In the firet place, he has confounded normal plethora with what I have, in § 355, distinguished as abnormal plethora. The cases of infertility accom panying fatness, which he cites in proof that over-feeding checks increase, are not cases of high nutrition properly so called ; but cases of such defective absorption or assimilation as constitutes low nutrition. In Chap. IX, abun dant proof was given that a truly plethoric state is an unusually fertile state. It may be added that much of the evidence by which Mr. Doubleday seeks to show that among men, highly-fed classes are infertile classes, may be out balanced by counter-evidence. Many years ago Mr. Lewes pointed this out : extracting from a book on the peerage, the names of 16 peers who had, at that time, 186 children ; giving an average of 11 "6 in a family. Mr. Doubleday insists much on the support given to his theory by the barrenness of very luxuriant plants, and the fruitfulness produced in plants by depletion. Had he been aware that the change from barrenness to fruit* fulness in plants, is a change from agamogenesis to gamogenesis — had it been aa well known at the time when he wrote as it is now, that a tree which goes on putting out sexless shoots, is so producing new individuals ; and that when it begins to bear fruit, it simply begins to produce r ew individuals after another manner — he would have perceived that facts of this class do not tell in big avour. In the law which Mr. Doubleday alleges, he sees a guarantee for the niah» MULTIPLICATION OF THE HUMAN RACE. 485 nutrition, we may suspect that it is in part due to greater muscular expenditure. A kindred fact, admitting of a kindred interpretation, may be added. Though the com paratively-low rate of increase in France is attributed to other causes, yet, very possibly, one of its causes is the greater proportion of hard work entailed on French women, by the excessive abstraction of men for non-productive occupations, military and civil. The higher rate of multiplication in England than in continental countries generally, is not im probably furthered by the easier lives which English women lead. That absolute or relative infertility is generally pro duced in women by mental labour carried to excess, is more clearly shown. Though the regimen of upper-class girls is not what it should be, yet, considering that their feeding is tenance of species. He argues that the plethoric state of the individuals con stituting any race of organisms, presupposes conditions so favourable to life that the race can be in no danger; and that rapidity of multiplication becomes needless. Conversely, he argues that a deplethoric state implies unfavourable conditions— implies, consequently, unusual mortality; that is— implies a necessity for increased fertility to prevent the race from dying out. It may be readily shown, however, that such an arrangement would be the reverse of self-adjusting. Suppose a species, too numerous for its food, to be in the resulting deplethoric state. It will, according to Mr. Doubleday, become unusually fertile ; and the next generation will be more numerous rather than less numerous. For, by the hypothesis, the unusual fertility due to tho deplethoric state, is the cause of undue increase of population. But if the next generation is more numerous while the supply of food has remained the same, or rather has decreased under the keener competition for it, then this next generation will be in a still more deplethoric state, and will be still more fertile. Thus there will go on an ever-increasing rate of multiplication, and an ever-decreasing supply of food, until the species disappears. Suppose, on the ether hand, the members of a species to be in an unusually plethoric state. Their rate of multiplication, ordinarily suffi cient to maintain their numbers, will become insufficient to maintain their numbers. In the next generation, therefore, there will be fewer to eat the already abundant food, which, becoming relatively still more abundant, will render the fewer members of the species still more plethoric, and still less fertile, than their parents. And the actions and reactions continuing, tW species will presently die out from absolute barr«nn*>ss. 486 LAWS OF MULTIPLICATION. better than that of girls belonging to the poorer classes^ while, in most other respects, their physical treatment is not worse, the deficiency of reproductive power among them may be reasonably attributed to the overtaxing of their brains — an overtaxing tvhich produces a serious reaction on the physique. This diminution of reproductive power is not shown only by the greater frequency of absolute sterility ; nor is it shown only in the earlier cessation of child-bearing ; but it is also shown in the very frequent inability of such women to suckle their infants. In its full sense, the re productive power means the power to bear a well- developed infant, and to supply that infant with the natural food for the natural period. Most of the flat-chested girls who survive their high-pressure education, are incompetent to do this. Were their fertility measured by the number of children they could rear without artificial aid, they would prove relatively very infertile. The cost of reproduction to males being so much less than it is to females, the antagonism between Genesis and Individuation is not often shown in men by suppression of generative power consequent on unusual expenditure in bodily action. Nevertheless, there are indications that this results in extreme cases. We read that the ancient athletes rarely had children ; and among such of their modern repre sentatives as acrobats, an allied relation of cause and effect is alleged. Indirectly this truth, or rather its converse, appears to have been ascertained by those who train men for feats of strength — they find it needful to insist on con tinence. Special proofs that in men, great cerebral expenditure di minishes or destroys generative power, are difficult to obtain. It is, indeed, asserted that intense application to mathematics, requiring as it does extreme concentration of thought, is apt to have this result ; and it is asserted, too, that this result is produced by the excessive emotional excitement of gambling. Then, again, it is a matter of common remark how frequently MULTIPLICATION OF THE HUMAN RACE. 487 .'Tien of unusual mental activity leave no offspring. But facts of this kind admit of another interpretation. The re action of the brain on the body is so violent — the overtaxing of the nervous system is so apt to prostrate the heart and derange the digestion ; that the incapacities caused in these cases, are probably often due more to constitutional dis turbance than to the direct deduction which excessive action entails. Such instances harmonize with the hypothesis ; but how far they yield it positive support we cannot say. § 368. An objection must here be guarded against. It is likely to be urged that since the civilized races are, on the average, larger than many of the uncivilized races ; and since they are also somewhat more complex as well as more active ; they ought, in conformity with the alleged general law, to be less prolific. There is, however, no evidence to prove that they are so : on the whole, they seem rather the reverse. The reply is that were all other things equal, these superior varieties of men should have inferior rates of in crease. But other things are not equal ; and it is to the inequality of other things that this apparent anomaly is attributable. Already we have seen how much more fertile domesticated animals are than their wild kindred ; and the causes of this greater fertility are also the causes of the greater fertility, relative or absolute, which civilized men exhibit when compared with savages. There is the difference in amount of food. Australians, Fuegians, and sundry races that might be named as having low rates of multiplication, are obviously underfed. The sketches of natives contained in the volumes of Livingstone, Baker, and others, yield clear proofs of the extreme depletion common among the uncivilized. In quality as well as in quantity, their feeding is bad. Wild fruits, insects, ' larvse, vermin, &c., which we refuse with disgust, often enter largely into their dietary. Much of this inferior food they eat uncooked ; and they have not ou? 488 LAWS OF MULTIPLICATION-. elaborate appliances for mechanically-preparing it, and rejecting its useless parts. So that they live on matters of less nutritive value, which cost more both to masticate anS to digest. Further, to uncivilized men supplies of food come very irregularly : long periods of scarcity are divided by short periods of abundance. And though by gorging when opportunity occurs, something is done towards compensating for previous want, yet the effects of prolonged starvation cannot be neutralized by occasional enormous meals. Bearing in mind, too, that improvident as they are, savages often bestir themselves only under pressure of hunger, we may fairly consider them as habitually ill- nourished — may see that even the poorer classes of civilized men, making regular meals on food separated from in- nutritive matters, easy to masticate and digest, tolerably good in quality and adequate if not abundant in quantity, are much better nourished. Then, again, though a much greater consumption in mus cular action appears to be undergone by civilized men than by savages ; and though it is probably true that among our labouring people the daily repairs cost more; yet in many cases there does not exist so much difference as we are apt to suppose. The chage is very laborious ; and great amounts of exertion are gone through by the lowest races in seeking and securing the odds and ends of wild food on which they largely depend. We naturally assume that because barbarians are averse to regular labour, their muscular action is less than our own. But this is not necessarily true. The monotonous toil is what they cannot tolerate ; and they may be ready to go through as much or more exertion when it is joined with excitement. If we remember that the sportsman who gladly scrambles up and down rough hill sides all day after grouse or deer, would think himself hardly used had he to spend as much effort and time in digging ; we ^hall see that a savage who is the reverse of industrious, nay nevertheless be subject to a muscular waste not verj MULTIPLICATION OF THE HUMAN RACE. 489 different in amount from that undergone by the indus trious. When it is added that a larger physiolo gical expenditure is entailed on the uncivilized than on the civilized by the absence of good appliances for shelter and protection — that in some cases they have to make good a greater loss of heat, and in other cases suffer much wear from irritating swarms of insects — we shall see that the total cost of self-maintenance among them is probably in many cases little less, and in some cases more, than it is among ourselves. So that though, on the average, the civilized are probably larger than the savage ; and though they are, in their nervous systems at least, somewhat more complex ; and though, other things equal, they ought to be the less prolific ; yet, other things are so unequal, as to make it quite conformable to the general law that they should be more prolific. In § 365 we observed how, among inferior animals, higher evolution sometimes makes self-preservation far easier, by opening the way to resources previously un available : so involving an undiminished, or even an in creased, rate of genesis. And similarly we may expect among races of men, that those whose slight further develop ments have been followed by habits and arts that immensely facilitate life, will not exhibit a lower degree of fertility, and may even exhibit a higher. § 369. One more objection has to be met — a kindred ob jection to which there is a kindred reply. Ca^es may be named of men conspicuous for activity, bodily and mental, who were also noted, not for less generative power than usual, but for more. As their superiorities indicate higher degrees of evolution, it may be urged that such men should, accord ing to the theory, have lower degrees of reproductive activity. The fact that here, along with increased powers of self-pre servation, there go increased powers of race-propagation, seems irreconcilable with the general doctrine. Reconcilia tion is not difficult however. 490 LAWS OF MULTIPLICATION. The cases are analogous to some before named, in which more abundant food simultaneously aggrandizes the indi vidual and adds to the production of new individuals — the difference between the cases being, that instead of a better external supply of materials there is here a better internal utilization of materials Creatures of the same species noto riously differ in goodness of constitution. Here there is some visceral defect, showing itself in feebleness of all the func tions; while here some peculiarity of organic balance, some high quality of tissue, some abundance or potency of the digestive juices, gives to the system a perpetual high tide of rich blood, that serves at once to enhance the vital activities and to raise the power of propagation. Such variations, however, are quite independent of changes in the proportion between Individuation and Genesis : this remains the same, while both are increased or decreased by the increase or decrease of the common stock of materials. An illustration will best clear up any perplexity. Let us say that the fuel burnt in the furnace of a locomotive steam- engine, answers to the food which a man consumes ; let us say that the produced steam expended in working the engine, corresponds to that portion of absorbed nutriment which carries on the man's functions and activities ; and let us say that the steam blowing off at the safety-valve, answers to that portion of the absorbed nutriment which goes to the propagation of the race. Such being the condi tions of the case, several kinds of variations are possible. All other circumstances remaining the same, there may be changes of proportion between the steam used for working the engine and the steam that escapes by the safety-valve. There may be a structural or organic change of proportion. By enlarging the safety-valve or weakening its spring, while the cylinders are reduced in size, there may be established a constitutionally-small power of locomotion and a constitu tionally-large amount of escape-steam ; and inverse variations so produced, will answer to the inverse variations between MULTIPLICATION OF THE HUMAN RACE. 491 Individuation and Genesis which different types of organisms show us. Again, there may be a functional change of pro portion. If the engine has to draw a considerable load, the abstraction of steam by the cylinders greatly reduces the discharge by the safety-valve ; and if a high velocity is kept up, the discharge from the safety-valve entirely ceases. Con versely, if the velocity is low, the escape-steam bears a large ratio to the steam consumed by the motor apparatus ; and if the engine becomes stationary the whole of the steam escapes by the safety-valve. This inverse variation answers to that which we have traced between Expenditure and Genesis, as displayed in the contrasts between species of the same typo but unlike activities, and in the contrasts between active and inactive individuals of the same species. But now beyond these inverse variations between the quantities of consumed steam and escape-steam, that are structurally and functionally caused, there are coincident variations, producible in both by changes in the quantity of steam supplied — changes that may be caused in several ways. In the first place, the fuel thrown into the furnace may be increased or made better. Other things equal, there will result a more active locomo tion as well as a greater escape ; and this will answer to that simultaneous addition to its individual vigour and its repro ductive activity, caused in an animal by a larger quantity, or a superior quality, of food. In the second place, the steam generated may be economized. Loss by radiation from the boiler may be lessened by a covering of non-conducting sub stances ; and part of the steam thus prevented from con densing, will go to increase the working power of the engine, while part will be added to the quantity blowing off. This variation corresponds to that simultaneous addition to bodily vigour and propagative power, which results in animals that have to expend less in keeping up their temperatures. In the third place, by improvement of the steam-generating apparatus, more steam may be obtained from a given weight of fuel. A better-formed evaporating surface, or boiler plates 492 LAWS OF MULTIPLICATION. which conduct more rapidly, or an increased number of tubes, may cause a larger absorption of heat from the burning mass or the hot gases it gives off; and the extra steam generated by this extra heat, will, as before, augment both the motive force and the emission through the safety-valve. And this last case of coincident variation, is parallel to the case with which we are here concerned — the augmentation of indi vidual expenditure and of reproductive energy, that may be caused by a superiority of some organ on which the utilizing or economizing of materials depends. Manifestly, therefore, an increased expenditure for Genesis, or an increased expenditure for Individuation, may arise in one of two quite different ways — either by diminution of the antagonistic expenditure, or by addition to the store which supplies both expenditures ; and confusion results from not distinguishing between these. Given the ratio 4 to 20, as expressive of the relative costs of Genesis and Individuation, and the expenditure for Genesis may be raised to 5 while the expenditure for Individuation is raised to 25, without any alteration of type ; merely by favourable circumstances or superiority of constitution. On the other hand, circumstances remaining the same, the expenditure for Genesis may be raised from 4 to 5, by lowering the expenditure for Indi viduation from 20 to 19 : which change of ratio may be either functional and temporary, or structural and per manent. And only when it is the last does it illustrate that inverse variation between degree of evolution and degree of procreative dissolution, which we have everywhere seen. § 370. There is no reason to suppose, then, that the laws of multiplication which hold of other beings, do not hold of the human being. On the contrary, there are special facts which unite with general implications, to show that these laws do hold of the human being. The absence of direct evidence in some cases where it might be looked for, we find fully explained when all the factors are taken into account -MULTIPLICATION OF THE HUMAN RACE. And certain seemingly-adverse facts, prove, on examination, to be facts belonging to a different category from that in which they are placed, and harmonize with the rest when rightly interpreted. The conformity of human fertility to the laws of multipli cation in general, being granted, it remains to inquire what effects must be caused by permanent changes in men's natures and circumstances. Thus far we have observed how, by their extremely-high evolution and extremely-low fertility, man kind display the inverse variation between Individuation and Genesis, in one of its extremes. And we have also observed how mankind, like other kinds, are functionally changed in their rates of multiplication by changes of conditions. But we have not observed how alteration of structure in Man entails alteration of fertility. The influence of this factor is BO entangled with the influences of other factors which are for the present more important, that we cannot recognize it. Here, if we proceed at till, wo must proceed deductively. CHAPTER XIII. HUMAN POPULATION IN THE FUTURE. § 371. Any further evolution in the most-highly evolved of terrestrial beings, Man, must be of the same nature as evolution in general. Structurally considered, it may consist in greater integration, or greater differentiation, or both — augmented bulk, or increased heterogeneity and definiteness, or a combination of the two. Functionally considered, it may consist in a larger sum of actions, or more multiplied varieties of actions, or both— -a larger amount of sensible and insensible motion generated, or motions more numerous in kind and more intricate and exact in co-ordination, or motions that are greater alike in quantity, complexity, and precision. Expressing the change in terms of that more special evolution displayed by organisms ; we may say that it must be one which further adapts the moving equilibrium of organic actions. As it was pointed out in First Principles, § 133, "the maintenance of such a moving equilibrium, re quires the habitual genesis of internal forces corresponding in number, directions, and amounts to the external incident forces — as many inner functions, single or combined, as there are single or combined outer actions to be met." And it was also pointed out that " the structural complexity accom panying functional equilibration, is definable as one in which there are as many specialized parts as are capable, separate!/ HUMAN POPULATION IN THE FUTURE. 495 and jointly, of counteracting the separate arid joint forces amid which the organism exists." Clearly, then, since all incompletenesses in Man as now constituted, are failures to meet certain of the outer actions, mostly involved, remote, irregular, to which he is exposed ; every advance implies additional co-ordinations of actions and accompanying com plexities of organization. Or once more, to specialize still further this conception of future progress, we may consider it as an advance towards completion of that continuous adjustment of internal to ex ternal relations, which constitutes Life. In Part I. of this work, where it was shown that the correspondence between inner and outer actions called Life, is a particular kind of what, in terms of Evolution, we called a moving equilibrium ; it was shown that the degree of life varies as the degree of correspondence. Greater evolution or higher life, implies, then, such modifications of human nature as shall make more exact the existing correspondences, or shall establish addi tional correspondences, or both. Connexions of phenomena of a rare, distant, unobtrusive, or intricate kind, which we either suffer from or do not take advantage of, have to be responded to by new connexions of ideas, and acts properly combined and proportioned : there must be increase of know ledge, or skill, or power, or of all these. And to effect this more extensive, more varied, and more accurate, co-ordina tion of actions, there must be organization of still greater heterogeneity and deliiiiteness. § 372. Let us before proceeding, consider in what par ticular ways this further evolution, this higher life, this greater co-ordination of actions, may be expected to show itself. Will it be in strength ? Probably not to any considerable degree. Mechanical appliances are fast supplanting brute force, and doubtless will continue doing this. Though at present civilized nations largely depend for self-preservation G4 496 LAWS OF MULTIPLICATION. on vigour of limb, and are likely to do so while wars con tinue ; yet that progressive adaptation to the social state which must at last bring wars to an end, will leave the amount of muscular power to adjust itself to the requirements of a peaceful regime. Though, taking all things into account, the muscular power then required may not be less than now, there seems no reason why more should be required. Will it be in swiftness or agility ? Probably not. In the savages these are important elements of the ability to main tain life ; but in the civilized man they aid self-preservation in quite a minor degree, and there seems no circumstance likely to necessitate an increase of them. By games and gymnastic competitions, such attributes may indeed be arti ficially increased ; but no artificial increase which does not bring a proportionate advantage can be permanent ; since, other things equal, individuals and societies that devote the same amounts of energy in ways that subserve life more effectually, must by and by predominate. Will it be in mechanical skill, that is, in the better-co ordination of complex movements ? Most likely in some degree. Awkwardness is continually entailing injuries and deaths. Moreover, the complicated tools which civilization brings into use, are constantly requiring greater delicacy of manipulation. All the arts, industrial and aesthetic, as they develop, imply a corresponding development of perceptive and executive faculties in men — the two necessarily act and react. Will it be in intelligence ? Largely, no doubt. There is ample room for advance in this direction, and ample demand for it. Our lives are universally shortened by our ignorance. In attaining complete knowledge of our own natures and oi the natures of surrounding things — in ascertaining the con ditions of existence to which we must conform, and in dis covering means of conforming to them under all variations of seasons and circumstances — we have abundant scope for intellectual progress. Will it be in morality, that is, in greater power of self- HUMAN POPULATION IN THE FUTURE. 