THE UNIVERSITY OF ILLINOIS LIBRARY COp.2 Return this book on or before the Latest Date stamped below. % A SYSTEM PF SYNTHETIC PHILOSOPHY. YOL. III. THE PRINCIPLES OF m BIOLOGY. BY HERBERT SPENCER, AUTHOR OF “SOCIAL STATICS,” “ TIIE PRINCIPLES OF PSYCHOLOGY.’ “essays: scientific, political, and speculative, ** “first principles,” etc. YOL. II. NEW YORK: D. APPLETON AND COMPANY, 549 & 551 BROADWAY. 1875. \ WOEKS BY HEEBEET SPENCEE. PUBLISHED BY D. APPLETON & CO. EVSiscsIlaneous Writings. EDUCATION— INTELLECTUAL, MORAL, AND PHYSICAL. 1 vol., 12nio. 2S3 pages. Cloth. ILLUSTRATIONS OF UNIVERSAL PROGRESS. 1 vol., large 12mo. 470 pages. Cloth. ESSAYS— MORAL, POLITICAL, AND AESTHETIC. 1 vol., large 12mo. 41S pages. SOCIAL STATICS / or, the Conditions Essential to Human Happi¬ ness Specified, and the first of them Developed. 1 vol.. large 12mo. 523 pages. THE CLASSIFICATION OF THE SCIENCES : to which is added Reasons for Dissenting from the Philosophy of M. Comte. A pamphlet of 50 pages. Fine paper. System of Philosophy. FIRST PRINCIPLES, in Two Parts— I. The Unknowable; II. Laws of the Ivnowable. 1 vol., large 12mo. 50S pages. Cloth. PRINCIPLES OF BIOLOGY. Yol. I. large 12mo. 475 pages. “ “ “ Yol. II. large 12mo. 566 payes. Entered, according to Act of Congress, in the year 1807, By D. APPLETON & CO., In the Clerk’s Office of the District Court of the United States for thj* •Southern District of New York. PEEFACE TO YOL. II. The proof sheets of this volume, like those of the last volume, have been 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 tinffe extremely precious to them, makes me feel how heavy is my obligation. Part IY., 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, Balfs, 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, 1865; No. 14 (pp. 81 — 160) in June, 1865 ; No. 15 (pp. 161 — 240) in December, 1865; No. !8 (pp. 241 — 320) in June, 1866 ; No. 17 (pp. 321 — 400) in November, 1866 ; and No. 18 (pp. 401 — 566) in March, 1867. * London , March 23rd, 1867. 439679 CONTENTS OF YOL. II. PART IV.— MORPHOLOGICAL DEVELOPMENT. CIJAP. VII. — THE GENERAL SHAPES OF PLANTS VIII. - THE SHAPES OF BRANCHES IX. — THE SHAPES OF LEAVES X. - THE SHAPES OF FLOWERS XI. - THE SHAPES OF VEGETAL CELLS XII. — CHANGES OF SHAPE OTHERWISE CAUSED ... XIII. - MORPHOLOGICAL DIFFERENTIATION IN ANIMALS XIV. - THE GENERAL SHAPES OF ANIMALS XV. - THE SHAPES OF VERTEBRATE SKELETONS .. XVI. - THE SHAPES OF ANIMAL CELLS ... XVII. - SUMMARY OF MORPHOLOGICAL DEVELOPMENT f Ana I. — THE PROBLEMS CF MORPHOLOGY • • • • • • 3 II. — THE MORPHOLOGICAL COMPOSITION OF PLANTS • • « 10 III. — THE MORPHOLOGICAL COMPOSITION OF PLANTS, CON- TINUED • • • • • • • • • • • • 28 IV.- — THE MORPHOLOGICAL COMPOSITION OF ANIMALS • • • 77 V. — THE MORPHOLOGICAL COMPOSITION OF ANIMALS, CON- TINUED • • • • • • • • • • • • 99 VI. — MORPHOLOGICAL DIFFERENTIATION IN PLANTS • • • 113 119 130 137 14G 159 162 166 169 192 210 213 PART Y.— PHYSIOLOGICAL DEVELOPMENT. i. — THE PROBLEMS OF PHYSIOLOGY... • • • • • • 221 ii. — DIFFERENTIATIONS BETWEEN THE OUTER AND INNER TISSUES OF PLANTS • • • • • • • • • 226 hi. — DIFFERENTIATIONS AMONG THE OUTER TISSUES OF PLANTS • • • • • • • • • • • • 213 rv. — DIFFERENTIATIONS AMONG THE INNER TISSUES OF PLANTS • • • • • • • • • 552 Vlll CONTENTS CHAP. Y. - PHYSIOLOGICAL INTEGRATION IN PLANTS ... VI. - DIFFERENTIATIONS BETWEEN THE OUTER AND INNER TISSUES OF ANIMALS VII. DIFFERENTIATIONS AMONG THE OUTER TISSUES OF ANIMALS ... ... ... ... Fill. DIFFERENTIATIONS AMONG THE INNER TISSUES OF ANIMALS ... ... ... ... IX. - PHYSIOLOGICAL INTEGRATION IN ANIMALS... X. — SUMMARY OF PHYSIOLOGICAL DEVELOPMENT PACI* 275 282 291 310 3G5 377 PART YI.— LAWS OF MULTIPLICATION I. - THE FACTORS II. - A PRIORI PRINCIPLE ... III. — OBVERSE A PRIORI PRINCIPLE ... IV. — DIFFICULTIES OF INDUCTIVE VERIFICATION V. - ANTAGONISM BETWEEN GROWTH AND ASEXUAL GENESIS VI. - ANTAGONISM BETWEEN GROWTH AND SEXUAL GENESIS VII. - ANTAGONISM BETWEEN DEVELOPMENT AND GENESIS, ASEXUAL AND SEXUAL VIII. — ANTAGONISM BETWEEN EXPENDITURE AND GENESIS ... IX.— COINCIDENCE BETWEEN IIIGII NUTRITION AND GENESIS X. - SPECIALITIES OF THESE RELATIONS XI. - INTERPRETATION AND QUALIFICATION XII. — MULTIPLICATION OF THE HUMAN RACE XIII. - HUMAN POPULATION IN THE FUTURE 391 397 404 412 419 428 440 446 454 463 470 479 494 APPENDICES. A. - SUBSTITUTION OF AXIAL FOR FOLIAR ORGANS IN PLANTS 511 B. — A CRITICISM ON PROF. OWENS THEORY OF THE VER¬ TEBRATE SKELETON ... ... ... 517 C. — ON CIRCULATION AND THE FORMATION OF WOOD IN PLANTS ... ... ... ... 536 PART IV. MORPHOLOGICAL DEVELOPMENT. CHAPTER L 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 MORPHOLOGICAL DEVELOPMENT. facts as the hypothesis of Evolution aims at, without contem¬ plating structures and functions in their mutual relations. Everywhere structures in great measure determine functions ; and everywhere functions are incessantly modifying 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 aspects 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. 5 before questions concerning the mode in which, these parts 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. Sucli 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 P — 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 is contained in an essay on The Classification of the Sciences , published in March, 1864. 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 poe- Besses. <5 MORPHOLOGICAL DEVELOPMENT. homologies of its different parts become problems. Under 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. J ust 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 rROBLEMS OF MORPHOLOGY. 