497 regulation ? Largely also : perhaps most largely . Right conduct is usually corne short of more from defect ot will than defect of knowledge. To the due co-ordination of those complex actions which constitute human life in its civilized form, there goes not only the pre- requisite — recognition of the proper course; but the further pre requisite — a due impulse to pursue that course. And on calling to mind our daily failures to fulfil often-repeated resolutions, we shall perceive that lack of the needful desire, rather than lack of the needful insight, is the chief cause of faulty action. A further endowment of those feelings which civilization is developing in us — sentiments responding to the requirements of the social state — emotive faculties that find their gratifications in the duties devolving on us — must be acquired before the crimes, excesses, diseases, improvi dences, dishonesties, and cruelties, that now so greatly diminish the duration of life, can cease. Thus, looking at the several possibilities, and asking what direction this further evolution, this more complete moving equilibrium, this better adjustment of inner to outer relations, this more perfect co-ordination of actions, is likely to take; we conclude that it must take mainly the direction of a higher intellectual and emotional develop ment. § 373. This conclusion we shall find equally forced on us if we inquire for the causes which are to bring about such results. No more in the case of Man than in the case of any other being, can we presume that evolution either has taken place, or will hereafter take place, spontaneously. In the past, at present, and in the future, all modifications; func tional and organic, have been, are, and must be immediately or remotely consequent on surrounding conditions. What, then, are those changes in the environment to which, by direct or indirect equilibration, the human organism has been adjusting itself, is adjusting itself now, and will continue to 498 LAWS OF MULTIPLICATION. adjust itself? And how do they necessitate a higher evolu tion of the organism ? Civilization, everywhere having for its antecedent the in crease of population, and everywhere having for one of ita consequences a decrease of certain race-destroying forces, has for a further consequence an increase of certain other race- destroying forces. Danger of death from predatory animals lessens as men grow more numerous. Though, as they spread over the Earth and divide into tribes, men become wild beasts to one another, yet the danger of death from this cause also diminishes as tribes coalesce into nations. But the danger of death which does not diminish, is that produced by augmentation of numbers itself —the danger from deficiency of food. Supposing human nature to remain unchanged, the mortality henre resulting would, on the average, rise aa human beings multiplied. If mortality, under such condi tions, does not rise, it must be because the supply of food also augments ; and this implies some change in human habits wrought by the stress of human needs. Here, then, is the permanent cause of modification to which civilized men are exposed. Though the intensity of its action is ever being mitigated in one direction, by greater production of food ; it is, in the other direction, ever being added to by the greater production of individuals. Manifestly, the wants of their redundant numbers constitute the only stimulus mankind have to obtain more necessaries of life : were not the demand beyond the supply, there would be no motive to increase the supply. And manifestly, this excess of demand over supply is perennial : this pressure of population, of which it is the index, cannot be eluded. Though by the emigration that takes place when the pressure arrives at a certain intensity, temporary relief is from time to time obtained ; yet as, by this process, all habitable countries must become peopled, it follows that in the end. the pressure, whatever it may then be, must be borne in full. This constant increase of people beyond the means of sub HUMAN POPULATION IN THE FUTURE. 4U9 sistence, causes, then, a never-ceasing requirement for skill, intelligence, and self-control — involves, therefore, a constant exercise of these and gradual growth of them. Every industrial improvement is at once the product of a higher form of humanity, and demands that higher form of humanity to carry it into practice. The application of science to the arts, is the bringing to bear greater intelligence for satisfying our wants ; and implies continued progress of that intelligence. To get more produce from the acre, the farmer must study chemistry, must adopt new mechanical appliances, and must, by the multiplication of processes, cultivate both his own Dowers and the powers of his labourers. To meet the requirements of the market, the manufacturer is per petually improving his old machines, and inventing new ones ; and by the premium of high wages incites artizans to acquire greater skill. The daily-widening ramifications of commerce entail on the merchant a need for more know ledge and more complex calculations; while the lessening profits of the ship-owner force him to build more scientifi cally, to get captains of higher intelligence, and better crews. In all cases, pressure of population is the original cause. Were it not for the competition this entails, more thought and energy would not daily be spent on the business of life ; and growth of mental power would not take place. Difficulty in getting a living is alike the incentive to a higher education of children, and to a more intense and long- continued application in adults. In the mother it in duces foresight, economy, and skilful house-keeping ; in the father, laborious days and constant self-denial. Nothing but necessity could make men submit to this discipline; and nothing but this discipline could produce a continued pro gression. In this case, as in many others, Nature secures each step in advance by a succession of trials ; which are perpetually repeated, and cannot fail to be repeated, until success is achieved. All mankind in turn subject themselves more 01 500 LAWS OF MULTIPLICATION. less to the discipline described ; they either may or may not advance under it ; but, in the nature of things, only those who do advance under it eventually survive. For, neces sarily, families and races whom this increasing difficulty of getting a living which excess of fertility entails, does not stimulate to improvements in production — that is, to greater mental activity — are on the high road to extinction ; and must ultimately be supplanted by those whom the pressure does so stimulate. This truth we have recently seen exem plified in Ireland. And here, indeed, without further illustration, it will be seen that premature death, under all its forms and from all its causes, cannot fail to work in the same direction. For as those prematurely carried -off must> in the average of cases, be those in whom the power of self- preservation is the least, it unavoidably follows that those left behind to continue the race, must be those in whom the power of self-preservation is the greatest — must be the select of their generation. So that, whether the dangers to existence be of the kind produced by excess of fertility, or of any other kind, it is clear that by the ceaseless exercise of the faculties needed to contend with them, and by the death of all men who fail to contend with them successfully, there is ensured a constant progress towards a higher degree of skill, intelligence, and self- regulation — a better co-ordina tion of actions — a more complete life.* * A good deal of this chapter retains its original form ; and the above paragraph is reprinted verbatim from the Westminster Review for April, 1852, in which the views developed in the foregoing hundred pages were first sketched out. This paragraph shows how near one may be to a great generaliza tion without seeing it. Though the process of natural selection is recognized ; and though to it is ascribed a share in the evolution of a higher type ; yet tha conception must not be confounded with that which Mr. Darwin has worked out with such wonderful skill, and supported by such vast stores of knowledge. In the first place, natural selection is here described only as furthering direct adaptation — only as aiding progress by the preservation of individuals in whom functionally-produced modifications have gone on most favourably. In the second place, there is no trace of the idea that natural selection may, by vO-operation with the cause assigned, or with other causes, produce divergences HUMAN POPULATION IN THE FUTURE. 501 § 374. The proposition at which we have thus arrived, is. then, that excess of fertility, through the changes it is ever working in Man's environment, is itself the cause of Man's further evolution ; and the obvious corollary here to be drawn, is, that Man's further evolution so brought about, itself necessitates a decline in his fertility. That future progress of civilization which the never ceasing pressure of population must produce, will be ac companied by an enhanced cost of Individuation, both in structure and function ; and more especially in nervous structure and function. The peaceful struggle for existence in societies ever growing more crowded and more complicated, must have for its concomitant an increase of the great nervous centres in mass, in complexity, in activity. The larger body of emotion needed as a fountain of energy for men who have to hold their places and rear their families under the inten sifying competition of social life, is, other things equal, the correlative of larger brain. Those higher feelings presupposed by the better self- regulation which, in a better society, can alone enable the individual to leave a persistent posterity, are, other things equal, the correlatives of a, more complex brain ; as are also those more numerous, more varied, more general, and more abstract ideas, which must also become increasingly of structure ; and of course, in the absence of this idea, there is no im plication, even, that natural selection has anything to do with the origin of species. And in the third place, the all important factor of variation — " spontaneous," or incidental as we may otherwise call it — is wholly ignored. Though use and disuse are, I think, much more potent causes of organic modification than Mi. Darwin supposes— though, while pursuing the inquiry in detail, I have been led to believe that direct equilibration has played a more active part even than I had myself at one time thought ; yet I hold Mr. Darwin to have shown beyond question, that a great part of the facts— perhaps the greater part — are explicable only as resulting from the survival of individuals which have deviated in some indirectly-caused way from the ancestral type. Thus, the above paragraph contains merely a passing recogni tion of the selective process ; and indicates no suspicion of the enormous range of its effects, or of the conditions under which a large part of its effects ire produced. 502 LAWS OF MULTIPLICATION. requisite for successful life as society advances. And the genesis of this larger quantity of feeling and thought, in a brain thus augmented in size and developed in structure, is, other things equal, the correlative of a greater wear of nerv ous tissue and greater consumption of materials to repair it. So that both in original cost of construction and in subse quent cost of working, the nervous system must become a heavier tax on the organism. Already the brain of the civi lized man is larger by nearly thirty per cent, than the brain of the savage. Already, too, it presents an increased hetero- geneity — especially in the distribution of its convolutions. And further changes like these wrhich have taken place under the discipline of civilized life, we infer will continue to take place. But everywhere and always, evolu tion is antagonistic to procreative dissolution. Whether it be in greater growth of the organs which subserve self-main tenance, whether it be in their added complexity of structure, or whether it be in their higher activity, the abstraction of the required materials, implies a diminished reserve of ma terials for race-maintenance. And we have seen reason to believe that this antagonism between Individuation and Genesis, becomes unusually marked where the nervous sys tem is concerned, because of the costliness of nervous struc ture and function. In § 346 was pointed out the apparent connexion between high cerebral development and pro longed delay of sexual maturity ; and in § § 366, 367, the evidence went to show that where exceptional fer tility exists there is sluggishness of mind, and that where there has been during education excessive expenditure in mental action, there frequently follows a complete or partial infertility. Hence the particular kind of further evolution which Man is hereafter to undergo, is one which, more than any other, may be expected to cause a decline in his power of reproduction. The higher nervous development and greater expenditure in nervous action, here described as indirectly brought about HUMAN POPULATION IN THE FUTURE. 503 by increase of numbers, and as thereafter becoming a check on the increase of numbers, must not be taken to imply an in tenser strain — a men tally -laborious life. The greater emotional and intellectual power and activity above con templated, must be understood as becoming, by small incre ments, organic, spontaneous and pleasurable. As, even when relieved from the pressure of necessity, large-brained Euro peans voluntarily enter on enterprises and activities which the savage could not keep up even to satisfy urgent wants ; so, their still larger-brained descendants will, in a still higher degree, find their gratifications in careers entailing still greater mental expenditures. This enhanced demand for materials to establish and carry on the psychical functions, will be a constitutional demand. We must conceive the type gradually so modified, that the more- developed nervous system irresistibly draws off, for its normal and unforced activities, a larger proportion of the common stock of nutri ment ; and while so increasing the intensity, completeness, and length of the individual life, necessarily diminishing the reserve applicable to the setting up of new lives — no longer required to be so numerous. Though the working of this process will doubtless be interfered with and modified in the future, as it has been in the past, by the facilitation of living which civilization brings ; yet nothing be}Toyid temporary interruptions can so be caused. However much the industrial arts may be im proved, there must be a limit to the improvement; while, with a rate of multiplication in excess of the rate of mortality, population must continually tread on the heels of produc tion. So that though, during the earlier stages of civiliza tion, an increased amount of food may accrue from a given amount of labour ; there must come a time when this relation will be reversed, and when every additional increment of food will be obtained by a more than proportionate labour : the disproportion growing ever higher, and the diminution of the reproductive power becoming greater. 504 LAWS OF MULTIPLICATION. § 375. There now remains but to inquire towards what limit this progress tends. So long as the fertility of the race is more than sufficient to balance the diminution by deaths, population must continue to increase. So long as population continues to increase, there must be pressure on (he means of subsistence. And so long as there is pressure on the means of subsistence, further mental development must go on, and further diminution of fertility must result Thus, the change can never cease until the rate of multiplication is just equal to the rate of mortality ; that is, can never cease until, on the average, each pair has as many children as are requisite to produce another generation of child-bearing adults, equal in number to the lust generation. At first sight, this would seem to imply that eventually each pair will rarely have more than two offspring ; but a little considera tion shows that this is a lower degree of fertility than is likely ever to be reached. Supposing the Sun's light and heat, on which all terres trial life depends, to continue abundant, for a period long enough to allow the entire evolution we are contemplating ; there are still certain slow astronomic and geologic changes which must prevent such complete adjustment of human nature to surrounding conditions, as would permit the rate of mul tiplication to fall so low. As before pointed out (§ 148) during an epoch of 21,000 years, each hemisphere goes through a cycle of temperate seasons and seasons extreme in their heat and cold - variations that are themselves alternately exaggerated and mitigated in the course of far longer cycles ; and we saw that these caused perpetual ebbings and findings of species over different parts of the Earth's surface. Further, by slow but inevitable geologic changes, especially those of elevation and subsidence, the climate and physical characters of every habitat are modified ; while old habitats are destroyed and new are formed. This, too, we noted as a constant cause of migrations and of consequent alterations of environment. Now though the human race differs fron> HUMAN POPULATION IN THE FUTURE. 505 other races in having a power 'f artificially counteracting external changes, yet there are limits to this power; and, even were there no limits, the changes could not fail to work their effects indirectly, if not directly. If, as is thought probable, these astronomic cycles entail recurrent glacial pe riods in each hemisphere, then, parts of the Earth that are at one time thickly peopled, will at another time, be almost de serted, and vice cersa. The goologically-caused alterations of climate and surface, must produce further slow re-distributions of population ; and other currents of people, to and from different regions, will be necessitated by the rise of successive centres of higher civilization. Consequently, mankind cannot but continue to undergo changes of environment, physical and moral, analogous to those which they have thus far been undergoing. Such changes may eventually become slower and loss marked ; but they can never cease. And if they can never cease, there can never arise a perfect adaptation of human nature to its conditions of existence. To establish that complete correspondence between inner and outer actions which constitutes the highest life and greatest power of self- preservation, there must be a prolonged converse between the organism and circumstances that remain the same. If the external relations are being altered while the internal rela tions are being adjusted to them, the adjustment can never become exact. And in the absence of exact adjustment, there cannot exist that theoretically-highest power of self- preservation with which there would co-exist the theoreticallv- lowest power of race-production. Hence though the number of premature deaths may ul timately become very small, it can never become, so small as to allow the average number of offspring from each pan to fall so low as two. Some average number between two and three may be inferred as the limit — a number, however, that is not likely to be quite constant, but may be ex pected at one time to increase somewhat and afterwards to decrease somewhat, according as variations in phyeica] 606 LAWS OF MULTIPLICATION. and social conditions lower or raise the cost of self- preservation. Be this as it may, however, it is manifest that in the end, pressure of population and its accompanying evils will dis appear; and will leave a state of things requiring from each individual no more than a normal and pleasurable activity. Cessation in the decrease of fertility implies cessation in the development of the nervous system ; and this implies a nervous system that has become equal to all that is demanded of it — has not to do more than is natural to it. But that exercise of faculties which does not exceed what is natural, constitutes gratification. In the end, therefore, the ob tain ment of .subsistence and discharge of all the parental and social duties, will require just that kind and that amount of action needful to health and happiness. The necessary antagonism of Individuation and Grenesis, not only, then, fulfils with precision the a priori law of maintenance of race, from the Monad up to Man, but ensures final attainment of the highest form of this maintenance — a form in which the amount of life shall be the greatest possible, and the births and deaths the fewest possible. This antagonism could not fail to work out the results we see it working out. The excess of fertility has itself rendered the process of civilization inevitable ; and the process of civiliza tion must inevitably diminish fertility, and at last destroy its excess. From the beginning, pressure of population has been the proximate cause of progress. It produced the original diffusion of the race. It compelled men to abandon predatory habits and take to agriculture. It led to the clearing of the Earth's surface. It forced men into the social state ; made social organization inevitable ; and has developed the social sentiments. It has stimulated to pro gressive improvements in production, and to increased skill and intelligence. It is daily thrusting us into closer contact and more mutually-dependent relationships. And after having caused, as it ultimately must, the due peopling of the globe, HUMAN POPULATION IN THE FUTURE. 50? and the raising of all its habitable parts into the highest state of culture — after having brought all processes for the satisfaction of human wants to perfection — after having, at the same time, developed the intellect into complete com petency for its work, and the feelings into complete fitness for social life — after having done all this, the pressure of population, as it gradually finishes its work, must gradually bring itself to an end. § 377. In closing the argument let us not overlook the Belf-sufficingness of those universal processes by which the results reached thus far have been wrought out, and which may be expected to work out these future results. Evolution under all its aspects, general and special, is an advance towards equilibrium. We have seen that the theo retical limit towards which the integration and differentia tion of every aggregate advances, is a state of balance be tween all the forces to which its parts are subject, and the forces which its parts oppose to them (First Prin. § 130). And we have seen that organic evolution is a progress towards a moving equilibrium completely adjusted to en vironing actions. It has been also pointed out that, in civilized Man, there is going on a new class of equilibrations — those between his ac tions and the actions of the societies he forms (First Prin. § 135). Social restraints and requirements are ever altering his activities and by consequence his nature ; and as fast as his nature is altered, social restraints and requirements undergo more or less re-adjustment. Here the organism and the con ditions are both modifiable ; and by successive conciliations of the two, there is effected a progress towards equilibrium. More recently we have seen that in every species, there establishes itself an equilibrium of an involved kind between the total race-destroying forces and the total race- preserving forces — an equilibrium which implies that where the ability to maintain individual life is small, the ability to propagate 508 LAWS OF MULTIPLICATION. must be great, and vice versd. Whence it follows that the evolution of a race more in equilibrium with the environment, is also the evolution of a race in which there is a correlative approach towards equilibrium between the number of new individuals produced and the number which survive and propagate. The final result to be observed, is, that in Man, all these equilibrations between constitution and conditions, between the structure of society and the nature of its members, be tween fertility and mortality, advance simultaneously towards a common climax. In approaching an equilibrium between his nature and the ever- varying circumstances of his inorganic environment, and in approaching an equilibrium between his nature and all the requirements of the social state, Man is at the same time approaching that lowest limit of fertility at which the equilibrium of population is maintained by the addition of as many infants as there are subtractions by death in old age. Changes numerical, social, organic, must, by their mutual influences, work unceasingly towards a state of har mony — a state in which each of the factors is just equal to its work. And this highest conceivable result must be wrought out by that same universal process which the simplest inor ganic action illustrates. THE END. APPENDIX APPENDIX A. SUBSTITUTION OF AXIAL FOR FOLIAR ORGANS IN PLANTS. I APPEND here the evidences referred to in § 190. The most numerous and striking I have met with among the Umbelliferce. Monstrosities having the alleged implication, are frequent in the common Cow-Parsnep — so frequent that they must be familiar to botanists; and wild Angelica supplies many over-developments of like meaning. Omitting numerous cases of more or less significance, 1 will limit myself to two. One of them is that of a terminal umbel, in which nine of the outer umbellules are variously transformed — here a single flower being made monstrous by the development of some of its members into buds ; there several such malformed flowers being associated with rays that bear imperfect umbellules ; and elsewhere, flowers being replaced by imbellules : some of which nre perfect, and others imperfect only in the shortness of the flower-stalks. The annexed Fig. 69, represent ing in a somewhat conventionalized way, a part of the dried sped- 05 512 men, will give an idea of this Angelica. At a is shown a single flower partially changed ; in the urabellule marked 6, one of the rays bears a secondary umbellule ; and there may be seen at c and d. several such over-developments. But the most conclusive instance is that of a Uow-Parsnep, in which a single terminal umbel, besides the transformations already men tioned, exhibits higher degrees of such transformations.* The com- ponpiits of this complex growth are ; — three central umbellules, ab normal only in minor points ; one umbellule, external to these, which is partially changed into an umbel; one rather more out of the centre, which is so far metamorphosed as to be more an umbel than an umbellule : nine peripheral clusters formed by the development of umbellules into umbels, some of which are partially compounded still further. Examined in detail, these structures present the fol lowing facts : — 1. The innermost umbellule is normal, save in having a peripheral flower of which one member (apparently a petal) is transformed into a flower-bud. 2. The next umbellule, not quite so central, has one of its peripheral flowers made monstrous by the growth of a bud from the base of the calyx. 3. The third of the central umbellules has two abnormal outer flowers. One of them carries a flower-bud on its edge, in place of a foliar member. The other is half flower and half umbellule : being composed of three petals, three stamens, and five flower-buds growing where the other petals and stamens should grow. 4. Outside of these umbellules comes one of the mixed clusters. Its five central flowers are normal. Surrounding these are several flowers transformed in different degrees : one having a stamen par tially changed into a flower bud. And then, at the periphery of this mixed cluster, come three complete umbellules and an incom plete one in which some petals and stamens of the original flower remain. 5. A mixed cluster, in which the umbel-structure pre dominates, stands next. Its three central flowers are normal. Surrounding them are five flowers over-developed in various ways, like those already described. And on its periphery are seven complete umbellules in place of flowers ; besides an incomplete umbellule that contains traces of the original flower, one of them being a petal imperfectly twisted up into a bud. 6. Of the nine external clusters, in which the development of simple into compound umbels is most decided, nearly all present anomalies. Three of them have each a central flower untransformed ; and in others, the central * For the information of those who may wish to examine metamorphoses of these kinds, I may here state that I have found nearly all the examples described, in the neighbourhood of the sea — the last-named, on the shore of Locheil, near Fort William. Whether it is that I have sought more dili gently for eases when in such localities, or whether it is that the sea-aii favours that excessive nutrition whence these transformations result, I air unable to say. 513 ambellule i.; composed of two, three, or four flowers. 7. But the most remarkable fact is, that in sundry of these peripheral clusters, resulting from the metamorphosis of simple umbels into compound umbels, the like metamorphosis is carried a stage higher. Some of the component rays, are themselves the bearers of compound umbels instead of simple umbels. In Fig. 70, a portion of the dried speci men is represented. Two of the central umbellules are marked a and b ; those marked c and d are mixed clusters ; at e and / are compound umbels replacing simple ones ; and g shows one of the rays on which the over-development goes still further. Does not this evidence, enforced as it is by much more of like kind, go far to prove that foliar organs may be developed into axial organs'? Even were not the transitional forms traceable, there would still, I think, be no other legitimate interpretation of the facts last detailed. The only way of eluding the conclusion here drawn, is by assuming that where a cluster of flowers replaces a single flower, it is because the axillary buds which hypothetically belong to the several foliar organs of the flower, become developed into axes ; and assuming this, is basing an hypothesis on another hypothesis that is directly at variance with facts. The foliar organs of flowers do not bear buds in their axils ; and it would never have been supposed that such buds are typically present, had it not been for that mistaken conception of " type " which has led to many other errors in Biology. Goethe writes: "Now as we cannot realize the idea of a leaf apart from the node out of which it springs, or of a node without a bud, we may venture to infer," &c. See here an example of a method of philosophizing not uncommon among the Germans 514 The method is this — Survey a portion of the facts, and draw from them a general conception ; project this general conception back into the objective world, as a mould in which "Nature casts her products; expect to find it everywhere fulfilled; and allege poten tial fulfilment where no actual fulfilment is visible. If instead of imposing our ideal forms on Nature, we are con* tent to generalize the facts as Nature presents them, we shall find no warrant for the morphological doctrine above enunciated. The only conception of type justified by the logic of science, is — that correlation of parts which remains constant under all modifications of the structure to be defined. To ascertain this, we must compare all these modifications, and note what traits are common to them. On doing so with the successive segments of a phaBnogamic axis, we are brought to a conclusion widely different from that of Goethe. Axillary buds are almost universally absent from the cotyledons; they are habitually present in the axils of fully-developed leaves higher up the axis ; they are often absent from leaves that are close to the flower ; they are nearly always absent from the bracts ; absent from the sepals ; absent from the petals ; absent from the stamens ; absent from the carpels. Thus, out of eight leading forms which folia assume, one has the axillary bud and seven are without it. With these facts before us, it seems to me not difficult to "• realize the idea " " of a node without a bud." If we are not possessed by a foregone conclusion, the evidence will lead us to infer, that each node bears a foliar appendage and may bear an axillary bud. Even, however, were it granted that the typical segment of a Phrenogam includes an axillary bud, which must be regarded as always potentially present, no legitimate counter-interpretation of the monstrosities above described could thence be drawn. If when an umbellule is developed in place of a flower, the explanation is, that its component rays are axillary to the foliar organs of the flower superseded ; we may fairly require that these foliar organs to which they are axillary, shall be shown. But there are none. In the last specimen figured, the inner rays of each such umbellule are without them ; most of the outer rays are also without them ; and in one cluster, only a single ray has a bract at its point of origin. There is a rejoinder ready, however : the foliar organs are said to be suppressed. Though Goethe could not " realize the idea" " of a node without a bud," those who accept his typical form appear to find no difficulty in realizing the idea of an axillary bud without anything to which it is axillary. But letting this pass, suppose we ask what is the warrant for this assumed suppression. Axillary buds normally occur where the nutrition is high enough to produce fully-developed leaves ; and when axillary buds are demonstrably present in flowers, they accompany foliar organs that are more leaf- like than usual — always greener if not always larger. That is tc 515 say. the normal and the abnormal axillary buds, are alike the con comitants of foliar organs coloured by that chlorophyll which habitually favours foliar development. How, then, can it be sup posed that when, out of a flower there is developed a cluster of flower-bearing rays, the implied excess of nutrition causes the foliar organs to abort ? It is true that very generally in a branched in florescence, the bracts of the several flower-branches are very small (•their smallness being probably due to that defective supply of certain chlorophyll-forming matters, which is the proximate cause of flowering) ; and it is true that, under these conditions, a flower ing axis of considerable size, for the development of which chloro phyll is less needful, grows from the axil of a dwarfed leaf. But the inference that the foliar organ may therefore be entirely sup pressed, seems to me irreconcilable with the fact, that the foliar organ is always developed to some extent before the axillary bud appears. Until it has been shown that in some cases a lateral bud first appears, and a foliar organ afterwards grows out beneath it, to form its axil, the conception of an axillary bud of which the foliar organ is suppressed, will remain at variance with the established truths of development. The above originally formed a portion of § 190. I have transferred it to the Appendix, partly because it contains too much detail to render it fit for the general argument, and partly because the inter pretations being open to some question, it seemed undesirable to risk compromising that argument by including them. The criticisms passed upon these interpretations have not, however, sufficed to con vince me of their incorrectness. Unfortunately, I have since had no opportunity of verifying the above statements by microscopic exami nations, as I had intended. Though unable to enforce the inference drawn by further facts more minutely looked into, I may add some arguments based on facts that are well known. One of these is the fact that the so- called axillary bud is not universally axillary — is not universally seated in the angle made by the axis and an appended foliar organ. In certain plants the axillary bud is placed far above the node, half-way between it and the succeeding node. So that not only may a segment of a phsenogamic axis be without the axillary bud, but the axillary bud, when present, may be removed from that place in which, according to Goethe, it necessarily exists. Another fact not congruous with the current doctrine, is the common occurrence of "adventitious" buds — the buds that are put out from roots and from old stems or branches bare of leaves. The name under which they are thus classed, is meant to imply that they may be left out of conside ration. Those, however, who have not got a theory to save b\ 616 patting anomalies out of sight, may be inclined to think that the occurrence of buds where they are avowedly unconnected with nodes, and are axillary to nothing, tells very much against the as sumption that every bud implies a node and a corresponding foliar organ. And they may also see that the development of these ad ventitious buds at places where there is excess of nutritive mute- rials, favours the view above set forth. For if a bud thus arises at a place where it is not morphologically accounted for, simply because there happens to be at that place an abundance of unorganized pro toplasm ; then, clearly, it is likely that if the mass of protaplasm from winch a leaf would usually arise, is greatly increased in mass bv excess of nutrition, it may develop into an axis instead of a leaf APPENDIX B. A CRITICISM OX PROF. OWEX'S TIIKORY OF THE VERTEBRATE SKELETON. From the BRITISH & FOREIGN MEDICO-CIIIRURGICAL REVIEW FOR OCT., 1858.] I. On. the Archetype and Ilomologies of the Vertebrate Skeleton. By RICHARD OWEN, F.R.S.— London, 1848. pp. 172. II. Principes d'Osteologie Comparer, ou Recherches sur I'Archetype et its Ilomologies da Squelette Vertebre. Par RICHARD OWEN. — Paris. Principles oj Comparative Osteology ; or, Researches on the Arcnettjpe and the, Ilomologies oj the Vertebrate Skeleton. By RICHARD OWEN. III. On the Nature of Limbs. A Discourse delivered on Friday, February 9, at an Evening Meeting of the Royal Institution oj Great Britain. By RICHARD OWEN, F.R.S. — London, 1841). pp. 119. JUDGING whether another proves his position is a widely different thing from proving your own. To establish a general law requires an extensive knowledge of the phenomena to be generalized; but to decide whether an alleged general law is established by the evidence assigned, requires merely an adequate reasoning faculty. Especially is such a decision easy where the premises do not warrant the con clusion. It may be dangerous for one who has but little previous acquaintance with the facts, to say that a generalization is demon strated ; seeing that the argument may be one-sided : there may be Qiany facts unknown to him which disprove it. But it is not dangerous to give a negative verdict when the alleged demonstra- 518 tioii is manifestly insufficient. If the data put before him do not bear out the inference, it is competent for every logical reader to say so. From this stand-point, then, we venture to criticize some of Professor Owen's osteological theories. For his knowledge of comparative osteology we have the highest respect. We belie re that no living man has so wide and detailed an acquaintance with the bony structure of the Vertebrata. Indeed, there probably has never been any one whose information on the subject was so nearly exhaustive. Moreover, we confess that nearly all we know of this department of biology has been learnt from his lectures and writ ings. We pretend to no independent investigations, but merely to such knowledge of the phenomena as he has furnished us with. Our position, then, is such that, had Professor Owen simply enun ciated his generalizations, we should have accepted them on his authority. But he has brought forward evidence to prove them. By so doing he has tacitly appealed to the judgments of his readers and hearers — has practically said, " Here are the facts ; do they not warrant these conclusions?" And all we propose to do, -is to consider whether the conclusions are warranted by the facts brought forward. Let us first limit the scope of our criticisms. On that division of comparative osteology which deals with what Professor Owen distinguishes as u special homologies," we do not propose to enter. That the wing of a bird is framed upon bones essentially parallel to those of a mammal's fore-limb ; that the cannon-bone of a horse's leg answers to the middle metacarpal of the human hand ; that various bones in the skull of a fish are homologous with bones in the skull of a man — these and countless similar facts, we take to be well established. It may be, indeed, that the doctrine of special homologies is at present carried too far. It may be that, just as the sweeping generalization at one time favoured, that the embryonic phases of the higher animals represent the adult forms of lower ones, has been found untrue in a literal sense, and is acceptable only in a qualified sense ; so the sweeping generalization that the skeletons of all vertebrate animals consist of homologous parts, will have to undergo some modification. But that this generalization is substantially true, all comparative anatomists agree. The doctrine which we are here to consider, is quite a separate one — that of " general homologies." The truth or falsity of this may be decided on quite apart from that of the other. Whether certain bones in one vertebrate animal's skeleton correspond with certain bones in another's, or in every other's, is one question ; and whether the skeleton of every vertebrate animal is divisible into a series of segments, each of which is modelled after the same type, is another question. While the first is answered in the affirmative, 519 the last may be answered in the negative ; and we propose to give reasons why it should be answered in the negative. In so far as his theory of the skeleton is concerned, Professor Owen is an avowed disciple of Plato. At the conclusion of his Archetype and limnologies of the Vertebrate Skeleton, he quotes ap provingly the Platonic hypothesis of iMxi, "a sort of models, or moulds in which matter is cast, and which regularly produce the same number and diversity of species." The vertebrate form ID general (see diagram of the Arclietypus), or else the form of each kind of vertebrate animal (see p. 172, where this seems implied), Professor Owen conceives to exist as an "idea" — an "arche typal exemplar on which it has pleased the Creator to frame certain of his living creatures." Whether Professor Owen hold* that the typical vertebra also exists as an " idea," is not so certain. From the title given to his figure of the " ideal typical vertebra," it would seem that he does ; and at p. 40 of his Nature of Limbs, and indeed throughout his general argument, this supposition is implied. But on the last two pages of the Archetype and Homologies, it is distinctly alleged that " the repetition of simi lar segments in a vertebral column, and of similar elements in a vertebral segment, is analogous to the repetition of similar crystals as the result of polarizing force in the growth of an inorganic body ; " it is pointed out that, " as we descend the scale of animal life, the forms of the repeated parts of the skeleton approach more and more to geometrical figures ; " and it is inferred that " the Platonic Ma, or specific organizing principle or force, would seem to be in antagonism with the general polarizing force, and to sub due and mould it in subserviency to the exigencies of the resulting specific form." If Professor Owen's doctrine is to be understood as expressed in these closing paragraphs of his Archetype and Jlomo- logies — if he considers that " the iaca " " which produces the diver sity of form belonging to living bodies of the same materials," is met by the " counter-operation" of " the polarizing force pervading all space," which produces " the similarity of forms, the repetition of parts, the signs of unity of organization,'' and which is " subdued " as we ascend " in the scale of being ; " then we may pass on with the remark that the hypothesis is too cumbrous and involved to have much vraisemb lance. If, on the other hand, Professor Owen holds, as every reader would suppose from the general tenor of his reasonings, that not only does there exist an archetypal or ideal vertebrate skeleton, but that there also exists an archetypal or ideal vertebra; then he carries the Platonic hypothesis much further than Plato does. Plato's argument, that before any specie? of object was created it must have existed as an idea of the Creative Intelligence, and that hence all objects of such species must be 520 copies of this original idea, is tenable enough from the anthropo- morphic point of view. But while those who, with Plato, think fit to base their theory of creation upon the analogy of a carpenter designing and making a table, must yield assent to Plato's inference, they are by no means committed to Professor Owen's expansion of it. To say that before creating a vertebrate animal, God must have had the conception of one, does not involve saying that God gratuitously bound himself to make a vertebrate animal out of seg ments all moulded after one pattern. As tnere is no conceivable advantage in this alleged adhesion to a fundamental pattern — as, for the fulfilment of the intended ends, it is not only needless, but often, as Professor Owen argues, less appropriate than some other construction would be (see Nature of Limbs, pp. 39, 40), to sup pose the creative processes thus regulated, is not a little startling. Even those whose conceptions are so anthropomorphic as to think they honour the Creator by calling him " the Great Artificer," will scarcely ascribe to him a proceeding which, in a human artificer, they would consider a not very worthy exercise of ingenuity. But whichever of these alternatives Professor Owen contends for -—whether the typical vertebra is that more or less crystalline figure which osseous matter ever tends to assume in spite of "the ^ea or organizing principle," or whether the typical vertebra is itself an " t^ea or organizing principle" — there is alike implied the belief that the typical vertebra has an abstract existence apart from actual vertebrae. It is a form which, in every endoskeleton, strives to embody itself in matter — a form which is potentially present in each vertebra ; which is manifested in each vertebra with more or less clearness ; but which, in consequence of antagonizing forces, is no where completely realized. Apart from the philosophy of this hypothesis, let us here examine the evidence which is thought to justify it. And first as to the essential constituents of the " ideal typical vertebra." Exclusive of "diverging appendages " which it "may also support," " it consists in its typical completeness of the follow ing elements and parts": — A centrum round which the rest are arranged in a somewhat radiate manner; above it two neurapophyses — converging as they ascend, and forming with the centrum a trian- guloid space containing the neural axis ; a neural spine surmounting the two neurapophyses, and with them completing the neural arch ; below the centrum two hamapophyses and a hcemal spine, forming a haemal arch similar to the neural arch above, and enclosing the hicmal axis ; two pleurapophyses radiating horizontally from the sides of the centrum ; and two parapophyses diverging from the centrum below the pleurapophyses. " These," says Professor Owen, " being usually developed from distinct and independent centres, I have termed * autogenous elements/ " The remaining elements, which he classes as " exogenous," because they " shoot out as continuations from some of the preceding elements," are the diapophyses diverging from the upper part of the centrum as the parapophyses do below, and the zygapophyses which grow out of the distal ends of the neurapophyses and hsemapophyses. If, now, these are the constituents of the vertebrate segment hi its typical completeness ;" and if the vertebrate skeleton consists oi a succession of such segments ; we ought to have in these con stituents, representatives of all the elements of the vertebrate skeleton — at any rate, all its essential elements. Are we then to conclude that the " diverging appendages," which Professor Owen regards as rudimental limbs, and from certain of which he considers actual limbs to be developed, are typically less important than some of the above-specified exogenous parts — say the zygapophyses ? That the meaning of this question may be understood, it will be needful briefly to state Professor Owen's theory of The Nature oj Limbs; and such criticisms as we have to make on it must be in cluded in the parenthesis. In the first place, he aims to show that the scapular and pelvic arches, giving insertion to the fore and hind limbs respectively, are displaced and modified lumnal arches, originally belonging in the one case to the occipital vertebra, and in the other case to some trunk-vertebra not specified. In support of this assumption of displacement, carried in some cases to the extent of twenty-seven vertebrae, Professor Owen cites certain acknow ledged displacements which occur in the human skeleton to the ex tent of half a vertebra — a somewhat slender justification. But for proof that such a displacement has taken place in the scapular arch, he chiefly relies on the fact that in fishes, the pectoral fins, which are the hornologues of the fore-limbs, are directly articulated to certain bones at the back of the head, which he alleges are parts of the occipital vertebra. This appeal to the class of fishes is avowedly made on the principle that these lowest of the Vertebmta approach closest to archetypal regularity, and may therefore be expected to show the original relations of the bones more nearly. Simply noting the facts that Professor Owen does not give us airy transitional forms between the alleged normal position of t!»e scapular arch in fishes, and its extraordinary displacement in the higher Vertebmta ; and that he makes no reference to the embryonic phases of the higher Vertebrate^ which might be expected to ex hibit the progressive displacement ; we go on to remark that, in the case of the pelvic arch, he abandons his principle of appealing to the lowest vertebrate forms for proof of the typical structure. In fishes, the rudimentary pelvis, widely removed from the spinal column, shows no signs of having belonged to any vertebra ; and nere Professor Owen instances the perennibranchiate JJatrac/iia 02 522 rxhibiting the typical structure : remarking that " mammals, birds and reptiles show the rule of connexion, and fishes the exception.