7 questions concerning its form — form in tlie ordinary sense. After tlie contrasts caused by variations in tbe process of integration, we have to consider tbe 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 these 8 MORPHOLOGICAL DEVELOPMENT. phenomena the process of evolution ; and to show how, as displayed in them, it conforms to those first principles which evolution in general conforms to. Two sets of factors have to he 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 of forces, 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. 9 forms. We 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 which 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. CHAPTER II. THE MORPHOLOGICAL COMPOSITION OF PLANTS. § 180. Evolution implies insensible modifications and gradual transitions, winch render definition difficult — which make it impossible to separate absolutely the phases of or¬ ganization from one another. And this indefiniteness of distinction, to be expected a priori, we are compelled to re¬ cognize a 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 the 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 is 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 form but a minute one could find nutriment. Obviously, too, 12 MORPHOLOG ICA L DEV ELOPMENT. it must have been structurelcsss ; 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 Protophyta 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. Prom the law of heredity, considered as extending to the entire succes¬ sion of living things during the Earth’s past history, it follows, 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. 13 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 tlio 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 Review 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 Diatomacece , 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. 33 ut, 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 Algcc, of which Codium adhcerens , Fig. 4, may serve as an example. In Hydrogastrum , an¬ other alga, Fig. 5, we have a structure which is described as THE MORPHOLOGICAL COMPOSITION OF PLANTS. 15 simulating a perfect plant, witli 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 Palmella botryoides. 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 Diatomacece , 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 be traced through phases of increasing permanence and de- 16 MORPHOLOGICAL DEVELOPMENT. finiteness, as well as increasing extent. In tlie yeast-plant, Tig. 7, we liave cells wliicli may exist singly, or joined into groups of several ; and wliicli 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 ; hut 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 Gonium perfor¬ ate > Fig. 16 {a being the front view, and b the edge view), and the Sarcina ventriculi , Fig. 17. Further, we have that spherical mode of aggregation of which the Volvox 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¬ ary aggregate : in proportion to the greater number of the TIIE 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 laminae 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 ao-orecrate 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 t] e ConfervcB, and their allies. In Fig. 18, there are re¬ 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 Vol. II. 2 18 MORPHOLOGICAL DEVELOPMENT. altered ; in tlie liiglier 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 vp'" way, unites all the parts into a whole. Even this slight individuation 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, r hollow frond may be produced — sometimes irregularly THE MORPHOLOGICAL COMPOSITION OF PLANTS. 19 spheroidal, and sometimes irregularly tubular ; as in Enter o * morpha intestinciUs, Fig. 27. And occasionally, as in Entero - morpha comprcssa, 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. 20 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. § 1S4. 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 Algce. 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 Confer ice and simple plants akin to them, as shown in Figs. 22, 23. Though, in some of the more developed Algce which exhibit the ramified arrangement in a higher degree, the component cells are, like those of the lower Algce , united to¬ gether end to end, in such way as but little to obscure their separate forms, as in Cladophora Ilutchinsice, Fig. 31 ; they nevertheless evince greater subordination to the whole of which they are parts, by arranging themselves more method- THE MORPHOLOGICAL COMPOSITION OF PLANTS. 21 ically. Still further pronounced becomes tlie 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 Pigs. 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 Phamogams make us familiar. Put 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 22 MORPHOLOGICAL DEVELOPMENT. integration still 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 Fuci of the Antartic seas, supply yet better instances. In some of these, the germ develops a very long slender stem, which4 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 fuscescens, 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 Algce, we do find some¬ thing more than 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 Rhodymenia 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 Phyllophora rubens, 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. Delesseria sanguined , 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 wre 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 Algae. , as 21 MORPHOLOGICAL DEVELOPMENT. tlie Sargassum, or common gulf- weed, tills tertiary degree of composition is far more completely displayed, so as to pro¬ duce among Tliallogens a type of structure closely simulating that of tlie 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 tho 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 Algcb, 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 unlikenesses 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 Acrogens — 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 progress towards a higher composition, which we have just observed among the more-developed AJgce. The Jungertnanniacece furnish us 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. Tho THE MORPHOLOGICAL COMPOSITION OF PLANTS. 25 frond grows as irregularly as tlie tliallus of a liclien : it is in¬ definite in size and outline, spreading hither or thither as the conditions favour. Moreover, it lacks the differentiations re¬ quired 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 epipliylla , 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 fur cat a, 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- 26 MORPHOLOGICAL D E VE L.O PMENT. gregate of the third order is thus produced. But though, along with increased definiteness 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 J. 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. Hot 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 MOXrilOLOGICAL 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 on 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 tliallus or frond ; so, by the integration of fronds, there is evolved a structure such as the above-delineated species possess P 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 Phanerogamia or Phoenogamia. CHAPTER III. THE MORPHOLOGICAL COMPOSITION OF PLANTS, CONTINUED. § 187. That advanced composition arrived at in tlio 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 he 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 wdll 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, wre shall not unfrequently find a simple or undivided leaf, at the insertion of one of the 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- TUB MORPHOLOGICAL COMPOSITION OF PLANTS. 29 posing tlie 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. quintuple leaves occur where the materials for growth, are supplied in greatest abundance ; that the leaves become less 30 MO C PROLOG I CAL 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-hunch : 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 furnished 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 thrown, 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. Put 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 Dear 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. Here 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 Ileracleum, 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 Ethiojpica. 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 steins, 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 cf 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 unit3 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 come to the Theorici 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 leaves, as in the calyx, with this differ- 34 MORPHOLOGICAL DEVELOPMENT. ence only, that the leaves which are merely placed in close contact in the calyx, are here united together ; * a view which he corroborates by referring to the manner in which many capsules open and sejmrate ‘ 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 with 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. Hut 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 of the flower ; at other times the bracts gradually assume more THE MORPHOLOGICAL COMPOSITION OF PLANTS. 35 and 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, tfley 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 internodes, the 86 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 vrill 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 ^ve 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 Phsenogams. I say a proximate interpretation, because there remain to be solved certain deeper problems ; one of wdiich 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 Phoenogam 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 occasional development of foliar organs into axial organs : the special case there described, being that of a fox- glove, in which some of the sepals were replaced by flower-buds. 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 of 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 suggestedto 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-barley ; 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 Guarea 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 Mahomet, 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 deductivety, is presented by the Cadacece. In this remark¬ able group of plants, deviating in such varied ways from the ordinary phsenogamic type, we find many highly instructive * See British and Foreign Medico- Chirurgical Review for January, 1862. 