*1 Thus in the case of the scapular arch, the evidence afforded by Qshes is held of great weight, because of their archetypal regularity ; while in the case of the pelvic arch, their evidence is rejected as exceptional. But now, having, as he considers, shown that these Lony frames to which the limbs are articulated are modified hcemal arches, Professor Owen points out that the haemal arches habitually bear certain " diverging appendages ;" and he aims to show that the " diverging appendages " of the scapular and pelvic arches re spectively, are developed into the fore and hind limbs. There are several indirect ways in which we may test the probability of this conclusion. If these diverging appendages are " rudimental limbs'" — " future possible or potential arms, legs, wings, or feet," we may fairly expect them always to bear to the haemal arches a relation such as the limbs do. 13 ut they by no means do this. " As the vertebrae approach the tail, these appendages are often transferred gradually from the pleurapophysis to the parapophysis, or even to the centrum and neural arch." (Arch, and Horn., p. 93.) Again, it might naturally be assumed that in the lowest vertebrate forms, where the limbs are but little developed, they would most clearly display their alliance with the appendages, or " rudimental limbs," by the similarity of their attachments. Instead of this, however, Professor Owen's drawings show that whereas the appendages are habitually attached to the pleurapophyses, the limbs, in their earliest and lowest phase, alike in fishes and in the Lepidosiren, are articu lated to the hsemapophyses. Most anomalous of all, however, is the process of development. When we speak of one thing as being developed out of another, we imply that the parts next to the germ are the first to appear, and the most constant. In the evolution of a tree out of a seed, there come at the outset the stem and the radicle ; afterwards the branches and divergent roots ; and still later the branchlets and rootlets ; the remotest parts being the latest and most inconstant. If, then, a limb is developed out of a " di verging appendage " of the haemal arch, the earliest and most con stant bones should be the humerus and femur ; next in order of time and constancy should come the coupled bones based on these ; while the terminal groups of bones should be the last to make their appearance, and the most liable to be absent. Yet, as Professor Owen himself shows, the actual mode of development is the very re verse of this. At p. 1C of the Archetype and Ifomologi&s, he says : — " The earlier stages in the development of all locomotive extremities are permanently retained or represented in the paired fins of fishes. First the essential part of the member, the hand or foot, appears : then the fore-arm ur leg, both much shortened, flattened, and expanded, as in all fins and all embryonic rudiments of limbs ; rinally come the humeral and femoral seg nients ; but this stage I have not found attained in any fish." 52H That is to say, alike in ascending through the Vertebrata gene rally, and in tracing np the successive phases of a mammalian em bryo, the last-developed and least constant division of the limb, is that basic one by which it articulates with the haemal arch. It seems to us that, so far from proving his hypothesis, Professor Owen's own facts tend to show that limbs do not belong to the vertebrae at all ; that they make their first appearance peripherally ; that their development is centripetal ; and that they become fixed to such parts of the vertebrate axis as the requirements of the case determine. But now, ending here this digressive exposition and criticism, and granting the position that limbs " are developments of costal appendages," let us return to the question above put — Why are not these appendages included as elements of the " ideal typical ver tebra? " It cannot be because of their comparative inconstancy ; for judging from the illustrative figures, they seem to be as con stant as the haemal spine, which is one of the so-called autogenous elements: in the diagram of the • Archetypus, the appendage is re presented as attached to every vertebrate segment of the head and trunk, which the haemal spine is not. It cannot be from their com parative unimportance ; seeing that as potential limbs they are essential parts of nearly all the Vertebrata — much more obviously so than the diapophyses are. If, as Professor Owen argues, " the divine mind which planned the archetype also foreknew all its modifications ;" and if, among these modifications, the development of limbs out of diverging appendages was one intended to charac terize all the higher Vertebrata; then, surely, these diverging ap pendages must have been parts of the " ideal typical vertebra." Or, if the " ideal typical vertebra" is to be understood as a crystal line form in antagonism with the organizing principle ; then why should not the appendages be included among its various offshoots ? We do not ask this question because of its intrinsic importance. We ask it for the purpose of ascertaining Professor Owen's method of determining what are true vertebral constituents. He presents us with a diagram of the typical vertebra, in which are included certain bones, and from which are excluded certain others. If re lative constancy is the criterion, then there arises the question — What degree of constancy entitles a bone to be included ? If re lative importance is the criterion, there comes not only the question — What degree of importance suffices t but the further question — How is importance to be measured ? If neither of these is the criterion, then what is it ? And if there is no criterion, does it not follow that the selection is arbitrary ? This question serves to introduce a much wider one : — lias the ideal typical vertebra " any essential constituents at all ? It might r>24 naturally be supposed that though pome bones art so rarely developed as not to seem worth including, and though some that are included are very apt to be absent , yet that certain others are invariable : forming, as it were, the basis of the ideal type. Let us see whether the facts bear out this supposition. In his "summary of modifications of corporal vertebras " (p. 96), Professor Owen 8,'iys — " The hcemal spine is much less constant as to its existence, and is subject to a much greater range of variety, when present, than its vertical homotype above, which completes the neural arch." Again he says — " The hcemapophyses, as osseous elements of a vertebra, are less constant than the pleurapophyses." And again— a The pleurapophyses are less constant elements than the neurapo- physes." And again — " Amongst air-breathing vertebrates the pleurapophyses of the trunk segments are present only in those species in which the septum of the heart's ventricle is complete and impcr- forate, and here they are exogenous and confined to the cervical and anterior thoracic vertebrae" And once more, both the neura- pophyses and the neural spine " are absent under both histological conditions, at the end of the tail in most air-breathing vertebrates, where the segments are reduced to their central elements." That is to say, of all the peripheral elements of the " ideal typical vertebra," there is not one which is always present. It will be ex pected, however, that at any rate the centrum is constant : the bone which " forms the axis of the vertebral column, and commonly the central bond of union of the peripheral elements of the vertebra (p. 97), is of course an invariable element. No : not even this is essential. "The centrums do not pas? beyond the primitive stage of the notochonl (undivided column) in the existing lepidosiren, and they retained the like rudimental state in every fish whose remains have been found in strata earlier than the permian tera in Geology, though the number of vertebrae is frequently indicated in Devonian and Silurian ichthyolites by the fossilized neur- and hsemapophyses and their spines " (p. 96). Indeed, Professor Owen himself remarks that "the neurapo- physes are more constant as osseous or cartilaginous elements of the vertebrae than the centrums" (p. 97). Thus, then, it appears that the several elements included in the " ideal typical vertebra " have various degrees of constancy, and that no one of them is essential. There is no one part of a vertebra which invariably answers to its exemplar in the pattern-group. How does this fact consist with the hypothesis ? If the Creator saw fit to make the vertebrate skeleton out of a series of segments, all formed on essentially the same model — if, for the maintenance of the type, one of these bony segments is in many cases formed out of a coalesced group of pieces, where, as Professor Owen argues, a single piece would have served as well or better ; then we ought to find this typical repetition of parts uai- 525 formly manifested. Without any change of shape, it would obvi ously have been quite possible for every actual vertebra to have contained all the parts of the ideal one — rudimentally where they were not wanted. Even ORe of the terminal bones of a mammal's tail might have been formed out of the nine autogenous pieces, united by suture but admitting of identification. As, however, there is no such uniform typical repetition of parts, it seem? to us that to account for the typical repetition which does occur, by sup posing the Creator to have fixed on a pattern- vertebra, is to ascribe to him the inconsistency of forming a plan and then abandoning it. If, on the other hand, Professor Owen means that the " ideal typical vertebra" is a crystalline form in antagonism with " the idea or organizing principle ;" then we might fairly expect to find it most clearly displaying its crystalline character, and its full com plement of parts, in those places where the organizing principle may be presumed to have " subdued " it to the smallest extent. Yet in the VeHebrata generally, and even in Professor Owen's Archetype, the vertebrae of the tail, which must be considered as, if anything, less under the influence of the organizing principle than those of the trunk, do not manifest the ideal form more com pletely. On the contrary, as we approach the end of the tail, the successive segments not only lose their remaining typical elements, but become as uncrystalline-looking as can be conceived. Supposing, however, that the assumption of suppressed or unde veloped elements be granted — supposing it to be consistent with the hypothesis of an " ideal typical vertebra," that the constituent parts may severally be absent in greater or less number, sometimes leaving only a single bone to represent them all ; may it not be that such parts as are present, show their respective typical natures by some constant character : say their mode of ossification 1 To this question some parts of the Archetype and Homologws seem to reply, " Yes ;" while others clearly answer, " No." Criticising the opinions of Geoffrey St. Hilaire and Cuvier, who agreed in thinking that ossification from a separate centre was the test of a separate bone, and that thus there were as many elementary bone.s in the skeleton as there were centres of ossification, Professor Owen points out that, according to this test, the human femur, which is ossified from four centres, must be regarded as four bones ; while the femur in birds and reptiles, which is ossified from a single centre, must be regarded as a single bone. Yet, on the other hand, he attaches weight to the fact that the skull of the human foetus presents " the same ossific centres " as do those of the embryo kan garoo and the young bird (Nature of Limbs, p. 40.) And at p. 104 of the Homoloyies, after giving a number of instances, he says — ' These and the like correspondences between the points of ossification ol 526 the human foetal skeleton, and the separate bones of the adult skeletons of inferior animals, are pregnant with interest, and rank among the most striking illustrations of unity of plan in the vertebrate organization." It is true that on the following page he seeks to explain this seeming contradiction by distinguishing "between those centres of ossification that have homological relations, and those that have, teleological ones — i.e., between the separate points of ossifica tion of a human bone which typify vertebral elements, often permanently dis tinct bones in the lower animals ; and the separate points which, without such signification, facilitate the progress of osteogeny, and have for their obvious final cause the well-being of the growing animal." But if there are thus- centres of ossification which have homo- logical meanings, and others which have not, there arises the ques tion — How are they always to be distinguished ? Evidently in dependent ossification ceases to be a homological test, if there are independent ossifications that have nothing to do with the homo- logies. And this becomes the more evident when we learn that there are cases where neither a homological nor a teleological meaning can be given. Among various modes of ossification of the centrum, Professor Owen points out that " the body of the human atlas is sometimes ossified from two, rarely from three, distinct centres placed side by side " (p. 89) ; while at p. 87 he says : — " In osseous fishes I find that the centrum is usually ossified from six points." It is clear that this mode of ossification has here no home- logical signification ; and it would be difficult to give any teleo logical reason why the small centrum of a fish should have more centres of ossification than the large centrum of a mammal. The truth is, that as a criterion of the identity or individuality of a bone, mode of ossification is quite untrustworthy. Though, in his " ideal typical vertebra," Professor Owen delineates and classifies as sepa rate " autogenous " elements, those parts which are " usually developed from distinct and independent centres ;" and though by doing so he erects this characteristic into some sort of criterion ; yet his own facts show it to be no criterion. The parapophyses are classed among the autogenous elements ; yet they are auto genous in fishes alone, and in these only in the trunk vertebra, while in all air-breathing vertebrates they are, when present at all, exogenous. The neurapophyses, again, " lose their primitive in dividuality by various kinds and degrees of confluence:" in the tails of the higher Vertebrata they, in common with the neural spine, become exogenous. Nay, even the centrum may lose its autogenous character. Describing how, in some batrachians, " the ossification of the centrum is completed by an extension of bone from the bases of the neurapophyses, which effects also the coalescence of these with the centrum," Professor Owen adds :— " In Pdobates fuscus and Pelobates cultripes, Muller found the en- 527 tire centrum ossified from this source, without any independent points of ossification " (p. 88). That is to say, the centrum is in these cases an exogenous process of the neurapophyses. We see, then, that these so-called typical elements of vertebne have no constant developmental character by which they can be identified. Not only are they undistinguishable by any specific test from other bones not included as vertebral elements ; not only do they fail to show their typical characters by their constant presence ; but, when present, they exhibit no persistent marks of individuality. The central element may be ossified from six, four, three, or two points ; or it may have no separate point of ossification at all : and similarly with various of the peripheral elements. The whole group of bones forming the " ideal typical vertebra" may severally have their one or more ossific centres ; or they may, as in a mam mal's tail, lose their individualities in a single bone ossified from one or two points. Another fact which seems very difficult to reconcile with the hypothesis of an " ideal typical vertebra," is the not infrequent presence of some of the typical elements in duplicate. Not only, as we have seen, may they severally be absent ; but they may seve rally be present in greater number than they should be. When we see, in the ideal diagram, one centrum, two neurapophyses, two pleurapophyses, troo hcemapophyses, one neural spine, and one haBinal spine, we naturally expect to find them always bearing to each other these numerical relations. Though we may not be greatly surprised by the absence of some of them, we are hardly prepared to find others multiplied. Yet such cases are common. Thus the neural spine " is double in the anterior vertebroe of some fishes " (p. 98). Again, in the abdominal region of extinct saurians, and in crocodiles, " the freely-suspended hsemapophyses are com pounded of two or more overlapping bony pieces" (p. 100). Yet again, at p. 99, we read — " I have observed some of the expanded pleurapophyses in the great Testudo elephantopns ossified from two centres, and the resulting divisions continuing distinct, but united by suture." Once more " the neurapophyses, which do not advance beyond the cartilaginous stage in the sturgeon, consist in that fish of two distinct pieces of cartilage ; and the anterior pleurapophyses also consist of two or more cartilages, set end on end" (p. 1)1). And elsewhere referring to this structure, he says : — " Vegetative repetition of perivertebral parts not only manifests itself in the composite neurapophyses and pleurapophyses, but in a small accessory (interneural) cartilage, at the fore and back part of the base of the neura- Dophysis ; and by a similar (interhsemal) one at the fore and back part of most of the parapophyses " (p. 87). Thus the nenral and hagmal spines, the, neurapophyses, the pleu- • 66 528 rapophyscs, the haemapophyses, may severally consist of two or more pieces. This is not all : the like is true even of the centrums. " In Ileptanchns (Sqnalus cinereus) the vertebral centres are feebly and vegetatively marked out by numerous slender rings of hard cartilage in the uotochordal capsule, the number of vertebrae being more definitely indicated by the neurapophyses and parapophyses. ... In the piked dog-fish (Acctnthias) and the spotted dog-fish (Scyllium,} the vertebral centres coin cide in number with the neural arches " (p. 87). Is it not strange that the pattern vertebra should be so little ail- hered to, that each of its single typical pieces may be transformed into two or three ? But there are still more startling departures from the alleged type. The numerical relations of the elements vary not only in this way, but in the opposite way. A given part may be present not only in greater number than it should be, but also in less. In the tails of hornocercal fishes, the centrums " are rendered by cen tripetal shortening and bony confluence fewer in number than the persistent, neural, and haemal arches of that part " — that is, there is only a fraction of a centrum to each vertebra. Nay, even this is not the most heteroclite structure. Paradoxical as it may seem, there are cases in which the same vertebral element is, considered under different aspects, at once in excess and defect. Speaking of the haemal spine, Professor Owen says : — - " The horizontal extension of this vertebral element is sometimes accom panied by a median division, or in other words, it is ossified from two lateral centres ; this is seen in the development of parts of the human sternum ; the same vegetative character is constant in the broader thoracic hiemal spines of birds ; though, sometimes, as e.y experiment it is demonstrated that the intermittent compressions caused by os- dilations urge the sap along the vessels and ducts. And it is also ex perimentally proved that the same intermittent compressions produce exudation of sap from vessels and ducts into the surrounding tissue. That the processes here described, acting through all past time, have sufficed of themselves to develope the supporting and distribut ing structures of plants, is not alleged. What share the natural selection of variations distinguished as spontaneous, has had in estab lishing them, is a question which remains to be discussed. Whether acting alone natural selection would have sufficed to evolve these vascular and resisting tissues, I do not profess to say. That it has been a co-operating cause, I take to be self-evident : it must all along have furthered the action of any other cause, by preserving the in dividuals on which such other cause had acted most favourably. Seeing, however, the conclusive proof which we have that another cause has been in action — certainly on individuals, and, in all proba bility, by inheritance on races — we may most philosophically ascribe the genesis of these internal structures to this cause, and regard natural selection as having here played the part of an accelerator EXPLANATION OF PLATE. Fig. 1. Absorbent organ from the leaf of Euphorbia neriifolia. The cluster of fibrous cells forming one of the terminations of the vascular system is here imbedded in a solid parenchyma. Fig. 2. A structure of analogous kind from the leaf of Ficits elastica. Here the expanded terminations of the vessels are im bedded in the network parenchyma, the cells of which unite to form envelopes for them. Fig. 3. Shows on a larger scale one of these absorbents from the leaf of Panax Lessonii. In this figure is clearly seen the way in which the cells of the network parenchyma unite into a closely- fitting case for the spiral cells. Fig. 4. Represents a m uch more massive absorbent from the same leaf, the surrounding tissues being omitted. Fig. 5. Similarly represents, without its sheath, an absorbent from the loaf of Clusiajiava. Fig. G. End view of an absorbent organ from the root of a Turnip. It is taken from the outermost layer of vessels. Its funnel- shaped interior is drawn as it presents itself when looked at from the outside of this layer, its narrow end being directed towards the centre of the Turnip. P'ig. 7 A longitudinal section through the axis of another such organ, showing its annuli of reticulated cells when cut through. Tin cellular tissue which fills the Ulterior is supposed to be removed. 