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, wre 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, wre 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, wdiich simulate Cactuses, show us the stages through which such abnormal structures are arrived at. In Euphorbia splendem , 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 neriifolia , 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 gpecies 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 Phy/Jocactus, they become flattened out into foliaceous shapes, having mid-ribs and something approaching to veins. So that here, and in the genus Epiphyllum, which has this character still more TIIE MORPHOLOGICAL COMPOSITION OF PLANTS. 45 marked, the plant appears to be composed of fleshy leaves growing one upon another. And then, in Uhipsalis, 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, we 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 Rhipsalis 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 fixes 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 4fi 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 Pheenogams 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 evolution 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 of 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 phsenogamic 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 THE MOUrHOLOG ICAL COMPOSITION OF PLANTS. 47 Mosses and Ferns, we find a distinctly marked stem.* Some Acrogens liave foliar expansions that are indefinite in their forms ; and some have quite definitely- shaped leaves. Roots 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 Acrogens, 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 Phmnogams. Thus, many facts imply that if the phaenogamic type is to be 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 of 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. But 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 kis strictures on the unphilosophical assumptions of other botanists. Vol. II. 3 43 MOltFHOLOG ICAL DE V ELOPM ENT. as they become integrated and definitely limited, aggregates 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 laminoe 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 away 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 Phcenogams, 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 OF PLANTS. 49 ing separation to be arrested ; and tlie fructifying elements will revert towards tlie ordinary form, and develop in con¬ nexion witb tbe parent. Turning to the Acrogens, we nnd among tliem, 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 (/. cxsecta) 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 androgynum 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 which are about to become fructifying organs, may similarly, under increased nutrition, develop into young fronds. As Sir W. Hooker says of the male fructification in Jungermannia furcata, — “ 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 Phocnogams, 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¬ mannia egnjphylla, Fig. 43 ; bearing, like it, the fructifying buds 50 MORPHOLOGICAL DEVELOPMENT. on its upper surface, and having a slightlv- marked mid-rib and rootlets. And sup¬ pose that, as shown, a secondary frond is proliferous!}^ produced from the mid-rib, and continues attached to it. Evidently, the ordinary discontinuous development, yg 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, yc 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 sjiores. 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 mjirk 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, which 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 MORPHOLOGICAL DEVELOPMENT is j ust the character of the successive leaves in tlie higher types. As shown in Eig. 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 belov/ 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 M0RTT10E0G ICAL COMPOSITION OF PLANTS. 53 tudinal 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 he 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 further facts supporting this view. In his Flora Antarctica , he describes the genus Lessonia (see Fig. 37) and especially L. ovata, as having a mode of growth simulating that of the Exogcns. 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 nearest to them. 51 MOKPHOLOGICAL DEVELOPMENT. 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 Acrogcns— a type is shown in Fig. 78, Riella lieUcophylla, 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 Junqermannia cord}folia% THE MORPHOLOGICAL COMROSITTON 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 campanulate flowers it is so with petals. And in some tribes of planfs 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 89 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. Eig. 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 Eig. 79 — the successive nodes answering to the successive points of origin of the fronds. Conversely, the stem of a grass, Eig. 91, dis¬ plays just such relations of parts, as would result from the de¬ velopment of the type shown in Eig. 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. Tor while, even more clearly than in the Dcndrobium, 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 sheath, instead of having its edges united as in De7idrobium, 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 b, 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 oi 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 De 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 P 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 Dcndro- 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 wray in -which the requisite stiffness appears producible, is by the thickening and hardening of the fused series of mid-ribs. The incqnent 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 MORPHO LOGICAL COMPOSITION OF PLANTS. 