566 Fig. 8. A less-developed absorbent, showing its approximate con nexion with a duct. In their simplest forms, these structures consist of only two fenestrated cells, with their ends bent round so as to m3et. Such types occur in the central mass of the Turnip, where the vascular system is relatively imperfect. Besides the compara tively regular forms of these absorbents, there are forms composed of amorphous masses of fenestrated cells. It should be added that both the regular and irregular kinds are very variable in their num bers : in some turnips they are abundant, and in others scarcely to be found. Possibly their presence depends on the age of the Turnip. APPENDIX D. ON THE ORIGIN OF THE VERTEBRATE TYPE. [When studying the development of tlie vertebrate skeleton, th?rc occurred to me the following idea respecting the possible origin of the notochord. 1 was eventually led to omit the few pages of Appendix in which I had expressed this idea, because it was unsupported by develop mental evidence. The developmental evidence recently discovered, how- ecer, has led Professor Ilaeckel and others to analogous views respecting the affiliation of the Yertebrata on the Molluscoida. Having fortu nately preserved a proof of the suppressed pages, I am able now to add them. With the omission of a superfluous paragraph, they arc reprinted verbatim from this proof, which dates back to the autumn oj 1865, at which tune the chapter on " The Shapes of Vertebrate Skeletons" was written. — December, 1869.] The general argument contained in Chap. XYT. of Part IV., 1 have thought it undesirable to implicate with any conception more speculative than those essential to it ; and to avoid so implicating it, I transfer to this place an hypothesis respecting the derivation of the rudimentary vertebrate structure, which appears to me worth considering. Among those molluscoid animals with which the lowest verte brate animal has sundry traits in common, it very generally happens that while the adult is stationary the larva is locomotive. The locomotion of the larva is effected by the undulations of a tail. In shape and movement one of these young Ascidians is not altogether unlike a Tadpole. And as the tail of the Tadpole disappears when its function comes to be fulfilled by limbs ; so the Ascidian larva's tail disappears when fixation of the larva renders it uselebS. This disappearance of the tail, however, is not without exception. The Appendicidaria is an Asuidian which retains its tail through out life ; and by its aid continues throughout life to swim about. Now this tail of the Appendicularia has a very suggestive structure. It is long, tapering to a point, and flattened. From end to end there runs a mid-rib, which appears to be an imbedded gelatinous rod, not unlike a notochord. Extending along the two sides of 568 APPENDIX. this mid-rib, are bundles of muscular fibres; and its top bears a gangliated nervous thread, giving off, at intervals, branches to the muscular fibres. In the Appendicularia this tail, which is inserted at the lower part of the back, is bent forwards, so as not to be adapted for propelling the body of the animal head foremost ; but the homologous tails of the larval Ascidians are directed backwards, so as to produce forward movement. If we suppose a type like Ihe Appendicularia in the structure and insertion of its permanent tail, but resembling the larval forms in the direction of its tail, it is, I think, not difficult to see that functional adaptation joined with natural selection, might readily produce a type approximating to that whose origin we are considering. It is a fair assumption that an habitually - locomotive creature would profit by in creased power of locomotion. This granted, it follows that such further development of the tail-structures as might arise from enhanced function, and such better distribution of them as spontaneous variation might from time to time initiate, would be perpetuated. AYhat must be the accompanying changes? The more vigorous action of such an appendage implies a firmer insertion into the body ; and this would be effected by the pro longation forwards of the central axis of the tail into the creature's back. As fast as there progressed this fusion of the increasingly- powerful tail with the body, the body would begin to partake of its oscillations ; and at the same time that the resistant axis of the tail advanced along the dorsal region, its accompanying muscular fibres would spread over the sides of the body: gradually taking'such modified directions and insertions as their new conditions rendered most advantageous. Without further explanation, those who examine drawings of the structures described, will, I think, see that in such a way a tail homologous with that of the Appendicularia, would be likely, in the course of that de velopment required for its greater efficiency, gradually to encroach on the body, until its mid-rib became the dorsal axis, its gangliated nerve-thread the spinal chord, and its muscular fibres the myocommata. Such a development of an appendage into a dominant part of the organism, though at first sight a startling supposition, is not without plenty of parallels : instance the way in which the cerebral ganglia, originally mere adjuncts of the spinal chord, eventually become the great centres of the nervous system to which the spinal chord is quite subordinate ; or instance the way in which the limbs, small and inconspicuous in fishes, become, in Man, masses which, taken together, outweigh the trunk. It may be added that these familiar cases have a further appropriateness • for they exhibit higher degrees of that yame increasing dominance of the organs of external relation, which the hypothesis itself implies. APPENDIX. 569 Of course, if the rudimentary vertebrate apparatus thus grew into, and spread over, a mollnscoid visceral system, the formation of the notochord under the action of alternating transverse strains, did not take place as suggested in § 255; but it does not therefore follow that its differentiation from surrounding tissues was not mechanically initiated in the way described. For what was said in that section respecting the effects of lateral bendings of the body, equally applies to lateral bendings of the tail ; and as fast as the developing tail encroached on the body, the body would become implicated in the transverse strains, and the differentiation would advance forwards under the influences originally alleged. Obviously, too, though the lateral muscular masses would in this case have a different history ; yet the segmentation of them would be eventually determined by the assigned causes. Fo." as fast as the strata of contractile fibres, developing somewhat in advance of the dorsal axis, spread along the sides, they would come under the influence of the alternate flexions ; and while, by survival of the fittest, their parts became adjusted in direction, their segmentation would, as before, accompany their increasing massiveness. The actions and reactions due to lateral undulations would still, therefore, be the causes of differentiation, with which natural selection would co operate. SUBJECT-INDEX. (For this Index the Author is indebted to F. HOWABD COLLINS, Esq., of Edgbaston, Birmingham.) ACACIA, foliar organs, 8, 32, 2-18. ^.calepha : motion of, 1, 55 ; water in, 1, 145. Acari : special creation and effects of, 1, 343 ; development, 1, 373 ; direct transformations in, 1, 489 j segmenta tion, 8, 99. Acrogens : morphology of junrjerman- niacece, 2. 24-27 ; growth and develop ment, 2, 46-53 ; name and growth of, 8, 52 ; tubular structure, 2, 54 ; sym metry, 2, 125 ; inner tissue differentia tion, *#, 256; vascular system, 2, 263; integration, 2, 276, 383 ; agamogenesis, 2, 421 ; growth and genesis, #,430 j genesis and development, 442. Actinophrys: a primary aggregate, 2y 78 j genesis, #, 431. Actinozoa : multiaxial development, 1, 137 ; reparative power, 1, 175 ; diffe rentiation, 1, 306 ; integration, 2, 85 ; symmetry, 2, 172, 174 j growth and genesis, 2, 424. Activity: nutrition and genesis, resume, 2, 470-2 ; and evolution, 2, 474-8. Adaptation : general truths, 1, 184-190, 190-2 ; botanical, 1, 184 ; physiologi cal 1, 185-190; psychological", 1, 186, 188, 189; structural, functional, and inter-dependence, i, 192-6,197-8, 255 ; social and organic stability, 1, 197-9 ; resume, 1, 199 ; multiplication of effects, 1, 424, 465-6 ; direct equili bration, 7, 435-7 ; natural selection and equilibration, 1, 4M-9, 466, 474; of iikin and skeleton, 8, 198, 200 ; outer ti-sue, 2, 295-9, 380 ; skin and mucous membrane differentiation, 2, 307-9, 382; vascular system, 2, 334 ; osseous, S. 343; muscular, 2, 360, 384; per sistence of force and physiological, 2, 387 ; plant multiplication, 2, 391-6 ; vertebrae development, 2, 532-5. Agamogenesis : of heterogenesis, 1, 211, 273 ; development of offspring, 1, 217 ; relation to vegetal growth, 1, 224; physiological units, 1, 288 ; in acti- nozoa, 2, 85 ; in fy/drozoa, 2, 9i ; in nutrition, 8, 163 ; 'laws of multiplica tion, 2, 395. (See also Multiplication) AgaricincB : symmetry, 2, 124 ; tissue differentiation, 2, 239. Agassiz, Prof. L. J. E., zoological classi fication, 1, 298. Aggregates : integration and orders of, 2, 5 ; primary vegetal, 2, 14, 74 ; morphology of vegetal, 2, 15-18, 71-6 ; primary animal, 2, 77-9, 111 ; second ary animal, 2, 79-83, 111 ; tertiary animal, 2, 83-5, 85-7, 111 ; annulose, 2, 97 ; symmetry of primary vegetal, 8, 119-22, 130 ; of secondary, 2, 122-5 ; of tertiary, 2, 125-8 ; morphological differentiation of primary vegetal, 2, 159-6 L ; summary of morphological development, 2, 214. Agrimony, floral symmetry, 2, 152, 154 Air .- in vegetal tissues, #, 536-7, 552, 560, 562.' Albumen : physical and chemical pro perties, 1, 12 ; formula, 1, 14 ; diffusi- bility, 1, 19 ; in animal and vegetal tissues, 1, 38. Alcohols : physical and chemical pro perties, 1, 10-12 ; transformation into acetic acid, 1, 40. Alga; multicentral development, 1, 135 ; axial development, 1, 136 ; uniform tissue and function, 1, 156; ga mo- genesis in conferva, 1, 219 ; unicellular forms, 2, 14 ; integration in conferva, 2, 17; pseudo-foliar and axial develop ment, #, 20-4 ; tubular structure, 2, 54; foliar development, 2, 73, 83; branch symmetry, 2, 130; cell meta morphoses, 2, 160; tissue differentia tion, 2, 228, 378-9 ; outer tissue differentiation, 8, 235, 239, 379 ; inner, #,255 ; integration. 2, 275 ; indefinite- ness, 8, 278; fci'ility, 8, 420, 421; sexual genesis, 2, 42 •*, 429, 430 ; genesis and development, .?, I- 12. Alimentary canal : f iu. -lion, /, 161 j differentiation, 9, 28G, 107-9, 310 ^ 57G SUBJECT-INDEX. 382 ; gizzard development in birds, 2, 312 ; development in ruminants, 2, 314-6 ; differentiation of liver, 2, 316- 21 ; muscularity, 2, 355. Allot ropism : of organic constituents, 1, 4, 9 ; muscular action, 1, 56. Alloys, melting point of, 1, 276. Alternation of generations (see Gamo- genesis). Amceba : central development, 1, 134 ; a primary aggregate, 2, 78 ; symmetry of encysted. 2, 169 ; genesis, #, 422. Ammonia : physical properties, 1, 6 ; chemical properties, /, 9 ; nerve stimu lant, 1, 52. Amphibia : classification of, 1, 308 ; structure and media, 1, 395 ; segmen tation, 2, 109 ; outer tissues, 2, 293 ; respiration, 2, 322, 326 j Owen on skeleton, 2, 521, 526, 527. Amphioxiis : classification, 1, 362; em bryonic development, 2, 108 ; genesis of vertebrate axis, 2, 196-9, 199-201, 205 ; development, 2, 533. Anal as scandens, the climbing fish, 1, 392, 394. Anackaris alsinastrum, individuality in, 1, 205. Anaesthetics, local and general effects, 1, 52. Animals : nutrition and molecular re arrangement, 1, 33-35 ; nitrogeneous character, 1, 37, 38 ; nitrogeneous food of carnivorous, 1, 45 ; sensible motion, 1, 54; changes showing life, 1, 72; length and complexity of life, 1, 84 ; degree of life and environment, 1, 85-8 ; growth, 1, 108 ; organization and size, 1, 110-12 ; growth and nutrition, 1, 112, 119-21, 122, 131; and initial and final bulks, 1, 115, 127- 31, 132 ; and structural complexity, 1, 117-9,132; structure, temperature, and self-mobility, 1, 145-50 ; functions, 1, 154-6, 306 ; functional and structural differentiation, 1, 157-60 ; functional differentiation and integration, 1, 161-4; functional specialization and vicariousneris, 1, 165-6 ; waste in, 1, 169-71, 176, 185; reparative power, Z, 175, 179-82; waste and activity, I, 175-7 ; organic polarity and physio logical units, 1, 182-3, 253 ; modified adaptability, 1, 188 ; stability of types, f,199; what is an individual? 7,203; heterogenesis, growth, and nutrition, 1, 228-33, 235-7 ; homo- nnd hetero genesis and natural selet-tion, 1, 233-7; general truths of heredity, 1, 238-41 ; heredity and breeding, 'l, 242, 252 ; functional alterations, structure and heredity, 1, 246-52, 255 ; variation, /, 257 ; domestication and variation, lt 261, 263 ; variation and adaptation, 1, 269 ; in-and-in breeding, 1, 282-4, 289 ; pure and mixed breeding, 1, 291 ; classification, 1, 298-304 ; distribution and migrations, 1, 312-7, 327; natural barriers and distribution, 1, 317-20, 328,388; distribution in time, 1,320-7, 328 ; special creation and parasitic, lt 342 ; evolution and classification, lt 358, 359, 364, 471; rudimentary organs in, 1, 386, 387, 472 ; evolutu a and varied media of, 1, 391-7, 401, 472 ; E. Darwin and Lamarck on evolution of, 1, 402-10 ; solar influence, 1,412, 472; geologic changes affecting, 1, 413-5, 464', 466, 472 ; interdepen dence with plants, 1, 416-8, 426; complexity of influences affecting, 1, 418 ; persistence of species, 1, 428 ; defensive plant appliances, 1, 437 ; direct equilibration, 1, 439-41, 442, 466, 474 ; natural selection and in direct equilibration, 1, 448-9, 466, 474; natural selection and equilibra tion, 1, 457-62, 474; importance of natural selection *, 468 ; distribution and age of plants and animals, 2, 280 ; muscular colour, 2, 356-9 ; laws of multiplication, #, 391-6; rhythm in numbers, 2, 399 ; law of weights and dimensions, 2, 414 ; growth and asexual genesis, 2, 422-6 ; growth and sexual genesis, 2, 431-6 ; nutrition and genesis, 2, 458. Annelida : phosphorescence, 1, 47 ; seg mentation, 2, 91-3; embryonic deve lopment, 2, 106; bilateral symmetry, 2, 180-3 ; genesis, 2, 425, 433. Annul oida : development, 1, 372 ; inte gration, 2, 94-7 ; symmetry, 2, l77-80j genesis, 2, 425. Annulosa : axial development, 1, 136, 137 ; definition, 1, 307 ; classification, 1, 363; segmentation, 2, 91-3, 111; integration, 2, 94-7, 108; unit and aggregate, 2, 97 ; embryonic develop ment, £, 107 ; bilateral symmetry, 2, 180-3 ; symmetry of vertebrata, 2, 186-9; segmental differentiation, SUBJECT-INDEX. 577 189-91; tmintegrated function in jdanaria, 2, 365; genesis, 2, 425, 43:2; development, and genesis, 2, 433 ; nutrition and genesis, 2, 466. (See also Articulata). Anthropomorphism, former prevalence of, 1, 335. Ants, nutrition and genesis, 1, 467. Aphides : axial development, 1, 137 ; individuality, 1, 206, 2i>7 ; pseudo-par thenogenesis, 1, 214; development of new individuals, 1, 217 ; het orogenesis, 1, 228 ; nutrition and genesis, 2, 455, 466. Araehnida : avoidance of danger, 1, 73 ; oviparous homogenesis, 1, 211 ; de velopment, 7, 371 ; integration and horology, 2, 99-102, 108; bilateral symnv. try, 2, 181. Arrella: symmetry, #, 169; outer tissue differentiation, 2, 291. Arm : development of human, 1, 140 ; functional vicariousness of human, 1, 166. Army, morphological analogy, 2, 6. Arteries (see Vascular system). Ai'thropoda (see Articulafd], Articulata : integration and homology. 2, 99-102, 108; embryonic develop ment, #, 106, 107 ; bila'eral symmetry, 2, 180-3 ; genesis, 2, 426, 433. Ascidians: integration, 2, 86, 88, 89; symmetry, 2, 176, 186 ; origin of vertebvate type, 2, 567. Assimilation, compared with reasoning, 1, 62-8. Aftteroidea, radial symmetry, 2, 178. Astronomy : and growth, 1, 107 ; Schleiden on individuality, 1, 202 ; evolution, 1, 347, 350; classification, 1, 359 ; rhythm of, and organic change, 1, 411-3, 473; law cf equilibration, 1, 433; " mechanical theory," 1, 490 ; co-operation of structure and function, & Q *5 °« Asymmetry (see Morphology). Atavism : occurrence of, 1, 243, 252 ; digital variation, 1, 258-60. Atoms : mechanically considered, 1, 14 ; etherial undulations and oscillations of, 1, 29-33. Axillary buds, origin and development, 2, 61-5. Axis : " neutral" of mechanics, 2, 193 ; genesis of vertebrate, 2, 195-9, 208. (See also Development). BAEB, K. E. TON : formula of, 1, 141-4, 3G5-9, 378 ; zoological classification, 1, 301 ; on animal transitions, 1, 892. SalanophorcB : wax on, 2, 244 ; inner tissue, 2, 257. Bark : varied development, 2, 229-31 ; physiological differentiation, 2, 231- 3, 2 10, 379, 380. Bat, expenditure and genesis, #, 452. Bean, vascular system, 2, 542, 560. Bees (see Insects). Begoniacecc : multiplication, 1, 180, IS I, 253, #, 422; individuality,.?, 207; de velopment from scales, 1, 221 ; symme try, 2, 144, 150-1 ; development, 2, 254. Berkeley, Rev. M. J. : inclefiniteness oi algce said fungi, 2, 278 ; of mosses and ferns, 2, 279. Bilateral symmetry (see Morphology). Biliverdine, function of, 2, 317, 320, 370. Binary compounds : physical properties, 1, 5-7; chemical properties, 1, 7-10 ; combining power and atomic weights, 1, 31. Biology : definition and divisions, 1, 94- 6 ; organic structural phenomena, 1, 96-8; also functional, 1, 98-100; actions and reactions of function and structure, 1, 100-2; genesis,.?, 102; limited knowledge of, 1, 103 ; evolu tion, 1, 347, 349. Birds : growth and expenditure of force, 1, 114, 127 ; size of egg and adult, 1, 116 ; temperature of, 1, 146, 147 ; self-mobility, 1, 147 ; functional and structural differentiation, 1, 157 ; food of starving pigeon, 1, 170 ; vivi parous, 1, 211 ; heredity and pigeon breeding, 1, 242 ; atavism in pigeon, 1, 252 ; osseous variation in pigeon, 1, 258 ; classification, 1, 308 ; distri bution and migrations, 1, 315, 316, 319 ; distribution in time, 1, 326 ; Darwin on petrels, 1, 368 ; rudimentary teeth, 19 370; vertebrse, lt 383, 8, 533; feather development, /, 385; habits of water-ouzel, 1, 397 ; migra tion, 1, 412; egg-shells and direct equilibration, 1, 440; bones and direct equilibration, 1, 441 ; sexual selection, 2t 253 ; wing spurs, 2, 296 ; outer tissue differentiation, 2, 300, 381 ; alimentary canal development, 2, 312, 314; muscular colour and activity, 2, 356-60 ; nutrition, 2, 413 ; cost of genesis, 2, 416; growth and SUBJECT-INDEX. genesis, £, 434, 437 ; heat expenditure and genesis, 2, 440-8, 453; muscular expenditxire and genesis, 2, 448-51, 453 ; mammalian fertility, 2, 449 ; eggs of wild and tame, 2, 457 ; nutri tion of blackbird and linnet, 2, 476 ; Owen on skeleton of, 3, 528, 529, 530. V lainville, H. M. D. de, definition of life, 1, 60, 74. Blister, nervous action in, 2, 299. Blood : similarity of iron peroxide,*.?, 17; segregation of abnormal consti tuents, 1, 152 ; changed by disease, 1, 177, 484 ; assimilative power and or ganic repair, 1, 177-9 ; respiratory tissue differentiation, 2, 292-5 ; pres sure in mammalia, 2, 329, 330. (See also Vascular system). Boismont, A.B.de, human fertility, 2, 484. Bone: adaptability, 1, 187, 2, 200-1; function and weight, 1, 246 ; mam mal1 an cervical vertebra), 1, 309 ; evolution and vertebral column, 1, 382 ; partial development, /, 385 ; size of head as influencing, 1, 424, 451-3 ; direct equilibration and strength, 1, 441 ; skull development, 2, 205 ; theory of supernumerary, 2, 20G ; membranous, cartilaginous, and osseous states, #, 207 ; differentiation, 2, 298, 334-46 ; integration, ,?, 375 ; Owen's theory of vertebrate skeleton, #, 517-35. Botany : influence of heat on plants, 1, 27 ; effect of solar rays, 1, 28-33, 412, 472 ; plants non-nitrogenous, 1, 37 ; fungi nitrogenous, 1, 38 ; generation of heat in plants, 1, 44 ; phospho rescence, 1, 46 ; vegetal electricity, 7, 48 ; sensible plant motion, 1, 53-5 ; vital plant changes, 1, 67, 68 ; changes showing life, 1, 72, 75 ; crystalli zation and vegetal life, 1, 78 ; vital adjustments, 1, 83 ; length and com plexity of life, 1, 84, 85 ; animal and vegetal biology, 1, 96 ; growth, 1, 108 ; protophytic structure, 1, 109 ; organization and growth, 1, 110, 117- 9, 132; growth and nutrition, 1, 112, 119-21, 130, 131 ; relation of initial to final bulks, 1, 115, 132 ; limits to growth, 1, 125 ; growth and expendi ture, 1, 130, 132 ; central protophytic development, 1, 134 ; insubordinate multicentral development, 1, 135 ; axial development, 1, 136, 211 ; uni- aad multi-axial development, 7, 136, 138 ; bud and leaf development, /, 138-41 ; weight, temperature, and self-mobility, 1, 145-50 ; function, 1, 154-6 ; functional and structural com plexity, 1, 156 ; vicarious function, lt 165-6 ; waste and repair, 1, 169, 176 ; multiplication of begoniacece, 1, 180, 181 , 253 ; organic polarity and physio logical units, 1, 182-3, 253 ; adapt a- tion, 1, 184 ; what is an individual? 1, 201-3, 207, 208; homogenesis ex. ceptional, 1, 211 ; heterogenesis, 1, 211, 212; parthenogenesis, 1, 214-6 ; disintegration of genesis, 1, 216-8; reproductive tissue structurally un- differentiated,^, 218-24; gamogenesis in protophyta, 1, 219 ; region of nutrition to growth and gnmogenesis, 1, 224-8, 232, 235-7, 2, 30 ; homo- logncs of ovules, 1, 228 ; nutrition and growth of willow, 7, 233 ; natural selection and homo- ar,d hetero-genesis, 1, 233-7 ; general truths of heredity, 1, 233-41; heredity and "change of habit," 1, 245 ; and " sports," 1, 246 ; variation, 1, 257 ; cultivation and variation, 1, 260, 261, 262-4; cross fertilization, 1, 278 ; self-fertilization, 1, 280-2 ; classification, 1, 29"-8, 3^1 ; distribution and migration, 1, 312-7, 327 ; natural barriers and distribution, 7, 3)7-20, 328, 388; distribution in time, 1, 320-7, 328 ; special creation and parasitism, 1, 343 ; evolution hypothesis, 1, 319; evolution and classification, 1, 358, 364, 471 ; Darwin on floral morphology, 1, 384 ; rudi mentary organs, 1, 886, 387, 472 ; European plants in Xew Zealand, 1, 389 ; distribution, 7, 389-91, 472 ; varied media, 1, 396, 472 ; E. Darrain and Lamarck on plant evolution, 1, 402-10 ; geologic changes affecting, 1, 413-5, 472 ; interdependence of animals and plants, 1, 416-8, 426; complexity of influences on plant?, lt 418 ; equilibration, nutrition, defence, and fertilization, 1, 437-9 ; natural selection and indirect equilibration, 1, 446, 448, 474 ; dimorphism, 1, 448 ; seed distribution, 1, 461 ; importance of natural selection, 1, 468 ; aquatic and terrestrial conditions, 2. 24; natural selection and nutrition, #,48; floral symmetry, 2, 117 ; wood de- velopment, 2, 258-62, 268-71, 272, SUBJECT-INDEX. 57l» 636-66 ; adaptation and multiplica tion, 2, 391-0 ; rhythm in numbers, 2, 399 ; growth and asexual genesis, 2, 419-22 ; growth and sexual genesis, 2, 428-31 ; expenditure, 2, 446 ; hor ticulture, nutrition, and genesis, 2, 456 ; tree development, 2, 522 ; cir culation and wood formation, 2, 536- 61 ; dye permeability and circulation, 2, 533-43, 546-50, 553, 555 ; resume on circulation and wood formation, £, 561—6. (See also Multiplication, Morphology, and Physiology.) Bothriocephalus, development, 2, 466. Botn/iis : uni- cellular, 2, 15 ; symme try, 2, 122. SraoMopoda (see Klolluscoida), Brain : natural selection and evolution, 1, 469 ; size in civilized and uncivil ized, 2, 502. Bramble : leaf development, 2, 28-33 ; leaf symmetry, 2, 139. Branches (see Botany and Morphology). Branchiae (see Respiratory system). Breeding: heredity, 1, 242 ; in-and-in, 1, 282-4, 289 ; pure and mixed, 1, 2'Jl. Bricks, changed equilibrium shown by, 1, 36, 40. Buds : development of axillary, 2t 61-5 ; effects of nutrition, 2, 70. CACTACE2E: foliar and axial develop ment, 2, 37-46 j differentiation in, 2, 240, 241, 259, 266 ; vascular system, 2, 265 ; dye permeability and circula tion, 2, 510, 541 ; wood formation, 2, 544, 546, 547, 559. Canadians, nutrition and genesis, 2, 482. Cancer: cesophageal, 2, 311; and vas cular system, 2, 334. Caoutchouc : analogy from, 2, 283, 310 ; leaf structure, 2, 558. Capillarity, and vegetal vascular system, 2, 262, 269, 537, 539, 554, 556, 561-5. Capilleries (see Vascular system). 