59 which thrive and have the best chances of leaving offspring, being, by the hj^pothesis, 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 : 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 Hoolteri, and J. decipiens. Thus the higher Acrogens show us how, alQng with an assumption of the upright attitude, there does 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 support GO MORPHOLOGICAL DEVELOPMENT. and a channel, being here unrestrained by the early-formed 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 internodes, 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 ; since tlie 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 comes to be a more marked contrast between it and the netioles, 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 this 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 unstipulated leaves on long petioles, and leaves that embrace the axis. It may be added that the distribution of these va¬ rious forms, is quite in harmony with the rationale above given. 62 MORPHOLOGICAL DEVELOPMENT. tlie forthcoming parts, while they are very small and un« 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 Pig. 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 Pig. 101, which is the bud of an exogen, we see how clear is the morphological correspondence : a being the rudiment of a foliar organ beginning to take shape ; h being the almost formless rudiment of the next foliar organ ; and c being the quite-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 Phaenogams — 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. Delesseria hypoglossum, Pig. 105, represents a seaweed of the same genus as one outlined in Pig. 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. Pesides being shown that the budding out of several fronds from one frond, may become habitual ; we are also shown that it may become a habit inherited by the fronds so produced, and also by the Tim MORP HOLOGICAL 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, is 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 b, but also another such secondary frond, V. Let us suppose, further, that the frond b is in like manner doubly proliferous : producing both c and c\ Lastly, let us suppose that in the second frond b' which a produces, as well as in the second frond c’ which b 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 show themselves at earlier and earlier stages of development. And in the same way that, by dwarfing and generalizing 64 MORPHOLOGICAL DEVELOPMENT. the original series, we arrive at a structure like that of the terminal bud ; by dwarfing and generalizing a lateral series, as shown in Figs. 107 — 110, we arrive at a structure an¬ swering in nature and position to the axillary bud. Facts confirming these interpretations, are afforded by the structure and distribution of buds. The pluenogamic axis in its primordial form, being an integrated series 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 phaenogamic 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 phaenogamic 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. 65 surface on which it occurs, must grow to some extent before the bud appears. On thus looking at the matter — on contemplating afresh 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 committed to the gratuitous assumption, that in the pha3no- 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 Phaenogams ; 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. tlie 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 phasnogamic 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 phaeno- gam, must be supposed rudely to indicate the type out of which the phccnogamic 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 the THE MORPHOLOGICAL COMPOSITION OP PLANTS. 6? exogenous modes of growth. P We shall find that they do. Starting, as before, with the proliferous form shown in Kg. 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 the first- 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- 08 MORPHOLOGICAL DEVE LOPMENT. sive fronds to eacli 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 monocotyledonous germ, will be seen on 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 wdth 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 of 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¬ bryo 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 a, 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 first and equal with it. TIIE MORPHOLOGICAL COMPOSITION OF PLANTS. 69 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 monocotyledonous 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 ; we are at the same time supplied with a rationale of those truths which vegetal morphologists have established. Those homo¬ logies which Wolff indicated in their chief outlines and Goethe followed out in detail, have a new meaning given to them when we regard the phaenogamic axis as having been evolved in the way described. Forming the modified con¬ ception which we are here led to do, respecting the units of which a flowering plant is composed, we are 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. The unit of composition of a Phsenogam, 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 internode, 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 tlie axis below it. And now observe liow, wben we take this for tbe unit of composition, tbe metamorphoses wbicb tbe