1'ai'bon : chemical and physical proper ties, 1, 3-5, 20, 22 ; binary compounds, 1, 5, 8, 9 ; ternary, 1, 10-12 ; quar- ternary, 1, 12-14, 23. Carbonic acid : properties, /, 6, 7, 9 ; excreted by animals, 1, 170 j diffusi- bility, 2, 319. Carbonic oxide, properties, 1, 5, 7. Carpenter, Dr. W. B. : on functional specialization, 1, j.6-4 j amphioxus, 1, 69 363 ; macrocysfis, £, 430 ; nutrition and reproductive function, 2, 439. Cartilage (see Bone). Catalysis, and vital metamorphosis, /, 36, 41. Cell : evolution and doctrine of, 2, 10- 13, 77 ; morphological differentiation, 2, 159-61 ; animal morphology, 2, 210-12 ; morphological summary, 2, 215 ; vegetal tissue differentiation, 2, 231-3, 379 j vascular development, 2, 262-8, 381. Centipede, bilateral symmetry, 2, 181-3. Cephalopoda : bilateral symmetry, 2, 186 ; vascular system, 2, 331. CercaricB (see Entozoa}. Cereus, tissue differentiation, 2, 259, 266. Cestoda (see Eniozoa). C/iaja, wing spurs, 2, 296. Change, and definition of life, 0, 62- 71. Chemistry : properties of organic ele ments, 1, 3-5, 20, 22 ; of binary com pounds, 1, 7-10; ternary, 1, 10-12; quaternary, 1, 12-14, 23; etherial undulations and atomic oscillation, 1, 2U-33 ; chemical affinity and organic change,.?, 33-5, 35-11 ; oxidation and generation of heat, 1, 43-6, 57 ; gene ration of nerve force, 1, 49-53, 57 ; physiology and organic, 1, 98 ; flesh constituents, 1, 125 ; composition of organisms and environment, 1, 145 ; organic development and differential assimilation, 1, 151-2 ; chemical units, 1, 182 ; pi'imitive ideas of elements, lt 333 ; evolution of organic compounds, 1, 479-84, 486. Chesnut, leaf symmetry, 2, 134, 138. Chiton : simulation of segmentation, 2t 104, 105 ; symmetry, 2t 185. Chlorophyll : nutrition and absence of, 2, 70 ; function, 2, 246. Circle, and evolution hypothesis, 1, 348. Circulatory system (see Vascular sys tem). Cirrhipedia, reproductive capacity, 9 417. Civilization : environment and degree of life, 1, 87 ; human evolution and genesis, 2, 501-3. Cladophora : integration, 2, 17 ; pseud- axial development, 2, 20. Classification : subjective conception, /, 59; two purposes of, 1, 292 ; a gradual process, 1, 293; botanical, /, 295-8, 301, 304 ; zoological, 7, 298-301 ; in- 580 SUBJECT-INDEX, complete equivalence of groups, 1, 304, 361, 364, 471 ; group attributes, 1, 305 8 ; the truths interpreted, 1, 308- 10 ; ethnologic and linguistic evolu tion, 1, 356-8, 360, 363; organic evolutior, 1, 358, 363, 471 ; differences in kind and degree, 1, 359-61 ; former structural similarity, 1, 362, 364; Von Baer's formula, 1, 366, 471 ; organic, not uniserial, 2, 102. Classification of the Sciences, The, and evolution and dissolution, 2, 5. Clay, physiological analogy from, #, 283. Clover : flower and axial development, 2, 36 ; symmetry, 2, 137. Coal, social effects of supply, 1, 195-6, 197-8. Cocoa-nut, growth and genesis, £, 437. Cod: ova of, 2, 415; growth and genesis, 2, 433. Codiiim : symmetry, #, 122 ; titssue dif ferentiation, 2, 228. Coelenterata : effect of shock on hydro~ zoa, 1, 55 ; changes in polype, 1, 76 ; axial development, 1, 136, 137 ; self- mobility, 1, 147 ; functional differen tiation, 1, 157, 306 ; vicarious func tion, 1, 165, 166 ; innutrition, 1, 169; reparative power, 1, 175, 180 ; indivi duality, 1, 203, 205, 207 ; hctero- genesi's, 1, 212, 217, 235 ; negative disintegration in htrdrozoa, 1, 216 j reproductive tissue,' 1, 221, 222 ; dif ferentiation in hi/drozoa, 1, 306 ; clas sification, 1, 363 ; functional co-ordi nation, 1, 3G8 ; development, 1, 372 ; development of detached parts, 2, 72 ; integration, 2, 82, 94, 97, 111; gem mation, 2, 83 ; tertiary aggregate, 2, 84, 88, 111 ; radial symmetry, 2, 171 ; symmetry of compound, 2, 174-6 ; cegmental differentiation, 2, 190 ; phy siological differentiation in hydra and analogy, 2, 283 ; tissue reduplication, g, 284-6, 382 ; outer tissue differentia tion, 2, 291 ; osmosis in hydra, 2, 328 ; vascular system in hydra^2, 330, 368 ; asexual genesis, 2, 424; growth and sexual genesis, 2, 432 ; development and genesis, 2, 442; nutrition and genesis, 2, 455. Colloids: T. Graham on, 1, 15-8 ; diffu- eibility, 1, 18-21 ; organic, 21, 23, 24 ; pliability and elasticity, 1, 25 ; capil lary affinity, 1, 26 ; isomerism, 1, 56 ; instability,' 1 , 287 j molecular mobility and diffusibility, 8, 318; nerve tissue differentiation, 2, 346-51, 352 ; mus cular differentiation, 2, 351-5. Colour : sensation of, 1, 51 ; pho3no» gamic, 2, 71, 249 ; light and vegetal, g, 245, 246 ; floral fertilization, g, 249-53; sexual selection, 2, 233 \ activity and muscular, 2, 356-60. Composites, floral symmetry, 2, 157. Comte, A., definition of life, 1} 74. Conferva (see Algce). Conjitgatece (see Alga) Consumption, hereditary transmission, 1, 250. Correspondence, use of word, 1, 79, Cow-parsnip (see Heracleum). Crab (see Crustacea). Creation (see Special creation). Crinoidea, symmetrv, 2, 177. Crocodile, growth, /, 125, 126, 231. Cruciferce, floral symmetry, 2, 149, 155. Crustacea: homogenesis, 1, 211 ; hetero- genesis and nutrition in daphnia, lt 229-30 ; growth and genesis, 1, 231 5 development of lernece, 1, 296 ; hermit crab parasite, 1, 313; amphibious and terrestrial, 1, 317, 393-4; retrograde development, 1, 371 ; segmentation, 1, 380 ; Darwin on jaws and legs, 1, 3S3 ; survival of cirrhipedia, 1, 429 ; renewal of limbs, 2, 72; integration and homology, 2, 99-102, 108; bi lateral symmetry, 2, 181-3; eyes, 2t 303 ; dermal structure of hermit-crab, g, 308, 380 ; fertility, 2, 433 ; nutri tion and genesis, 2, 464. Crystalloids : Prof. Graham on, 1, 15-8 ; diffusibility,.?, 18-21 ; organic, 1, 21-4. Crystals : of " storm glass," 1, 77 ; growth, 1, 107, 108, 109 ; segregation, I, 151, 152, 177, 179, 181 ; equilibra tion, 1, 274 ; physiological units and polarity, 1, 484-92 ; time and fonna- tion, g, 73. Cube, bilateral symmetry, 2, 117. Cuttle fish, individuality of hectocotylus> 1, 207. Cuvier, Baron GK L. C. F. D., zoological classification, 1, 299. Cyanogen, physical and chemical proper ties, 1, 6, 8, 9. Cyclicthys, dermal structure, 2t 288. Dapknia, heterogenesis and nutrition, 229-30. SUBJECT-INDEX. 581 Darwin, C. : Origin of Species, 1, 101 ; natural selection and function, 1, 246 ; atavism, 1, 252 ; osseous variations in pigeons, 1, 258 ; plant variation and domestication, 1, 262 ; " spontaneous variation," 1, 264, 480 ; plant fertiliza tion, 1, 278 ; intercrossing and self- f civilization,!, 281,282: intercrossing, 1, 284 ; plant fertilization and distri bution, 1, 313 ; habits of birds, 1, 316 ; distribution and natural barriers, 1, 318, 388 ; disappearance and non -re appearance of species, 1, 322 ; distri bution in time and space, 1, 326 ; linguistic classification, 1, 357 ; classi fication of organisms, ly 358 ; classifi cation and descent, 1, 364 ; on petrels, 1, 368 ; suppression of organs, 1, 370 ; development of cirrhi'pedia, 1, 371 ; jaws and legs of cru-stacta, 1, 383 ; aborted organs, 1, 386 ; vegetal distri bution, 7,390; opinions of E. Darwin and Lamarck, 1, 403 ; natural selection and indirect equilibration, 1, 444-9, 466 ; changes without natural selec tion, 1, 449-57 ; A. R. Wallace, 1, 469; floral fertilization, 2, 153, 250, 571 ; sexual selection, 2> 253 ; attach ment of climbing plants, 2, 259 ; vegetal fructification, 2, 277 ; animal sterility and domestication, 2, 461 ; natural selection of, 2, 500. Darwin, Dr. E.,modifiability of organisms, 1, 402, 404-9. Death : and vital correspondence, 1, 83, 88 ; only limit to vegetal growth, !, 125; physiological integration, 2, 366, 385 ; cause of natural, 2, 393 ; relation to births, 2, 397. Definiteness : of vital change, 1, 68-71, 88, 91 ; developmental, 1, 150 ; func tional,!, 168 ; segregation of evolution, 1, 426-8, 429-31. Definition, difficulties of, !, 59, 0, 10. 2)endrobium (see Orchids). Desmidiacece : unicellular, #, 14; linear and central aggregation, 2, 16 ; natural selection and symmetry, 2, 119, 120 ; morphological differentiation, 2, 160 ; tissue, 2, 226 ; genesis, 2, 420, 429. Development : an increase of structure, 1, 133, 2, 440 ; primarily central, 1, 133, 137 ; uni- and multi-central, 1, 134, 135, 137, 138; axial, 1, 135, 138; uni- and multi-axial, 1, 136-7, 138 ; a change to coherent definite hetero geneity, 1, 138-41, 151 ; Von Baor's formula, 1, 141-4; individual diffe rentiation from environment, 1, 145-50; cell formation, 1, 182 ; discontinuous, and agamogenesis, 1, 215 ; Prof. Hux ley's classification, 1, 215 ; direct and indirect, 1, 371-8; continuous and discontinuous vegetal, #, 49; summary of physiological, 2, 377-88; nutrition and genesis, resume, 2, 170-2; evolu tion, 2, 474-8 ; commencement of genesis, 2, 479 ; of vertebrate limbs, #, 522. (See also Multiplication). Development ki/vothesis, T/te, arguments from, 1, 333,' 348. Dialects (*ee Language). Dialysis, and ditfusibility, 1, 19, 20. Diastase, decomposition of, 1, 36, 37. DiatomacecE : unicellular forms, /, 14 ; aggregation, 1, 15 ; natural selection and symmetry, 1, 120; tissue, 2, 226 ; genesis, 2, 420, 428. Differentiation (see Morphology and Physiology) . Dljjlugia: primary aggregate, 2, 78; symmetry, 2, 1 69 ; outer tissue diffe rentiation, 2, 291. Diffusion, of colloids and crystalloids, 1, 18-20 ; 2, 318. Digestion : and obesity, 2, 459-62 ; fer tility, 2, 487. Diphi/es: individuality,!, 203; symmetry, 2, 175. Disease : segregation of blood constitu ents, !, 152 ; blood changes from, 1. 177, 484 ; heredity, !, 244, 250, 252 j special creation, 1, 335, 342 ; dermal structure, 2, 289 ; exudation of dropsy, 2, 298 ; hypertrophy and vasculai system, 2, 299; intestinal musculai hypertrophy, 2, 312 ; indigestion and alimentary canal development, 2, 315 ; jaundice and biliverdine, 2, 317, 321 ; localization of excretion, 2, 319 ; membranes of croup, 2, 333; osseous differentiation in rickets, 2, 343 ; jaundice and functional specialization, 2, 370; fatty degeneration, 2, 460. Disintegration, physiological (see Phy siology) . Distoma : metagenesis, /, 213 ; disin tegration of genesis, 1, 216. Distribution : the two kinds, !, 311 ; and migration of plants and animals, 1, 312-7, 327; natural barriers, 1, 317-20, 327, 388; of animals and 582 SUBJECT-INDEX. plants in time, 1, 320-7, 328; botanical, in New Zealand, 1, 389 ; region and organism, lt 389-91, 401; through Taried media, 1, 391-7, 401, 472; past and present organic forms, 1, 399-401, 472; vegetal and animal complexity, 2t 280. Division of labour, physiological, (see Physiology). Dog : lives of tortoise and, 1, 84, 85 ; heredity and habits, 1, 247 ; abnormal digits, 1, 262 ; expenditure and genesis, #, 452 ; nutrition and genesis, 2, 458. Domestication : animal and vegetal variation, 1, 260-2, 264; and fertility (see Multiplication). Doublcday, E., on nutrition and genesis, 2, 483-5. Dropsy (see Disease). Drosera: individuality, /, 208; prolife rous growth, 2, 71. Du Bois-Eeymond, E. H., electricity from muscles and nerves, 1, 47. Dualism, and evolution, 1, 491. Dye : tissue absorption, 2, 262 ; vegetal circulation and staining, 2, 538-43, 546-50, 553, 555. EAE, development of vertebrate, 2, 304, 306. Earth, climatic rhythm and organic change, 1, 411-3, 473. Earth-worm, bilateral symmetry, 2, 182, 183. EcJiinococcus (see Entozoa). EcJiinodermata, symmetry, #, 177-80. Ectoderm : functional differentiation, 1, 158, 159 ; functional vicariousness, 1} 165. Effects, multiplication of: variation, 1, 265, 269 ; organic evolution, 1, 423-6, 428, 430, 465, 473 ; morphological de velopment, 2, 7-9, 216 ; physiological differentiation, 2, 383-4, 385. Eggs (see Embryology) . Electricity : genesis in organic matter, 1, 47-9, 57 ; muscular action, 1, 56. Elephant, growth and genesis, 2, 439. Embryology : as aiding biology, 1, 97 ; simulation of growth, 1, 108 ; initial and final organic bulks, 1, 115, 128, 132 ; foetal flesh constituents, 1, 125 ; human arm development, 1, 140 ; Von Baer'g formula, 1, 141-4, 365-9, 378 ; embryonic heat, 1, 149 ; spherical organic form, /, 149 ; functional diffe rentiation, 1, 159; individuality, 1, 204; unspecialized reproductive tissue, 1, 219-24, 254; changes following impregnation, 1, 223 ; nutrition and vegetal growth, 1, 224-8, 235-7 ; and animal growth, 1, 228-33, 235-7 ; physiological units and heredity, lt 253-6 ; variation and parental func tional condition, 1, 261 ; physiological units and variation, 1, 264-7, 267-8, 269-70 ; fertilized and unfertilized ova, .?, 277; hermaphrodism, 1, 278-9, 280 ; animal and vegetal self-fertili zation, lt 279-82, 290; in-and-in breeding, 1, 282-4, 289 ; eggs of entozoa, 1, 297 ; evolution hypo thesis, 1, 349, 351, 471 ; petrel de velopment, 1, 368 ; substitution and suppression of organs, 1, 369-71, 378, 384 ; direct and indirect development, 1, 371-8 ; size of ova and develop ment, 1, 373; egg-shell function, 1, 440 ; direct transformations and phy siological units, 1, 489 ; transforma tion of blastema, 2, 13 ; arrest of growth and innutrition, 2, 70 ; &&• velopmentofdorsibranchiata, and luli- colce, 2, 92 ; annul osa, 2, 94; adaptive vertebrate segmentation, 2, 105-10, 111, 208 ; animal cell morphology, 2, 210; lung development,^, 321, 322; mammalian ova maturation, 2, 333 ; movements of ova, 2, 346, 354; modi fications in mole, j?, 384 ; genesis and nutrition, 2, 404, 405 ; fish ova, 2, 415, 433 ; cost of genesis, 2, 415-6 ; num ber of birds' eggs, 2, 435, 457; heat and genesis, 2, 447, 453 ; muscular expenditure and genesis in birds, 2, 418-51, 453 ; vertebrate limb develop ment, 2, 522 ; ossification in vertebrates, 2, 525 ; Owen's vertebrate theory, 2, 532 ; development of vertebrae, 2, 533. (See also Multiplication). Endoderrn: functional differentiation, /, 158, 159 ; functional vicariousness, lt 165. Endogens : mode of growth, 2, 56-8, 65-9, 75, 165 ; growth and genesis, #, 430; physiological integration, 2, 276, 383. Entozoa: metagenesis, 1, 213; self-fer tilization, 1, 280 ; eggs, 1, 297 ; dis tribution, 1, 314 ; and special creation, 1,342-3; development, 1, 378 ; direct SUBJECT- INDEX. 583 transformation, 1, 489; integration, 2, 94 ; genesis, 2t 425 ; nutrition and genesis, 2, 465. Environment : degree of life and com plexity of, 1, 85-8 ; relation to organic structure and function, 1, 145-50. Holis : bronchia?, 2, 105 ; outer tissue, 2t 293. Epiclei'mis (see Skin). Epilepsy, and heredity, 1, 251. Epithelium: reproductive tissue,!, 221; "pavement" and "cylinder," 2, 211. Epizoa : development oi lernece, 1, 290; distribution, 1, 314 ; special creation and effects of, lt 343 j nutrition and genesis, #, 464. /Ljuilibration : variation and law of, 1, 263, 270 ; molecular arrangement, 1, 274-8 ; of organic change, 1, 285, 462, 474 ; direct and indirect, 1, 432-5 ; adaptation and direct, 1, 435-7, 4C6, 474 ; nutrition, defence, and fertiliza tion of plants, 1, 437-9 ; direct, of animals, 1, 439-41, 442, 466, 474; natural selection and indirect, 1, 443-9, 4f>6, 474; of natural selection, 1, 457- 62, 469, 474 ; increasing importance of direct, 1, 468 ; tissue differentia tion, 2, 227 ; functional, 2, 381-7; laws of multiplication, 2, 391-6 ; of forces acting on species, 2, 397-9, 410; human and social evolution, 2, 507. (See also Natural selection). Ethnology : heredity, 1, 240, 248 ; plas ticity of mixed races, 2, 291 ; primi tive ideas, 1} 333 ; evolution and classification, 1, 356-8, 363 ; natural selection, 1, 468. JEuphorbiacecB : foliar and axial de velopment, 2, 36-46 ; physiological differentiation, 2, 240 ; dye permea bility and circulation, 2, 540 ; wood formation, 2, 544, 546, 547 ; foliar vascular system, 2, 558-61, 565. Evaporation : organic change, 2, 27 ; vegetal circulation, 2, 556. Evolution : definitions of life, 1, 89-93 ; implies growth and development, 1, 107, 133 ; formula supported by de velopment, 1, 138-41, 150 ; show's life prior to organization, 1, 167 ; formula Bupp jrted by function, 1, 168 ; sta bility of species, 1, 199, 428, 430; individuality, 1, 204 ; genesis, heredity, and variation resulting from, 2, 291 ; hypothesis of special creation, 1, 331, 344; derivation, 1, 346, 355, 470; gradual development of hypothesis, 1> 346-8, 355 ; circle and straight line, 1, 348; conceivability, 1, 348-51, 355 ; support from direct evidence, 1. 351-3, 355 ; malevolence not im plied by, 1, 353-5, 355 ; Von Baer's formula, 1, 365-9, 378; substitu tion and suppression of organs, 1, 369-71, 378, 384; segmentation of articulata, 1, 380-2; vertebral column development, 1, 382-4 ; rudimentary organs, 1, 384-7, 472; vegetal ani animal changes of media, 1, 391-7, 472 ; considered historically, 1, 402-4, 408-10 ; instability of the homo geneous, a cause, 1, 421-3, 428, 465 ; multiplication of effects, 1, 423-6, 429-31, 465 ; segregation, and hetero geneity and definiteness of, 1, 426-8, 429-31, 465 ; natural selection and general doctrine, 1, 457-62, 47-4 ; classification, embryology, morpho logy, and distribution, 1, 471-2 ; in organic and the System of Philosophy, 1, 479 ; " spontaneous generation," 1, 479-84,485 ; materialism and dualism, 1, 490-2 ; dissolution and problems of morphology, 2, 4-6 ; morphology and formula, 2, 7-9, 213-7 ; difficulties of definition, 2, 10 ; cell-doctrine, 2, 10-13, 77; first plants unicellular 2, 14; resume of vegetal morphology, 2, 74-6 ; physiological problems, 2, 221-5; tissue' differentiation, #, 226-8, 378 ; race and individual multiplica tion, 2, 408-10 ; declining fertility of, 2, 411, 501-3 ; individuation and gene sis, 2> 474-8 ; human life, prospective, 2, 494-7 ; forces influencing human, #, 497-500 ; future of population, 2, 504-7 ; self-sufficingness of, 2, 507 ; vertebral, 2, 532-5. Excretion, localization of, 2, 319-21. Exogens: growth, 2, 58-61, 65-9, 75; outer stem and leaf tissues, 2, 240 ; physiological integration, 2, 276, 383; growth and genesis. 2, 430 Expenditure (see Multiplication). Eye, the : waste and repair, 1, 173-4 ; transmission of defects, 1, 244; uso and disuse, 1, 247, 249 ; migration in pleuronectidee, 2, 188 1 differentia tion, 2, 303-5. 584 SUBJECT-INDEX. FATS, the : physical and chemical pro perties, 1, 10-12 ; non-nitrogenous, 1, 38 ; action of bile, 8, 317. Feathers, development, 1, 385 ; 2, 299- 302. Feet, heredity and size, 1, 248. Ferments, changes and nitrogenous character of, 1, 35, 37, 39. Ferns : foliar development and nutrition, 8, 73 ; inner tissue differentiation, 2, 256 ; indefiniteness, 2, 279 ; genesis, 8, 421, 442. Fertility (see Multiplication). Ficus, foliar structure, 2, 558, 565. Fingers : development of human, 1, 140 ; heredity and abnormal, 1, 243, 252, 258-60 ; abnormal number, /, 385. Fish : sizes of ova and adult, 1, 115 ; growth of pike, 1, 126, 231 ; tempera ture, 1, 146, 147 ; self-mobility, 1, 147; alimentation, 1, 170, 8, 314; symmetry, 1, 188 ; genesis, 1,211; 2, 415, 416 ; growth and genesis, 1, 231 ; 8, 433 ; classification, 1, 308 ; change of media, 1, 317, 392 ; distribution in time, 1, 324, 325; the climbing, 1. 392, 394 ; migrations, 1, 412 • dermal structure, 1, 440 ; 8, 288, 300-2, 380 ; segmentation, 2, 109 ; bilateral sym metry, 2, 186-9; eye of pfeuronectidce, 2, 188 ; genesis of vertebrate axis, 2, 195-9,202-4; ossification of palaeozoic, #, 201 ; respiratory organs, 2, 322-7 ; activity and muscular colour, 2, 356- 60; Owen on skeleton, 8, 521, 526, 527-9, 531, 533. Flowers, shape of (see Morphology). Food (see Nutrition). Foraminifera : primary aggregate, 2, 78, 111 ; progressing integration, #, 80-3, 111 ; symmetry, 2, 170. Force : action on like and unlike units, 1, 5 ; expenditure and organic growth, lt 121-6, 131, 132; functional ac cumulation, transfer, and expenditure, 1, 154-6, 306; waste and expenditure, 1, 170-2 ; conception of, 1, 491 ; distri bution during strain, 2, 192-5 ; persis tence cf (tee Persistence of force). Fossils (see Paleontology). Fowls (see Birds). Foxglove: abnormal development, 1, 226, 228 ; 8, 36 ; floral distribution, 8, 126 ; nutrition and growth, 8, 163. France, rate of multiplication, 8, 482, 485. Frankland, Dr. E., on isomerism of pro tein, 1, 483. Fries, E. : on indefiniteness of alga and fungi, 2, 278 ; reticularia, 2, 430. Fuci : cell multiplication, #, 19 ; pseudo- foliar and axial development, 2, 22 ; undifferentiated outer tissue, 8, 239. Function : biology and phenomena of, 1, 94-6, 98-100; co-ordinate structural modifications, 1, 100-3; precedes structure, 1, 153, 167 ; divisions of, 1, 151-6,306; structural complexity,/, 156, 167 ; progressive structural diffe rentiation, 1, 157-60 ; differentiation and integration, 1, 160-4; speciali zation and vicariousness, 1, 164-7 ; formula of evolution, 1, 168 ; dimi nished ability and overwork, 1, 171 ; growth and increased, 1, 185-90, 190- 2 ; interdependence of social and organic, 1, 192-6, 197-9; structure and heredity, 1, 244-52, 255-6 ; aids natural selection, 1, 246 ; organic interdependence, 1, 255 ; parental con dition and variation, 1, 261, 263 ; variation and altered, 1, 262-4, 269- 70 ; as causing variation, 1, 270-2 ; effect on physiological units, 1, 289, 291 ; zoological classification, 1, 305- 8 ; multiplication of effects, 1, 424 ; law of equilibration, 1, 432-5, 473 ; correlation of changes in, 1, 443 ; structiual effects of changing, 1, 455- 7 ; structural co-operation, 2, 3, 200 ; vicarious vegetal, 2, 254; vicarious- ness and specialization, 2, 276 ; epi dermic structure, 8, 295-9, 380; structure and muscular, 8, 360, 384 ; structural repair and growth, 8, 361- 4 ; equilibration and adaptation, 2t 385 ; persistence of force and adapta tion, 2, 387. (See also Physiology). Fungi: nitrogenous character, 1, 38; multicentral development, 1, 135 ; axial development, 1, 136 ; unicellular, 2, 14 ; integration, 2, 17, 276 ; syn> metry, 2, 122-5, 130; puffball tissue, 2, 228, 235, 379 ; tissue differentia* tion, 2, 239; inner tissue, 2, 262; in- definiteness, 2, 278 ; sexual genesis, #, 429, 430 ; growth and genesis, 2, 438 ; nutrition and genesis, 8, 464. Gillinacece (see Birds). Grauaogenesis : hornogenesis, 1, £10 f SUBJECT-INDEX. 585 helerogcnesis, 7, 211, 273 ; offspring, development in, 1, 217 ; reproductive tissue structure, 1, 218-2i; vegetal nutrition, 1, 224-8, 232 ; 2, 30 ; animal nutrition, 1, 228-33, 230 ; when and why does it recur? 1, 233-7, 273-8; effect on species, 1, 281-6 ; leaf forma tion, 2, 30 ; molluscan, 2, 103, 105 ; vertebrate, 2, 105 ; growth, 2, 249. (See also Multiplication). Gasteropoda (see Mollusca), Gemmation : and genesis, lt 212-6 ; annulose, 2, 93-7, 98. Generation, and genesis : the words, 1, 209. Genesis (see Multiplication). Gentiana : floral arrangement, #, 571-4. Genus: indefinite value, 1, 305, 361 ; in stability of homogeneous and heteroge neity of, 1, 421-3, 428, 429-31, 465, 473. Geology: growth displayed in, /, 107, 108; distribution in time, 1, 320-7, 328 ; special creation, 1, 335, 340 Devolution, 1, 347, 352 ; record congruous with evo lution, 1, 397-401, 472 ; organic influ ence of changes, 1, 413-5, 464, 466, 473 ; climatic influence of changes, /, 415 ; human evolution and changes, #, 504. Germ cell : unspecialized, 1, 219-24, 253 ; dissimilarity, 1, 265-7, 267-8, 269-70, 280 ; equilibrium, 1, 277. Gizzard, development of birds, 2, 312. Glass, molecular rearrangement, 1, 274, 289, 487. Glove : structural analogy, 2, 285 ; strain analogy, 2, 544. Goethe, J. \Y . von : foliar homology, 2, 34, 513, 514, archetypal hypothesis, 2, 109 ; vegetal fructification and nutrition, 2, 164 ; theory of supernu merary bones, 2, 206 j on the skull, 2, 530. Gurilla, callosities, 2, 295. Gould. J., Birds of Australia, 2, 443. Gout (-see Disease). Graham, T. : properties of water, 1, 9 ; 2, 350 ; colloids and crystalloids, 1, 15-8; their diffusibility, 1, 18-21; sapid and insipid substances, 1, 51. Qr amines : foliar surfaces, 2, 57, 246 ; floral symmetry, 2, 150 j physiological differentiation, 2, 240. Gravity : effect on vascular system, 2, 298 ; vegetal circulation, 2, 555. Gregarina ; central development, 1, 134 j primary aggregate, 2, 78 ; symmetry, #, 169 ; asexual genesis, 2, 423. Growth : organic and inorganic, 1, 107- 9 ; simulation of, 1, 108 ; limits to, 1, 109; structural complexity,!, 110- 12, 117-9, 132 ; null-it ion, 1, 112, 119- 21, 131 ; expenditure of force, 1, 113- 5, 131 ; initial and final bulks, /, 115-6, 127-31, 132 ; final arrest of, 1, 121-6 ; unceasing, 1, 126 ; resume, witli generalizations, 1, 131 ; defined,. 1, 133 ;. 2, 440 ; increased function, lt 185-90, 190-2; functional interde pendence, 1, 192-6, 197-9; nutrition and vegetal, 1, 224-8, 2:32, 233-7, 273, 2, 30 ; heterogenesis and animal nu trition, 1, 228-31, 236, 273; homo- and hetero-genesis r,nd natural selec tion, 1, 233-7; o. acrogens, 2, 52; cylindrical form of vegetal, 2, 53-61 ; endogenous, 2, 56-8, 75; exogenous, 2, 58-61, 75 ; plant differentiation, 2, 114-6 ; tissue differentiation, 2, 3G1-4 ; vegetal, and asexual genesis, 2, 419-22 ; animal, and asexual genesis, 2, 422-6 ; antagonistic to asexual genesis, 2t, 426 ; vegetal and eexual genesis, 2, 428-31 ; animal and sexual genesis, 2, 431-6 ; antagonistic to sexual genesis, 2, 436-8 ; nutrition and genesis, resume, 2, 470-2 ; evolution, 2, 474-8 ; commencement of genesis, 2, 479. Gunpowder, nitrogenous instability, 1, 8, 40. Gymnotus, electricity of, 1, 43. HATES : non-conductors of heat, 1, 440 ; vegetal, and natural selection, 1, 446 ; development, 2, 299-302 j tactual organs, 2, 302. Hand : development of human, 1, 140 ; heredity and size of, 1, 248. Hare : activity and muscular colour, 2t 356-60; expenditure and genesis, 2t 451. Uarley, Dr. T., on biliverdhio, 2, 317. Head, structural influence of size, /, 424, 451-3. Heaving, sense of, 1, 51. Heart (s<-e Vascular system). Heat : absorption by water vapour, 1, 7 ; action on binary compounds, 1, 8-9, 22 ; on ternary, 1, 10-12 ; on colloids and crystalloids, 1, 24 ; organic changes from evaporation, 2, 27 i 585 SUBJECT-INDEX. decomposition by, 1, 31 ; organic oxida tion, 1, 43-6, 57 ; growth aucl organic, 2, 124 ; animal, vegetal, and environ ment, 1, 145-6, 149 ; alloy melting points, 1, 276 ; organic change and rhythm in terrestrial, 1, 411, 473 ; effect on physiological units, 1, 488 ; fish respiratory organs, 2, 325 ; evolved by lion -nitrogenous substances, 2, 353 ; animal preservation, 2, 414 ; verte brate expenditure and genesis, 2t 447— 8, 453 ; insect genesis, 2, 455. Hectocotylus, individuality, 1, 207. Uepaticce : Schleideu on, ^, 47, 49 ; continuous and discontinuous develop ment, 2, 49 ; vascular system, 2, 263 ; genesis and development, 2, 412. Meracleum : leaf symmetry, 2, 139-40 ; floral symmetry, 2, 156 ; axial and foliar organs, 2, 511-16. Heredity : structural modification, 1, 189 ; general truths, 1, 233-41; trans mission of congenital peculiarities, 1, 211-4; atavism, or recurrence of an cestral traits, 1, 243, 252 ; structure and altered function, 1, 241-52, 255- 6; physiological units, resume, 1, 286- 91; natural selection, 1, 460-2, 469, 474 ; ethnology and natural selection, 1, 469 ; cell doctrine, 2, 12 ; physio logical development, 2, 224, 229; wood formation, 2, 270 ; tissue diffe rentiation, 2, 286, 295-9, 380 ; respira tory system, 2, 293 ; osseous differen tiation, £, 342 ; muscular adaptation, 2, 359 ; persistence of force and phy siological adaptation, 2, 3^7 ; vegetal vascular system, 2, 543, 551, 557, 565. Hernia phrodism, vegetal and animal, 1, 278-9, 280. Heterogeneity : of vital change, 1, 65-71, 88, 90; of development, 1, 133-41, 150; functional, 1, 160-4, 168 ; of organic matter, 1, 286-91 ; organic, and in stability of homogeneous, 1, 421-3, 430, 465, 473 ; segregation of evolu tion, 1, 426-8, 429-31, 465. Ueterogenesis : classification, 1, 210-6, 273; animal nutrition, 1, 228-33, 235-7 ; natural selection, 1, 233-7 ; heredity, 1, 238. Histology (see Physiology). Hollyhock, floral symmetry, 2, 152, 154. Homogeneous, instability of the : vari ation, 1, 264-7, 267-8, 269-70, 280; evolution, 1, 4*21-3, 428, 465, 473 j morphological development, 9, 7-9, 216 ; direction of vegetal growth, 2t 164 ; radial symmetry, 2, 173 ; phy siological differentiation, 2, 377, 385. Homogeuesis (see Gamogenesis). Honiology, articulate integration, 2t 99- 102, 108. Hooker, Sir J. D. : European plants in New Zealand, 1, 389 ; vegetal dis tribution, /, 391 ; amphibious and terrestrial plants, 1, 396; vegetal growth, 2, 53 ; balanophora, 2, 244 j balanophorts and rafflesiacecs, 2, 257; structural complexity, 2, 278, 280; vegetal and animal distribution ard age, 2, 280 j bean vascular system, 2t 543. Hooker, Sir W., onjungermanniacea, 2, 49. Horns, natural selection and size, /, 451-3. Huxley, Prof. T. H. : "continuous" and " discontinuous " development, 1, 135 ; life without organization, 1, 154; pseud-ova, 1, 214; classification of development, 1, 215 ; hermaphro- disin, 1, 281 ; zoological classification, 1, 3;)l-3, 307 ; " On persistent types/' 1, 324-6; the cell -doctrine, 2, 13; SjJOiigiJa, 2, 81 ; articulata, 2, 101 ; vertebrate embryo, 2, 106, 108; eckino- dermata, 2, 179 ; molluscan sym metry, 2, 185 ; ossification, 2, 207 ; cozleiiterata, 2, 285 ; tegurnentary organs, 2, 280, 297, 300 ; vertebrate sensory organs, 2, 304, 305 ; "Kyber on aphis, 2, 455; Owen's vertebrate theory, 2, 532. Hyacinth, symmetry, 2, 126, 147. Hydra (see CoBlenterata). Hydro-carbons : physical properties, 1, 6 ; chemical properties, 1, 8, 9 ; of living tissue, 1, 10 j tissue differentia tion, 2, 363. Hydrogaslrum : symmetry, 2, 122 ; sexual genesis, 2, 429. Hydrogen: chemical and physical pro perties, 1, 3-5, 20, 22 ; binary com pounds, 1, 6, 8, 9 ; ternary, 1, 10-12 j quaternary, 1, 12-14, 23. Nijdrozoa (see Coelenterata), Hymenoptera (see Insects). Hypertrophy (see Disease). IDEAS (see Psychology), SUBJECT-INDEX. 587 Individuality: the botanical, J?, 201-3; the zoological, 1, 203-4 ; the fertilized germ product, 1, 204-6 ; definition of life, 1, 20G-8. Individuation : genesis, 2, 408-10, 472 ; total cost, 2, 415-7 ; genesis and evolution, 2, 474-8. Infusoria: functional specialization, 1, 306 ; primary aggregate, 2, 79 ; lack ing symmetry, 2, 170, 171 ; tissue differcntiMion, 2t 282, 378; genesis, 2, 423, 432. Injuries, repair of animal, 1> 175 179- 82, 253. Insanity, and heredity, 1, 252. Insects : temperature, 1, 44, 146 ; phos phorescence, 1, 46 ; self-mobility, 1, 147 ; homogenesis, I, 211 ; partheno genesis in lepidoptera, 1, 214, 217, 233 ; growth and reproduction, 1, 231 ; vegetal and animal distribution, 1, 313 ; distribution in time, 1, 324 ; development, 1, 371, 373; segmenta tion, I, 380 ; aborted organs, 1, 386, 387; East Indian distribution, 1, 390; floral fertilization, 1, 438; 2, 153-5, 158, 250-3, 571; integration and homology, 2, 99-102, 108 ; bilateral symmetry, 2, 181 ; sexual selection, 2, 253 ; eyes, £ 303 ; environment, 2, 413 ; cost of genesis, 2, 416, 417 ; development and genesis, 2, 441 ; nutrition and genesis, 2, 455. 406-8. Instability of the homogeneous (see Homogeneous). Integration : morphological composition, 2, 4-6; ariicv.lata, 2, 99-102, 108; vegetal physiological, 2, 2-75-8, 278-81, 383 ; genesis, 2, 404, 406-8. Internodes : varied development, 2, 35- 6 ; nutrition and length, 2, 162. Intestine (see Alimentary canal). Irish, nutrition and genesis, 2, 483. Iron : isomerism of compounds, 1, 4 ; colloidal form of peroxide, 1, 17, 20 ; molecular rearrangement, 1, 274, 289, 487 ; vegetal absorption, 2, 542. Iron industry, interdependence of social function, 1, 194-6, 197-8. Isomerism : of organic constituents, 1, 4, 9, 23 ; ternary compounds, 1, 11 ; quaternary, 1, 13, 23 ; muscular action, 1, 56 ; organic evolution, 1, 483, 486 ; differentiation of nerve tissue, 2 316-51, 352 ; of muscular tiesue, 8, 351-5. JAUNDICE (see Disease). Jaws, of uncivilized and civilized, lt 455. JinigermanniacecB : morphology, 2, 21- 7 ; continuous and discontinuous de velopment, 2, 49-52, 84; tubular structure, 2, 54, 59 ; proliferous growth, 2, 63, 83 ; colour, 2, 71, 249 ; symmetry, 2, 125 ; fertility and growth, 2, 421. Jussieu, A. de, Tegetal classification, 1, 296. LABOTJE, physiological division of, 1, 1GO, 163 ; 2, 365. Lamarck, J. 13. P. A. de M. : zoological classification, 1, 300 ; opinions of E. Darwin and, 1, 403, 405-9. Laminaria : pseudo-foliar and axial development, 2, 22 j tissue, 2, 229, 239, 255. Language and evolution, .?, 347, 357-8, 360. Laurel, leaves of, S, 134, 231. Leaves : development and aggregation, 2, 28-33, 73 ; stem-like stalks, 2, 32 ; homology, 2, 33-6, 71-4; foliar and axial development, 2, 36-46, 511-6; " aclnate," 2, 55 ; proliferous growth, 2, 63, 83 ; nutrition and develop ment, 2, 73-4; symmetry, and of branches, 2, 133-5* 136 ; size and distribution of leaflets, 2, 137-40; transition from compound to simple, 2, 140-3 ; unsymmefcrical form. 143- 4; morphological summary, 2. 216; tissue differentiation, 2, 229 ; distri bution, 2, 231 ; outer tissues of stem and, 2, 239-42. 253, 380; distribution of stomata, 2, 243 ; wax deposit on, 2, 243-5 ; light and colour, 2, 245 ; superficial differentiation, 2, 246-8, 254,380; root nourishment from, #, 257 ; inner tissue differentiation, 2t 261, 381 ; vascular tissue differentia* tion, 2, 269, 272, 381 ; dye absorption and circulation, 2, 539-43, 546 ; vas cular system, £, 557-61, 565; arrange ment, 8, 571-4. Lepidoptera (see Insects). Lepidosiren: ossification, 2, 201; res piration, 2, 326 ; skeleton, 2, 522, 524, 529. Lepldosteus : armour, 1, 4-10 j air blad« der, 2t 322. 588 SUBJECT-INDEX. Lessonia : Hooker on growth, 2y 53 ; branch symmetry, 2, 131. Lewes, G. H.5 definition of life, 1, 61. Lichens : cell multiplication, 2, 19 ; Hooker on growth, 2, 53 ; tubular structure, 2, 54 ; integration, 2, 276 ; indefiniteuess, 2, 278 ; sexual genesis, 2, 430. Liebig, Baron J. von, nitrogenous food stuffs, 1, 44, 45. 1 -ife : co-ordination of actions, 1, GO, 70 ; defined by Schellinsj, 1, 60, 150 ; Eicheraud, .Z, 60; De^Blainville, 1, CO, 74 ; a. H. Lewes, 1, 61 ; the definite combination of heterogeneous changes, &c., 1, 62-7 ; changes showing, 1, 72 ; defined by Con.te, 1, 74; the definite combination, &c., in correspondence with external co-existences and se quences, 1, 74, 263 ; correspondence of external and internal relations, 1, 74-7, 81 ; continuous adjustment of internal relations to external relations, 1, 80 ; completeness of, proportionate to correspondence, 1, 82-5 ; length and complexity, 1, 84 ; degree, and complexity of environment, 1, 85-8; perfect, is perfect correspondence, 1, 88, 92 ; definitions of evolution and life, 1, 89-93 ; definition of science of, 1, 94-6 ; is organization produced by? 1, 153; precedes organization, 1, 167 ; definitions of individuality and, 1, 207 ; effect of incident forces on, 1, 286, 291 ; length in individuals and species. 1, 333 ; equilibration of, 1, 462, 474; "absolute" commence ment of, 1, 482, 485 ; integration and augmentation, 2, 406 ; prospective, 2, 494-7. I/ glit : influence on animals and plants, 1, 28-33 ; 2, 413 ; nitrogenous plants, 1, 38 ; animal and vegetal phos phorescence, 1, 46, 57 ; heliotropism, /, 73, 2, 145 ; effects on organic matter, 1, 121 ; plant adaptation, 1, 184 ; organic change and rhythm in terrestrial, 1, 411, 473 ; vegetal in fluences, 2, 115, 116, 132, 134, 139, 143 ; influence on flowers, 2, 152, 571- 4; vegetal tissue differentiation, 2, 236-8, 241, 242 ; action on leaves, 2, 243-8 ; on vegetal vascular system, 2, 271, 280, 556 ; development of sensory organs, 2, 307. Lime, leaf forms, 2, 143, 144. Lindley, J., vegetal classification, t, 295-7. Linnaeus, C. : plant classification, 1, 295 ; animal, 1, 298 ; indefiniteness of alace andfuiiffi, 2, 278. Liver, the, development, 1, 375 ; 2, 316- 21. Liverworts, (see Hepaticce). Logic, reasoning and definition of life, 1, 62-71. Logwood, vegetal staining, 2, 538-43, 546-50, 553. LongeTity, characteristic of develop ment, 1, 84. Lubbock, Sir J. : on DapJinia, 1, 229- 30 ; insecta and Crustacea, 1, 231. Lungs (see Kespiratory system). MAGENTA, vegetal staining by, 2, 538- 43, 546-50, 553. Magnetism, and muscular action, 1, 56. Maillet, B. de, modifiability of organisms, 1, 402, 408. Mammalia: nutrition and growth, /, 113 ; growth and expenditure of force, 1, 114, 127 ; flesh constituents, 1,125; embryonic development and Von Baer's formula, 1, 142—4; temperature, 1, 146, 149; self-mobility, 1, 147; functional and structural differentia tion, 1, 157 ; viviparous homogenesis, 1, 211 ; classification, 1, 308 ; cervical vertebrae, 1, 309 ; 2, 533 ; aquatic, jf, 317 ; distribution in time, 1, 324, 326 ; embryonic respiratory system, 1, 369; suppression of teeth, 1, 370 ; arrested development, 1, 385-6 ; symmetry, 2, 187 ; tegunientary structure, 2, 297 , outer tissue differentiation, 2, 300 ; blood pressure, #, 329, 330; ova iratii- ratior., ,£, 333 ; osseous differentiation, 2, 335-46 ; activity and muscular colour, 2, 356-60 ; functional integra tion, 2, 367 ; growth and genesis, 2, 435, 438 ; development and genesis, #, 444 ; heat expenditure and genesis, 2t 416-8 ; fertility of birds, 2, 449 ; mus cular expenditure and genesis, 2t 451 ; nutrition and genesis, 2, 458. Manatee, nailless paddles, 1, 385. Marchanti^cecB: symmetry, 2, 125 ; outef tissue differentiation, 2, 235. Marmot, hibernation and waste, 1, 170, 171. SUBJECT-INDEX. Marriage (see Multiplication). Masters, Dr. M. T., on foliar homology, 2, 33, 37-43. Materialism, and evolution, 1, 490-2. Mechanics : transverse strains, 2, 102-5 ; genesis of vertebrate axis, 2, 195-9, 199-201, 208 ; osseous differentiation, g, 335-42; disintegrated motion, 2, 867 ; analogy from locomotive, 2, 490-2 ; future human evolution, £,496; strain and vegetal structure, 2, 543- 57, 561-5. Medusidce : contractile function, lt 55, 2, 366 ; symmetry, 2, 171-4. Metagenesis : of Prof. Owen, 1, 213 ; in imects, 2, 446. Metals, melting of alloys, ly 276, 289. Meteorology : changing phenomena, 1, 63, 66, 88 ; crystalli/ation of " storm glass," 1, 77 ; special creation, 1, 335 ; rhythm in, and organic change, lt 41 1-13, 473 ; effect of geologic change, 1, 415. Migrations : eolar influences, 1, 412 ; causes, 2, 504. Milne-Edwards, II. : "physiological divi sion of labour," lt 160 ; ocular struc ture, 2, 303. Milk, expenditure and genesis, 2, 447. Mind (see Psychology). Mobility, molar and molecular, 1, 14. Mole, function and multiplication of effects, 2, 384. Molecules : mechanically considered, 1, 14; stability, 1, 274-8; nerve diff erentiation, 2, 346-51, 372-5. Mollusca: axial development, 1, 136, 337; vascular system, 1, 158, 161-2, 2, 330-2 ; individuality, 1, 204 ; genesis, 1, 211, 2, 425 ; hermaphro- ciism, 1, 279 ; definition, 1, 307 ; dis tribution in time, 1, 321, 324, 326; classification, 1, 363 ; development, 1, 372 ; amphibious and terrestrial, 1, 393 ; indirect equilibration, 1, 448 ; secondary aggregate, 2, 102-4 ; sym metry, 2y 184-6 ; outer tissue, 2, 292, 380 ; alimentary system, 2, 312. Molluscoida : differentiation, 1, 307 ; integration, 2t 85-7 ; a tertiary aggre gate, 2, 111 ; contrasted with moliusca, 2, 103; symmetry, 2, 176; vascular system, 2, 330-2 ; genesis, 2, 425 ; origin of vertebrate type, 2, 567. Monocotyledon (see Enclogens). Monstrosities, vertebrate, £, 105. Morphology : as aiding biology, 1, 97 ; morphological units, 1, 182; rudimen tary organs, 1, 384-7, 472 ; structural and functional co-operation, £, 3 ; inte gration, 2, 4-6 ; change of shape, 2> 6 ; formula of evolution, 2, 7-9 ; evolution and cell doctrine, 2, 10-13. Morphology, animal : evolution and seg mentation of arliculata, 1, 380-2; vertebral column development, 1, 382- 4 ; primary aggregates, 2, 77-9, 111 ; secondary, 2, 79-83, 111 ; tertiary, 2, 83-5 ; integration in mulluscoida, 2, 85-7, 111 ; integration and indepen dence of individuality, 2, 87-91, 111 ; segmentation in ann-ulosa, 2, 91-3, 111 ; also integration, #, 03-7, 99-102, 108, 111, 208 ; unintegrated molluscan form, 2, 102-4 ; adaptive segmentation in vertebrafa, 2, 104-10, 111, 208 ; motion and symmetry, 2, 166-8 ; primary aggregate symmetry, 2, 169 ; secondary, 2, 170-4 ; symmetry of com pound coelenterata, 2, 174-6; simula tion of plant shapes, 2, 174 ; symmetry of molluscoida, 2, 176 ; of annuloida •with echinodermata, 2, 177-80 ; of anindosa, 2, 180-3 ; of wolhtsca, 2, 181-6; of vertebrata, 2, 186-9, 190; similarity of animal and vegetal, 2, 189 ; cell shapes, 2, 210-12 ; evolu tion and generalizations summarized, 2, 213-7. (See also Vertebral). Morphology, vegetal : unicellular plants, 2, 14 ; aggregation and integration, 2, 15-8, 74-6; pseudo-foliar develop ment, 2, 18-20 ; pseud-axial, 2, 20 ; pseudo-foliar and axial, 2, 21-4 ; composition of jwngermanniacecB, 2, 24-7 ; leaf development and aggrega tion, 2, 28-33, 71-4; foliar homologies, 2, 33-6, 71-4; foliar and axial de velopment, 2, 36-46, 511-6; grov.th and development of acroirens, 2, 46- 53 ; of endogens and exogens, 2, 53- 61, 74-6 ; origin and development of axillary buds, 2, 61-5 ; growth of endogens and exogens, 2, 65-9 ; phoenogamic axis and unit, 2, 69-71 ; development of foliar into axial organs, 2, 71-4; resume, 2, 74-6; can plant shapes be formulated ? 2, 113 ; grow th and differentiation, 2, 114-6 ; kinds of symmetry, 2, 116-8 ; symmetry of primary aggregates, 2, 119-22 ; of 590 SUBJECT-INDEX. secondary, £, 122-5; of tertiary, #, 125-8; symmetry and environing influences, 2, 128-9; symmetry of primary branches, 2, 130 ; of secon dary, 2, 130 j of tertiary, 2, 131-3 ; leaf and branch symmetry, #, 133-5, 136 ; phoenogamic unit homology, 2, 136 ; size and distribution of leaflets, 2, 137-40 ; transition from compound to simple leaves, 2, 140-3 ; unsymme- trical leaf development, 2, 143-4 ; differentiation of homologous units, 2, 144 ; floral cluster symmetry, 2, 146, 158; radial floral symmetry, 2, 147-9, 158, 571 ; bilateral floral symmetry and fertilization, 2, 149-55 ; floral clusters and component flowers, 2, 155-8 ; cell differentiation and meta morphosis, 2, 159-61 ; nutrition and differentiation,^, 162; and inflorescence, 2, 163 ; helical growth of phoenogams, 2, 164 ; summary of symmetry, 2, 216 ; stress and structure, 2, 258-62, 381 ; interdependence with physiology, 2, 221. (See also Leaves). Mosses : varied development, 2, 47, 49 ; indeflniteness, 2, 279 j multiplication, 2, 421. Motion : organic and environment, 1, 145-50 ; of animals and waste, 2, 170, 176 ; conception of, 1, 491. Mountains : climatic effects, 1, 415 j growth of trees on, 2, 127. Mouse: fertility, 2, 401 ; expend'ture and genesis, 2, 452 ; tape-worm develop ment, 2, 466 ; and rat, 2, 476-7. Mucous membrane, differentiation, £, 307-9, 332. Mulder, G. J., on chlorophyll, 2, 244. Multiplication : declining fertility of evolution, 1, 84; 2, 411 ; biology and phenomena of, 1, 102 ; reasons for the word " genesis," 1, 209 ; classifica tion of processes, 1, 210-6, 273; a process of disintegration, 1, 216-8 ; reproductive tissue in gamogenesis, 1, 218-24 ; nutrition and growth, 1, 224- 33, 235-7 ; natural selection as aiding, 1, 233-7 ; hermaphrodism, animal and Tegetal, 1, 278-9, 280; in-and-in breeding, 1, 282-4, 289 ; physiological units, resume, 1, 286-91 ; four factors of law of, 2, 395, 415; destructive end preservative forces, 2, 397-9, 410 ; rhythm of species, 2, 399 ; fertility and preservation, 2t 400-3, 410 ; nutri tion and disintegration of, 2, 404, 405, 410 ; integration and genesis, 2, 406- 8 ; individuation, 2, 408-10, 415-7, 472, 474-8 ; difficulties from environ- ment, 2, 412 ; from individual expen diture, 2, 413-5; plant growth and asexual genesis, 2, 419-22 ; and animal growth, 2y 422-6 ; character of asexual and sexual, 2, 428 ; vegetal growth and sexual genesis, 2, 428-31 ; also animal, #, 431-6; antagonism of growth and sexual genesis, 2, 436-8 ; of development and genesis, 2, 440 j vegetal development, 2, 441, 443; animal development, 2, 442, 444 ; vegetal expenditure, 2, 446 ; verte brate expenditure, 2, 446-8, 451 ; muscular expenditure in birds and mammals, 2, 448-51; vegetal nutri tion, 2, 454, 484 ; animal nutrition and agamogenesis, 2, 455 ; nutrition and effect of conditions, 2, 455-9 ; obesity and nutrition, 2, 459-62, 484 ; nutrition, resume, 2, 463, 470-2 ; vegetal parasitic nutrition, 2, 463; and animal, 2, 464-6 ; insect nutrition, 2, 466-S; nutrition of blackbird and linnet, 2, 176; human, 2, 479-81, 493 ; nutrition and human, 2, 481-3 ; Doubleday on, 2, 483-5 ; human bodily and mental expenditure, 2, 484-7, 489-92, 502; civilized and uncivilized, 2, 487-9 ; human evolu tion and decline in, 2, 501-3 ; the future of popidation, 2, 504-7 ; equili bration and evolution, 2, £07. Muscle : electricity from, 1, 47 ; action of, 1, 56 ; growth and function, 1, 123 ; development, 1, 141 ; functional differentiation, 1, 159 ; vicarious func tion, 1, 166 ; waste and repair, 1, 171-3 ; modifiability and adaptability, 1, 185, 187, 189, 191 ; natural selection and increase, 1, 450-3 ; differentia- tion, 2, 351-61 ; activity and colour, 2, 356-60; repair and growth, 2, 361-4; integration, 2, 368, 375 ; equilibration in action, 2, 386 ; expenditure and bird genesis, 2, 448-51 ; power, as square of dimensions, 2, 449 ; future human evolution, 2, 495 ; origin of vertebrate type, 2, 567-9. Music, faculty of, and heredity, 1, 249, 260. Myocommata, and vertebrate skeleton, '?, 199,201,205. SUBJECT-INDEX. 591 Myriapoda : integration and homology, 2, 99-102 ; genesis, 2, 425. , mammalian, 1, 385. Natural selection : structural modifica tion, 1, 3 68 ; homogenesis and hetero- genesis, 1, 233-7 ; aided by function, 2, 246-52 ; special creation, 1, 340-4 ; indirect equilibration, 1, 444-9, 4G6, 474; changes without, 1, 449-57; economical tendency, 1, 450 ; general doi-irine of evolution, 1, 457-62, 474 ; unceasing, 1, 468 ; ethnologic, 1, 468 ; vegetal nutrition, 2, 48 ; upright Tegctal growth, 2, 53 ; endogenous growth, 2, 54 ; exogenous, 2, 61 ; gemmation, 2, 90 ; navicula symme try, 2, 12D; foliar position, 2, 143; foliar distribution, 2, 152 ; floral fer tilization and symmetry, 2, 153-5, 571-4 ; helical phoenogamic growth, 2, 164; ecninodennata and bilateral symmetry, 2, 179; vertebrate axis segmentation, #, 204; phoenogamic tissue differentiation, 2, 230 ; physio logical differentiation, 2, 235, 239; foliar wax deposit, 2, 244; foliar surfaces, 2, 246 ; floral fertilization, 2, 252 ; sexual selection, 2, 253 ; inner vegetal tissue differentiation, 2, 262 ; wood formation, 2, 270-1, 273-4 ; animal tissue differentiation, 2, 286- 90; differentiation of respiratory sys tem, 2t 293-5 ; epidermic differentia tion, 2, 295-9 ; sensory organ deve lopment, 2, 307 ; skin and mucous membrane differentiation, 2, 308; localization of excretion, 2, 320-1 ; respiratory organs of fish, 2, 324-7 ; heart and vascular system, 2, 332, 334 ; osseous differentiation, 2, 345 ; also muscular, 2, 354, 359-61 ; insect nutrition and genesis, 2. 467 ; genesis and individuation,#, 472; economics of evolution, 2, 474-8 ; Darwin, 2, 500 ; vegetal tissue formation, 2, 551, 563-5 ; origin of vertebrate type, 2, 568. Kavicula, symmetry, 2, 120. Kemertidce : integration, 2, 94 ; bilateral symmetry, 3, 178. Nerves : electricity from, 1, 48 ; genera tion of nerve-force, 1, 49-53, 57 ; differentiation, 1, 159; 2, 346-51, 352 ; vasculo-motor system, 1, 162 ; "''carious function, 1, 166; over-exer tion and waste, 1, 171 ; adaptability, 1, 186, 189, 193 : hereditary epilepsy, 1, 251 ; muscular differentiation, #, 354. (See also Psychology). Nervousness, and heredity, i, 244, 251. New Zealand, European plants in, /, 389, 401. Nitric acid, properties, 1, 6, 8, 9. Nitrogen : chemical and physical proper ties, 1, 3-5, 20, 22 ; binary compounds, 1, 6, 8, 9 ; instability of compounds, 1, 8, 37, 39 ; 2y 232 ; quaternary com- pounds, 1, 12-] 4, 23 ; organic impor tance, 1, 39-41 ; evolution of heat and oxidation, 1, 44 ; tissue differentiation, 2, 362-4 ; nutrition and genesis, 2, 4G1. North American Review: "Philosophical Biology," 1, 479-84 j "Physiological units," 1, 484-94. Notochord : formation, 2t 199-301, 569 ; segmentation, 2, 202-5. Nutrition: organic re-arrangement, t, 34; nitrogenous and non-nitrogenous, 1, 44, 45 ; 2t 353; food assimilation and reasoning, 1, 62-8 ; needful for vital change, 1, 75 ; relation to growth, 1, 112, 114, 116, 119-21, 122, 127, 131 ; expenditure of force, 1, 154, 306 ; fluid, 1, 165 ; vegetal growth, /, 224-8, 232, 235-7 ; animal growth, 1, 228-33, 235-7; Dr. E. Darwin on procuring, 1, 407 ; leaf development, 2, 30, 71-4; vegetal development, 2, 48, 162, 259 ; axillary buds, 2, 61-5 ; defective, 2, 70; vegetal inflorescence, 2, 163; helical phoenogamic growth, 2, 164; vegetal fructification, 2, 250; action of bile, 2, 317 ; osseous develop ment, 2, 340, 344; genesis, 2, 399, 402,407,415-7,432; of young, a pa ren tal loss. 2, 404, 408, 409 ; distribution, 2, 413 ; reproductive system, 2, 439 ; animal development and genesis, 2, 414 ; expenditure and genesis, 2, 447 ; vegetal genesis, 2, 454, 484; agamo- genesis, 2, 455 ; genesis and etlect of conditions, #, 455-9; obesity and genesis, 2, 459-62, 484; general doctrine of genesis, 2, 463 ; genesis and vegetal parasitism, 2, 463 ; al«o animal, 2t 464-6; insect genesis, 2t 466-8 ; genesis, resume, 2, 470-2 ; and evolution, 2, 474-8 ; of blackbird and linnet, 2, 476 ; genesis in human race, 2, 481-3, 487-9 ; Doubleduy on, 2, 483-5: future human evolution, ,592 SUBJECT-INDEX. 