phoenogamic axis displays, are inferable from known laws of development. Embryology teacbes us tbat arrest of development shows itself first in tbe absence of those parts tbat have arisen latest in tbe course of evolution ; tbat if defect of nutrition causes an earlier arrest, parts tbat are of more ancient origin abort ; and tbat tbe part alone produced wben tbe supply of materials fails near tbe outset, is tbe prim* ordial part. We must infer, therefore, tbat in each seg¬ ment of a Pbsenogam, tbe foliar organ, wbicb answers to tbe primordial frond, will be tbe most constant element ; and that tbe internode and tbe 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 tbe flower, the J2& 42G asj J28 J2Q axillary bud disappears ; and tbe segment is reduced to a small foliar surface, with an internode wbicb is in most cases very short if not absent, as in 125 and 126. In tbe flower itself, axillary buds and internodes are both want¬ ing: there remains only a foliar surface (127), wbicb, though often larger than tbe immediately preceding foliar surface, shows failing nutrition by absence of chlorophyll. And then, in tbe quite terminal organs of fructification (129), we have tbe 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 i£ 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. Nay, 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 homologies among the parts of the phaenogamic 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 phaenogamic 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 recognize in the anomaly, a resumption of an original mode of growth : fronds frequently do this. When we learn that a flowering plant, as the Drosera 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- Vol. II. 4 72 MORPHOLOGICAL DEVELOPMENT. bils by Phoenogams, ceases to be so surprising when w*e find it to be habitual among the inferior Acrogens ; and when we 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 on 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 wrill be clearly understood on remembering howr, on 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. 73 tins inference. In tlie compound leaf, Fig. 65, tlie 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 wdiere the nutrition is greatest ; namely, next the stem. But the clearest evidence is afforded among the Algce, which, not drawing nutriment from roots, have their paHs much less mutually dependent ; and are therefore capable of showing more clearly, how any part may remain an append¬ age or may become the parent of appendages, according to circumstances. In the annexed Fig. 130, representing a branch of Ptilotcc plumosa, we see how a wring grows into a wing-bear¬ ing branch, if its nutrition passes a certain point. This form, so strikingly like that of the feathery crystallizations of many inor¬ ganic substances, proves to us that, as in such crystallizations, the simplicity or com¬ plexity of structure at any place, depends on the quantity of matter that has to be polarized at that place in a given time.* * now the element of time modifies the result, is shown by the familiar fact that crystals rapidly formed are small ; and that they become larger when they are formed more slowly. If the quantity of molecules contained in a solution is rela¬ tively great, so that the mutual polarities of the molecules crowded together in every place throughout the solution are intense, there arises a crystalline aggre¬ gation around local axes ; whereas, in proportion as the local action of molecules on one another is rendered less intense by their wider dispersion, they becoma 74 MORPHOLOGICAL DEVELOPMENT. Hence, then, we are not without an interpretation of those over-developments which the phamogamic axis occasionally undergoes. Fig. 104, represents the phacnogamic bud in its rudimentary state. The lateral process b, which ordinarily becomes a foliar appendage, differs very little from the terminal process c, which is to become an axis — differs mainly in having, at this period when its form is being determined, a smaller bulk. If while thus undifferentiated, its nutrition remains inferior to that of the terminal process, it becomes moulded into a part that is subordinate to the general axis. But if, as sometimes happens, there is supplied to it such an abundance of the materials needful for growth, that it becomes as large as the terminal process ; then we may naturally expect it to begin moulding itself round an axis of its own : a foliar organ will be replaced by an axial organ. And this result will be especially liable to occur, when the growth of the axis has been previously under¬ going that arrest which leads to the formation of a flower ; that is, when, from defect of materials, the terminal process has almost ceased to increase, and when some concurrence of favourable causes, brings a sudden access of sap, which reaches the lateral processes before it reaches the terminal process. § 198. The general conclusion to which these various lines of evidence converge, is, then, that the shoot of a flowering plant is an aggregate of the third degree of composition. Taking as aggregates of the first order, those small masses of protoplasm which ordinarily assume the forms under which they are known as cells ; and considering as aggregates of the second order, those assemblages of such cells which, in the lower cryptogamia, compose the various kinds of thal- lus ; then that structure, common to the higher cryptogams and tj phcenogams, in which we find a series of such groups relatively more