2, 49S, 503 ; abnormal vegetal growth, 2, 512 ; organ of, (see Alimentary canal). OBESITY, nutrition and genesi?, 2, 459- 62, 484. Odours : floral fertilization, 2, 252, 253 ; animal protection, 2, 414. Offspring, influence of age on, 2, 480. Oken, L. : archetypal hypothesis, 2, 109 ; theory of supernumerary bones, 2, 206 ; on the skull, 2t 530. Opuntia (see Cactacect], 0/chids : pollen propulsion, /, 54; leaf formation, 2t 56 ; aerial roots and physiological differentiation, 2> 238, 240 ; foliar surface, 2, 247. Orginic matter: properties of elements, i, 3-5, 22 ; of binary compounds, 1, 5-10; of ternary, 1, 10-12; quaternary 1, 12-14, 23 ; molar and molecular mobility, 1, 14; colloid and crystalloid form, 1, 15-8, 23 ; their diffusibility, 1, 18-21, 24 ; extreme complexity, 1, 21 ; modifiability, 1, 25, 41 ; capillarity and osmosis, 1, 26 ; effects of heat, 1, 27 ; of light, 1, 28-33 ; nitrogenous, 1, 35-41 ; oxidation and evolution of heat, 1, 43, 57 ; genesis of electricity, 1, 47- 9, 57 ; transformations and persistence of force, 1, 57 ; sensible motions in, 1, 57 ; inorganic matter, 1, 89-93 ; resume of generalizations, /, 94; instability, 1, 121, 420, 473 ; and heterogeneity, 1, 286-91 ; "spon taneous generation," and evolution of, 1, 479-34, 486; cell-doctrine and evolution of, 2, 10-13. Organization (see Structure), Osmosis : organic effects, 1, 26, 27 ; in animals, 1, 55 ; 2, 361-4; in vascular system, 2, 298, 327-34; in vegetal tissue, 2, 537, 544, 546, 554, 561-5. Ossification (see Bone). Owen, Sir K.: metagenesis and partheno genesis, 1, 213 ; fossil mammalia, 1, 326; human parasites, 1, 342; con tinuous operation of creative power, 1, 404 ; theory of vertebrate skeleton, 2, 110, 517-35 ; theory of supernumerary bones, 2, 206 j Eschricht on ascaris, 2, 465. Oxalis : radial symmetry, 2, 137 ; foliar surface, g, 248. Oxen and sheep, growth of,l, 128, 13 L Oxidation (see Oxygen). Oxygen : chemical and physical propel* ties, 1, 3-5, 20, 22 ; binary compounds, 1, 5-7, 10, 22; ternary, 1, 10-12; quaternary, 1, 12-14, 23 ; a crystal loid, 1, 21 ; combining power and atomic weight, 1, 31 ; organic change from, 1, 34; heat generation, 1, 43-6; phosphorescence, 1, 46 ; nerve force dependent on, 1, 50 ; necessary to animal life, 1, 75-6 j amount inhaled, 1, 170. PAGET, SIB J., blood changes, from disease, 1, 177, 484. Paleontology : distribution in time, lt 320-7, 328 ; special creation, 1, 340 ; record congruous with evolution, lt 397-401, 472. Palmella : tissues, 2, 226 ; sexual genesisj 2, 429. Parasites : aberrant type forms, 1, 296 ; distribution, 1, 313 ; special creation and human, 1, 342, 354; retrograde development, 1, 370 ; nutrition and genesis in vegetal, 2, 463, 468 j in animal, 2, 464-6, 468. Parthenogenesis : true and pseudo-, lt 213-5 ; laws of multiplication, 2, 395 j of articulata, 2, 426. Peloria : in gloxinia, 2t 151; phceno- gams, 2, 163. Penguin, dermal structure, 2, 300. Persistence of force : properties of com pounds, 1, 3 ; organic transformation, 1, 57; growth, 1, 122, 131, 132; organic energy, 1, 176 ; organic repair, 1, 177 ; variation, 1, 271 ; genesis, heredity, and variation, 1, 291; mor phological summary, 2," 217 ; vegetal tissue differentiation, 2, 227 ; physio logical development, 2, 3S7. Petals: foliar homology, 2, 33-6 1 "adnate," 2, 55. Petrels, Darwin on, 1, 368. Phanerogatnia (see Phsenogams). Phsenogams : uni- and multi-axial eym* metry, 2, 126-8 ; unit of composition, 2, 136; helical growth, #,164; tissue and leaf differentiation, 2, 229-31, 379; also bark and cambium, 2, 231-3, 379 ; also outer tissue, 2, 285, 239-42, 253, 380 ; wax deposit on leaves, 2, 243-5 ; differentiation of inner tissues, #, 256-8, 381 i vascular system develop* SUBJECT-INDEX. 593 ihent, g, 263-8, 381 ; integration, 2, 277, 280, 3s3 ; multiplication, 2, 421, 422 ; genesis and growth, 2, 431, 437 ; and development, 2, 443 ; and nutri tion, 8, 455, 45G, 484. Philology, and evolution, 1, 347, 357-8, 360. Philosophy, (see Psychology). Phosphorescence, animal and vegetal, 1, 46. Phosphorus : allotropic, lt 4 ; organic evolution, 1, 486. Photogenes, visibility of, 1, 174. JPh'ysalia : motion, 1, 55 ; individuality, 1, 204 ; tertiary aggregate, 2, 84. Physiological units : definition, 1, 183 ; genesis, 1, 220 ; heredity, 1, 253-6 ; variation, 1, 265-7, 267-8, 269-70; stability, 1, 277-8 ; hermaphrodism, J?, 278-9,280; self-fertilization, 1, 279- 82, 290 ; interbreeding, 1, 282-4, 289 ; genesis, heredity, and variation, re sume, 1, 286-91; organic development, 1,373,376-8; "mechanical theory," 2, 484-9 ; morphological development, #,7-9; cell -doctrine, 2, 10-13; foliar development, 2, 72. Physiology : and psychology, 1, 98 ; comparative and general, 1, 100 ; vicarious function, 1, 165 ; primitive interpretations, 1, 333 ; multiplication of effects, 1, 425 ; structural and functional co-operation, 2, 3 ; verte brate organic symmetry, 2, 190 ; and morphology, 2, 221-3 ; and evolution, 2, 223-5 ; tissue differentiation and evolution, £, 226-8, 377-83 ; tissue differentiation in secondary aggregates, g, 228, 378; in phamogjims, 2, 229- 31, 379 ; in bark and cambium, 2, 231-3, 379 ; on free and fixed surfaces, g, 23 1-9, 253, 379 ; outer stem and leaf tissue, 2, 239-42, 253, 380 ; super ficial differentiation in leaves, 2, 212- 8, 254, 380; floral tissue differentia tion, 2, 248-53, 254, 381 ; outer plant tissue, rs.-iume, 2, 253 ; inner plant tissue differentiation, 2, 256-8, 381; supporting plant tissue, 2, 258-62, 268-71, 381 ; vascular system develop ment, 2, 262-8, 268-71, 381 ; inner plant tissue, summary, 2, 271-4, 381 ; vegetal integration, 2, 275-8, 278-81, 883 ; tissae differentiation in protozoa, g, 282, 378 ; analogy of tissue differen tiation in coelenterata, 2, 283-4, 382 ; tissue reduplication in coelentcrata, 2, 284-6, 382 ; natural selection and animal tissue, 2, 286-90; outer tissue in coelenterata, 2, 291 ; respiratory organs, 2, 292-5, 321-7, 380 ; diffe rentiation of animal epidermic tissue, 2, 295-9, 380 ; development of tegu- mentary organs, 2, 299-302, 380 ; of sensory, 2, 302-7 ; inner and oute tissue 'transition, 2, 307-9, 382; all mentary canal differentiation, 2, 310- 12 ; gizzard development in birds, 2t 312 ; alimentary canal of ruminants, 2, 314-6 ; differentiation of liver, 2, 316-21 ; of animal vascular system, 2, 327-34 ; of osseous system, 2, 334- 46; of nerve tissue, £, 316-51, 352; of muscle, 2, 351-61 ; tissue repair, and growth, i?, 361-4; correlation of integration and differentiation, 2, 365 ; differentiation and integration iu animals, 2, 365-8; in vascular system, 2, 368-72, 375 ; in nerves, 2, 372-5 ; origin of development, 2, 377 ; animal differentiation and instability of homo geneous, 2, 377-83, 385 ; summary of development, #, 377-88; vegetal diffe rentiation and instability of homo geneous, 2, 377-83, 385; and multi plication of effects, g, 383-4, 385; animal differentiation and multiplica tion of effects, 2, 383-4, 385 ; equili bration, 2, 384-7 ; persistence of force and development, 2, 387 ; vegetal circulation and wood formation, 2, 533-66; also dye permeability, 2, 538-43, 546-50, 553, 555. (See also Function, and Multiplication). Pigeons (see Birds). Pike, unceasing growth, 1, 126, 231. Planaria : integration, 2, 94 ; symmetry, g, 178 ; liver, 2, 316 ; unintegrated function, 2, 365 ; growth and genesis, 0, 432. Plants (see Botany, Morphology, and Physiology). Plato, iofo of, 2, 519. Plethora, nutrition and genesis, 2, 459- 62, 48 1. PteuroneclidoB : symmetry and eye, 2, 188 ; outer tissue, 2, 380. Plumatella : metagenesis, 1, 217 ; sym« metry, g, 177. Polarity : organic, 1, 182, 253, £8G-91 j physiological units, 1, 484-92. Polvmerism : of binary compounds, /, 9, 594 SUBJECT-INDEX. 23 ; ternary, 1, 11, 23 ; nerve tissue, 2, 847-51. Polype (see Ccelenterata). Polyzoa, : structural indefiniteness, J?, 145 ; functional differentiation, 1, 158 ; integration, 2, 85, 88 ; sym metry, 2, 177, 189 ; functional co-ordi nation, #, 368 ; genesis, 2, 425. Potato : simulation of growth, 1, 108 ; physiological differentiation, 1, 238. Preservation : fertility and self-, 2, 403, 410 ; nutrition, 2, 469. Protein, isoraerism, 1, 483, 486, 487. Protococcus (see Protophyta). Protophyta : central development, 1, 134 ; axial, 1, 135 ; structure, 1, 145 ; self -mobility, 1, 147 ; individuality, 1, 202 ; spontaneous fission, 1, 216 ; genesis, 1, 2*9, 2, 419, 442; hetero- genesis and nutrition, 1, 235 ; uni cellular, 2, 14; symmetry, 2t 119; tissues, 2, 226, 231. Protozoa : locomotion, 7, 54, 147 ; cor respondence shown by, 7, 75 ; struc ture, 1, 111, 144, 145 ; development, 7, 134, 135, 372 ; spontaneous fission, 1, 216 ; genesis, 1, 219 ; 2, 422, 431 ; heterogenesis and nutrition, 1, 235 ; undifferentiated, 1, 306 ; distribution, 7, 312; "spontaneous generation," 1 480-4 ; primary aggregate, 2, 77-9, 111 ; progressing integration, 2, 79- 83, 111 ; symmetry, 2, 169 ; differen tiation, 2, 282, 291, 378 ; genesis in rotifer a, 2, 432, 439. Pseud-axial development, vegetal, 2, 20, 22. Pseudo-foliar development, vegetal, #, 18-20, 22. Pseudo - parthenogenesis, animal and vegetal, 1, 214-6 ; 2, 466. Pseud-ova, of Huxley, 1, 214. Psychology : reasoning and definition of life, 1, 62-71 ; correspondence shown by recognition, 1, 77 ; contrasted with physiology, 1, 98 ; subjective, and objective, 1, 99; comparative and general, 1, 100 ; vicarious function, 1, 166 ; waste and repair in sensory organs, 1, 173-4; sensory adaptability, 1, 186, 188, 189 ; sensory organs and heredity, 1, 244 ; heredity and musical talent, 1, 249; primitive ideas and progress of knowledge, 1, 333 ; in conceivability of special creation, 1, 336, 344, 348, 470 ; conceivability of evolution hypothesis, /, 348-51, 355, 470 ; persistent formative power, un representable, 1, 404 ; E. Darwin and Lamarck on desires, 1, 406 j natural selection and brain evolution, 1, 469 ; " mechanical theory" and the unknow able, 1, 490-2 ; vitiation of evidence, 2t 80 ; repetition and perception, #, 128 j sensation and vascular system, 2, 299 ; differentiation of sensory organs, 2t 302-7 ; differentiation of nerve tissue, 2, 346-51, 352 ; functional integration, 2, 368 ; also integration, 2, 372-5 ; equilibration of nerve discharge, 2, 386 ; genesis and nerve development, 2, 415, 502; mental activity and genesis, 2, 485-7, 489-92, 502 ; future human evolution, 2, 495-7, 499 ; human evolution and genesis, 2, 501- 3 ; future mental development, #, 506 ; origin of vertebrate type, #, ,567-9. Pteropoda : bilateral symmetry, 2, 181 ; outer tissue, 2, 292. Pyrosoma : phosphorescence, 7, 47 ; in tegration, 2t 89. QUATERNARY compounds, propevties, 1, 12-14, 23. Quills, development, 2, 299-302. RABBIT : activity and muscle colour, 2, 356-60 ; expenditure and genesis, 2, 451. Eadial, definition, 2, 133. Raffiesiacece : tissue differentiation, 2, 235, 257 ; nutrition and genesis, 2, 463. Rathke, H., on vertebrate embryo, 2, 106. Ray, J., plant classification, /, 296. Reasoning, compared with assimilation, 1, 62-8. Rernak, R., on vertebrate embryo, £, 108. Repair: continuity of, 1, 171-4; of! animal injuries, 1, 175, 179-82 ; organic, and assimilative power in blood, 177-9 ; of differentiated tissue, 2, 361-4. Reproduction (see Multiplication). Eeptilia: growth and expenditure of force, 1, 114, 127 ; sizes of ova and adult, 1, 116 ; temperature, 1, 146 ; SUBJECT-INDEX. 595 waste, 1, 170 ; distribution in time, lt 325, 328 ; bilateral symmetry, 2, 186, 187 ; outer tissue differentiation, 2, 301, 380 ; activity and muscular colour, #, 3.36 ; functional integration, *, 367 ; hind limbs, 2, 382 ; vertebral column, 2, 385 ; Owen on skeleton, 2, 529 ; vertebra?, 2, 533. Respiratory system : effect of light, 1, 28 ; orgaric rearrangement, 1, 34 ; cutaneous, 1, 165 ; differentiation, 2, 292-5, 321-7,380; osmosis and deve lopment, 2, 362 ; physiological inte gration, 2, 366-7, 375 ; vascular dif ferentiation and integration, 2, 369. Jthizopoda : structure, 1, 145 ; life with out organization, 1, 153, 156 ; un- differentiated, 1, 306 ; a primary aggregate, 2, 77 ; symmetry, 2, 169 ; tissue differentiated, 2, 282, 378; motion of sarcode, 2, 346. Ehythm : astronomic and organic, 1, 411-3, 473 ; law of equilibration, 1, 4?2-5; of multiplication, 2, 399. Kicheraud, Baron A., deliuition of life, ly 60. Kivinus, A. Q., plant classification, 1, 295. Boots : physiological differentiation, 2, 236-8, 253 ; nutrition from leaves, 2, 257 ; size and function, 2, 259. Ruminants, alimentary canal develop ment, 2, 314-6. Salmonidce, reproduction and growth, 1, 231. Salpidce : hetcrogenesis, 1, 212, 217 ; in tegration, 2, 89. Sap (see Vascular system) . Sarcina : central aggregation, 2t 16 ; fertility, 2, 420. Scent : floral fertilization, 2, 252, 253 ; animal protection, 2, 414. Schelling, E. W. J. von, definition of life, 1, 60, 150. Schleiden, J. M. : on individuality, 1, 202 ; hepatica, 2, 47, 49 ; algal inde- finiteness, 2, 279. Sea : organic motion, 1, 65 ; life in, lower than terrestrial, 1, 85 ; dislri- bution, 1, 312, 429 ; change of media caused by, 1, 393 ; geologic influence, 7, 414. Seals : n^il-bearing, 7, 385 ; vibrissa, 8, 3°2- 70 Sedgwiclr, Wm., on heredity, 1, 243, 252. Seeds : nitrogenous, 1, X8 ; temperature of germinating, 1, 44 ; natural selec tion of, 1, 447. Segmentation : in annttlosa, 2, 91-8, 111 ; simulated in mollu-ica, 2, 104, 111 ; in vertebrata, 2, 104-10, 111, 202-5, 208. Segregation : of growth, /, 108 ; of lite units, 1, 151 ; organic repair, 1, 177; variation, 1, 264-7, 267-8, 269-70; morphologic development, 2, 7-9 ; heterogeneity, and definiteness of evo lution, 2, 426-8, 429-31. Self-fertilization, animal and vegetal, 1, 279-82, 290. Senses, the (see Psychology). Sexual selection (see Natural selection). Sheep : and oxen, growth of, 1, 123 ; nutrition and genesis, 2, 459. Ship-building, interdependence of social functions, 1, 194-6, 197-8. Silica, colloid and crystalloid, 1, 17. Silicic acid : properties, lt 16, 17 ; iso- merism, 1, 56. Silicon, allotropic, 1, 4. Skeleton, vertebrate (see Verfbrata). Skin: adaptability, 1, 185; 2, 295-9, 380; differentiation, 2, 198, 200, 286-90; tegumentary development,!?, 299-302, 380; differentiation of sen sory organs, 2, 302-7 ; and mucous membrane, 2, 307-9, 382. Skull (see Vertelrata). Sleep, repair favoured by, 1, 172. Small -pox, blood changes from, 1, 177. Smith, Prof. W., on fertility of dialo- macece, 2, 420. Snakes (see Sept ilia). Sociology : functional differentiation, 1, 160; division of labour, 1, 162; func tional interdependence, 1, 193-6, 197-9 ; evolution 1, 347 ; organic de velopment, 1, 373, 376 ; natural selec tion, 1, 469 ; integration and differen tiation, 2, 371 ; effects of population, 2, 506 ; equilibration, 2, 507. Solatium : organs of attachment, 2t 260 ; inner tissue, 2, 262. Special creation : and evolution, 1, 331, 344 ; improbable, 1, 334-6, 344, 354, 470; inconceivability, 1, 336, 344, 348, 470; of individuals and species, 1, 337-40 ; implication of benevolence, 1, 340-4, 354 ; summary, 1, 314, 470 ; Von Baer's formula, 7, SUBJECT-INDEX. 365-9 ; vertebrate skeleton, 2, 520, 525, 534. Species : adaptation and stability, 1, 199 ; change in, 1, 209 ; hereditary transmission, 1, 238-41 ; variation in wild and cultivated, 1, 260-2, 262-4 ; gamogenesis, 1, 284-6 ; indefinite value, 1, 335, 361; special creation, 1, 337-40; instability of homogeneous and heterogeneity of, 1, 421-3, 428, 429-31, 46% 473 ; persistence of, 1, 428, 430 ; natural selection and equi libration, 1, 457-62, 469, 474. Specific gravity, of organisms and en vironment, 1, 145, 149. Sperm cell : unspecialized form, 1, 219- 24, 253 ; dissimilarity of, 1, 265-7, 2G7-8, 2G9-7J, 280; equilibrium, 1, 277. Sphere : tendency of units to form, 1, 15 ; the embr> onic form, 1, 143 j symmetry, 2, 116. Spheroid, symmetry, 2, 117. Spine (see Vertebraia), Sponge : multicentral development, 1, 135 ; reproductive tissue, 1, 222 ; morphological integration. 2, 81-3, 111 ; pbysiological differentiation, 2, 283. 379; physiologically unintegrated, 2, 375; development and genesis, 2, 442; analogy from, 2, 545. Spontaneous generation: and hetero- genesis, 1, 210; and evolution, 1, 479- 84, 485. Stamens, and foliar homology, 2, 33-6. Starches: properties, 1, 10-12; saccha rine transformation, 1, 36, 37 ; 2, 5J2. Steens'irup, J. J. 8., on eye of pleura- nei-tidce, 2, 188. Stickleback: ova, 2,433; bothriocephalus in, 2, 466. Stomach (see Alimentary canal). Stomata, distribution, 2, 243. Straight line, and evolution hypothesis, 1, 348. Strain : compression and tension of, 2, 192-5 ; vegetal structure, 2, 543-57, 561-5; origin of vertebrate type,!?, 569. Strawberry : multiaxial development, 1, 137 ; multiplication, 2, 421. Structure : biology and organic, 1, 94-6, 96-8 ; functional co-ordinate modifi cation, 1, 100-3 ; size and organic, 1, 110-12 ; growth and complexity ol, 1, 117-9, 132; relation to environment, 1, 145-50; precedes function^ 1, 153, 167 ; functional complexity, /, 156, 167; also differentiation, 1, 157-60; reparative power, 1, 175 ; social and organic functional interdependence, 1, 192-6, 197-9 ; reproductive tissue, 1% 219-22; heredity and function, lt 244-52, 255-6 ; varied by function, 1, 270-2, 455-7, 2, 200; zoological classification, 1, 305-8; equilibration, 1, 432-5, 474 ; co-operation with function, 2, 3 ; evolution and in creased, 2, 4 ; earliest organic forms, 2, 12 ; cylindrical vegetal, 2, 54-8 ; permanence and complexity, 2, 278, 280 ; function and epidermic, 2, 295- 9, 380 ; and muscular, 2, 360, 384 j and repair and growth, 2, 361-4 ; adaptation and equilibration, 2, 385 5 persistence of force and physiological adaptation, #, 387 ; evolution, 2, 474- 8. (See also Morphology). Strut hers, Dr. J. : on heredity, 1, 213, 252 ; digital variation, 1, 258-60. Sugars: properties, 1, 10-12; vegetal transformation, 1, 36, 37 ; 2, 562. Sulphur: allotropic, 1, 4, 56; organic evolution, 1, 486. Sun (see Light). Survival of the fittest (see Natural se lection). Symmetry (sec Morphology). TS:NIA (see Enfasm). Taste, dependent on chemical action, 2, 51. Teeth : hereditary transmission, 1, 244 ; suppression of mammalian, 1, 370; oi uncivilized and civilized, 1, 455. Temperature (see Heat). Tension (see Strain). Thallassicolla : unicentral development 1, 134 ; secondary aggregate, 2, 80-3, 111 ; symmetry, 2, 170. Tide (see Sea). Tissue (see Physiology). Tortoise: HTe of dog and, 1, 84, 85 natural selection and carapace, 1, 4 18. Tree (see Botany). Tremblay, A., on the polype, 1, 180. Ti'ichiniasis, in Germany, 1, 343. Tubicolce: development, 2, 92 ; bilateral symmetry, 2, 180. Turnip : outer tissue, £, 237 ; vasculai system, 0, 264, 268, 547, 560, 565. Twins, similarity ol;, 1, 261, 264. SUBJECT-INDEX. 59' l, Prof. J., on heat absorption by wat.T vapour, 1, 7. 14 Types, jX.-ifeistent," Iluxley on, 1 , Ur.ciB, dermal structure, 2, 2SD. Viva : cell multiplication, #, 18 ; outer tissue, #, 239. Umbelliferce : floral symmetry, #, 150 ; axial and foliar organs, 2, 511-G. Units : differentiation and dissimilarity, 1, 21 ; segregation and organic repair, j?, 177-9, 179-82 ; chemical, morpho logical, and physiological, 1, 182-3 ; stability, 1, 275 ; instability and hete rogeneity of organic, 1, 287 ; niorpbo- logical composition, 2, 5, 7-9, 13, 75, 77; phsenogamic, 2, 69-71, 136; annulose, 2, 97 ; incident force and homologous, 2, 144; morphological summary, #, 215. Physiological, (see Physiological units). Unknowable, the, manifestations of, 1, 491. Up-symmetrical, definition, 2} 11G. VAN BENEDEN, P. J., on tonta, 2, 95. Variation : Struthers on digllal, 1, 258- 60 ; effects of parental condition, 1, 260-2 ; of altered function, 1, 2G2-4 270-2; "spontaneous," 1, 26 (, 272, 425, 480; 2, 500; dissimilarity of initial conditions, I, 264-7,267-8,269- 70 ; persistence of force, 1, 271 ; phy siological units, resume, 1, 286-91; equilibration and vegetal, 1, 437- 9 j equilibration of favourable, 2, 387. Vascular system : nutrition, 1, 118, 120 ; development, 1, 140 ; function, 1, 154-6 : of molliwa, 1, 158 ; func tional differentiation and integration, 1. 161-3 ; organic repair, 1, 173, 177- i); adaptability of, 1, 186; and in creased function, 1, 191, 193 ; heart development, 1, 375, 377; equilibra tion, 1, 450 ; development of vegetal, £, 256-8, 262-8, 268-71, 381; diffe rentiation of, summary, 2, 271-4, 381 ; effect of gravity, 2, 298; differentia tion of animal, 2, 3^7-34 ; mammalian biood pressure, 2, 329 ; osseous deve lopment, 2, 338-42 ; muscularity, #, 855; muscular colour, 2, 357-60; repair and growth, 2, 361-4 j heart- motor apparatus, 2, 3CH ; differentia tion and integration in animal, £, 368- 72, 375 ; wood forv.iation, 2, 536-61; resume oi wood formation, 2, 561-6. Vegetal (see Botany and Morphology). Velocity, of moving bodies, 2, 202-4. f'ertebrata: size, i, 111; si/e at birth and maturity, 1, 116 ; axial structure 1, 136 ; embryonic development and Von Baer's formula, 1, 142-4 ; self- mobility, 2, 147; functional differen tiation, 1, 160; reparative power, 1, 175, 179 ; homogenesis universal, 1, 210 ; Huxley's deflnit'on, 1, 307 ; classification of higher, I, 308; distri bution in time, 1, 325 ; classiflcatoiy value, 1, 361 ; embryonic mammalian respiratory system, 1, 369 ; direct and indirect development, 1, 372, 375 ; evolution and vertebral column, 1, 382 ; rudimentary organs, 1, 385; evo lution and varied media, 1, 391-7 ; size of head and vertebrse, 1, 421 ; segregation and evolution of vertebra3, 1, 427 ; adaptive segmentation, 2, 104-10, 111 , 208 : bilateral symmetry, 2, 186-9 ; internal organic symmetry, 2, 191 ; genesis of rudimentary axis, 2, 195-9 ; origin of notochord, 2, 19J- 201 ; spinal segmentation, 2, 202-5, 208 ; skull development, 2, 205 ; membranous, cartilaginous, and osseous skeleton, 2, 207 ; resume, of axis development, 2, 208 ; sensory organs, 2, 304; air chambers, 2, 322; osseous differentiation, 2, 334-46 ; activity and muscular colour, 2, 3 5 6- GO ; heart-motor apparatus, 2, 366 ; cost of genesis, 2, 416 ; ngamogenesis unknown, 2, 426; growth an 1 genesis, 2, 433 ; heat expenditure and genesis, 2, 446-8, 453 ; Owen, theory of skele ton, 2, 517-35; evolution of vertebrae, 2, 532-5 ; origin of type, 2, 567-9. Viviparous: homogenesis, 1, 210, 218; heterogenesis, 1, 214-5. Volcano, definition of life, 1, 67, 70. Volvocinee : individuality, 1, 202 ; disin tegration of genesis, 1, 216 ; spherical aggregation, 2. 16 ; symmetry, 2, 122 ; outer Ussue, 2, 234, 379; fertility,;?, 421. Vomiting, alimentary canal development, 2, 315. Voriicella: secondary aggregate, %y 82 » symmetry, 2, 171. 503 SUBJECT-INDEX. WALLACE, A. B., "The Origin of the Human Races," 1, 469. Waste: vegetal, 1, 169, 176; animal, 1, 169-71, 185 j relation to activity, 1, 175-7. Water : properties, 1, 7, 9 ; colloidal affinity for, 1, 26; organic change from, 1, 27 ; organic need for, 1, 119, 121 ; in mammalian flesh, 1, 125 ; in organic matter, 1, 145, 149 ; terres trial organisms inhabiting, 1, 316; change of media by animals, 1, 391-7 ; vegetal tissue differentiation, 2, 236 ; molecular rearrangement, 2, 350; colloidal contraction, 2, 352. Watts, Dr., on The Principles of Biology, 1, v. Was, folier deposit, 2, 2 13-5, Weight : relation to environment of organic, 1, 145, 149 ; varying as cubt of dimensions, 2, 414, 449. Wind : and vegetal bilateral symmetry 2, 127 ; and inner vegetal tissue differentiation, 2, 258-62, 269, 272 381 ; and vegetal multiplication, 2 278 ; and vegetal sap movement, 2 552, 553, 556, resume, 561-5. Wolff, C. : vegetal fructification an< nutrition, 1, 224, 2, 163-4; vegela vascular system, 2, 266. Women (see Multiplication). Wood (see Botany). YEAST: fermentation, 1, 35, 27 ; line* aggregation, 2t 13 ; fertility, 2, 420. Spencer, Herbert B 1652 The principles of .A6- biology vol.3