subordinate to tbe forces exerted on them by the larger aggre¬ gates of molecules that are at greater distances, and thus are left to arrange themselves round fewer axes into larger crystals. THE MORPHOLOGICAL COMPOSITION OF PLANTS. of cells bound up into a continuous whole, must be regarded as an aggregate of tbe third order. The inference drawn from analysis, and verified by a synthesis that corresponds in a remarkable manner with the facts, is, that those compound parts which, in Endogens and Exogens, are called axes, have really arisen by integration of such simple parts as in lower plants are called fronds. Here, on a higher level, ap¬ pears to have taken place a repetition of the process already observed on lower levels. The formation of those small groups of physiological units which compose the lowest protophytes, is itself a process of integration ; and the con¬ solidation of such groups into definitely-circumscribed and coherent cells, is a completing of the process. In those coalescences, variously carried on, by which many such cells are joined into threads, and discs, and solid or flattened- out masses, we see these morphological units aggregating into units of a compound kind — the different phases of the transition being exemplified by groups of various sizes, various degrees of cohesion, and various degrees of definite¬ ness. Once more do we now find evidences of a like process on a larger scale : the compound groups are again com¬ pounded. And, as before, there are not wanting types of organization by which the stages of this higher Integration are shadowed forth. From fronds that occasionally produce other fronds from their surfaces, we pass to those that habitually produce them. From those that do so in an in¬ definite manner, to those that do so in a definite manner. And from those that do so singly, to those that do so doubly and triply through successive generations of fronds. Even within the limits of a sub-class, we find gradations between fronds irregularly proliferous, and groups of such fronds united into a regular series. Hor does the process end here. The flowering plant is rarely uniaxial — it is nearly always multiaxial. From its primary shoot, there grow out secondary shoots of like kind. Though occasionally among Phienogams, and frequently 76 MORPHOLOGICAL DEVELOPMENT. among the higher Cryptogams, the germs of new axes detach themselves under the form of bulbils, and develop separately instead of in connexion with the parent axis ; yet in most Phaenogams, the germ of each new axis maintains its con¬ nexion with the parent axis : whence results a group of axes — an aggregate of the fourth order. Every tree, by the pro¬ duction of branch out of branch, shows us this integration repeated over and over again : forming an aggregate having a degree of composition too complex to be any longer defined. / CHAPTER IY. THE MORPHOLOGICAL COMPOSITION OF ANIMALS* § 199. "W hat was said in § 180, respecting tlie ultimate structure of organisms, holds more manifestly of animals than of plants. That throughout the vegetal kingdom the cell is the morphological unit, is a proposition admitting of a better defence, than the proposition that the cell is th'e mor¬ phological unit throughout the animal kingdom. The qualifi¬ cations with which, as we saw, the cell-doctrine must he taken, are qualifications thrust upon us more especially by the facts which zoologists have brought to light. It is among the Protozoa that there occur numerous cases of vital activity displayed by specks of protoplasm ; and from the minute anatomy of all creatures above these, up to the Teleozoa , are drawn the numerous proofs that non-cellular tissues may arise by direct metamorphosis of structureless colloidal sub¬ stance. Our survey of morphological composition throughout the animal kingdom, must therefore begin with those undiffer¬ entiated aggregates of physiological units, out of which are formed what we call, with considerable license, morphological units. § 200. In that division of the Protozoa distinguished as Pliizojpoda, are presented, under various modifications, these minute portions of living organic matter, so little differenti- 78 MO R PIIOLOG ICAL DEVELOPMENT. ated, if not positively undifferentiated, tliat animal individu¬ ality can scarcely be claimed for them. Figs. 131, 132, and 133, represent certain nearly-allied types of these — Amoeba , Adinophrys, and Lieberkuhnia. The viscid jelly or sarcode, comparable in its physical properties to white of egg, out of which one of these creatures is mainly formed, shows us in various ways, the feebleness with which the component physio¬ logical units are integrated — shows us this by its very slight cohesion, by the extreme indefiniteness and mutability of its form, and by the absence of a limiting membrane. Though unqualified adherents of the cell-doctrine assert that the Amoeba has an investment, yet since this investment, com¬ pared by Dujardin to the film which forms on the surface of paste, does not prevent the taking of solid particles into the mass of the body, and does not, in such kindred forms as Fig. 133, prevent the pseudopodia from coalescing when they meet, it cannot be anything deserving the name of a cell- wall. A considerable portion of the body, however, in Difflu- gia, Fig. 134, has a denser coating; so that the protrusion of the pseudopodia is limited to one part of it. And in the solitary Foraminifera, like Gromia , the sarcode is covered over most of its surface by a delicate calcareous shell, pierced with minute holes, through which the slender pseudopodia are thrust. The Greg anna exhibits an advance in integration, and a consequent greater definiteness. Figs. 135 and 136, exemplifying this type, show the complete membrane in which the substance of the creature is con¬ tained. Here there has arisen wdiat may be properly called a cell : under its solitary form this animal is truly unicellular. Its embryology has considerable significance. After passing through a certain quiescent, “ encysted ” state, its interior breaks up into small portions, which, after their exit, assume THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 79 forms like that of the Amoeba ; and from this young condi¬ tion in which they are undifferentiated, they pass into that adult condition in which they have limiting membranes. If this development of the individual Gregarina typifies the mode of evolution of the species, it yields further support to the belief, that homogeneous fragments of sarcode existed earlier than any of the structures which are properly called cells. Among aggregates of the first order, there are some much more highly developed. These are the Infu¬ soria ; constituting the most numerous of the Protozoa, in species as in individuals. Figs. 137, 138, and 139, are ex¬ amples. In them we find, along with greater definiteness, a considerable heterogeneity. The sarcode of which the body consists, has an indurated outer layer, bearing cilia and some¬ times spines ; there is an opening serving as mouth, a per¬ manent oesophagus, and a cavity or cavities, temporarily formed in the interior of the sarcode, to serve as one or more stomachs ; and there is a comparatively specific arrangement of these and various minor parts. Thus in the animal kingdom, as in the vegetal kingdom, there exists a class of minute forms having this peculiarity, that no one of them is separable into a number of visible com¬ ponents homologous with one another — no one of them can be resolved into minor individualities. Its proximate units are those physiological units of which we conclude every or¬ ganism consists. The aggregate is an aggregate of the first order. § 201. Among plants are found types indicating a transi¬ tion from aggregates of the first order to aggregates of the second order ; and among animals we find analogous types. But the stages of progressing integration are not here so dis¬ tinct. The reason probably is, that the simplest animals, having individualities much less marked than those of the simplest plants, do not afford us the same facilities for ob¬ servation. In proportion as the limits of the minor indi- 80 MORPHOLOGICAL DEVELOPMENT. viclualities are indefinite, the formation of major individu¬ alities out of them, naturally leaves less conspicuous traces. Be this as it may, however, in such types of Protozoa as the Thalassicollce, we find that though there is reason to re¬ gard the aggregate as an aggregate of the second order, yet its divisibility into minor individualities like those just de¬ scribed, is by no means manifest. Fig. 140, representing Splicer ozoum punctalujji, one of this group, illustrates the diffi¬ culty. Only by some license of interpretation, can we regard the “ cellteform bodies ” contained in it, as the morphological units of the animal. The jelly-like mass in which they are imbedded, shows no signs of being divisible into portions having each a cell or nucleus for its centre.* Comparison of the various forms assumed by creatures of this type, suggests, contrariwise, that the homogeneous sarcode is primary, and its included structures secondary. Among the Foramimfera , we find evidence of the coalescence of aggre¬ gates of the first order, into aggregates of the second order. There are solitary Foraminifers, allied to the creature repre¬ sented in Fig. 134. Certain ideal types of combination * This statement seems at variance •with the figure ; but the figure is very in¬ accurate. Its inaccuracy curiously illustrates the vitiation of evidence. When I saw the drawing on the block, I pointed out to the draughtsman, that he had made the surrounding curves much more obviously related to the contained bodies, than they Avere in the original (in Dr Carpenter’s Foraminifera) ; and having looked on while he in great measure remedied this defect, thought no further care was needed. Now, however, on seeing the figure in the printer’s proof, I find that the engraver, swayed by the same supposition as the draughtsman that such a relation was meant to be shown, has made his lines represent it still more de¬ cidedly than those of the draughtsman before they were corrected. Thus, vague linear representations, like vague verbal ones, are apt to grow more definite when repeated. Hypothesis warps perceptions as it warps thoughts. THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 81 among them, are shown in Fig. 141. And setting out from tliese, we may ascend in various directions to kinds com¬ pounded to an immense variety of degrees in an immense variety of ways. In all of them, however, the separability of the major individuality into minor individualities, is very in¬ complete. The portion of sarcode contained in one of these calcareous chambers, gives origin to an external bud ; and this presently becomes covered, like its parent, with calcareous matter : the position in which each successive chamber is so produced, determining the form of the compound shell. But the portions of sarcode thus budded ont one from another, do not become distinctly individualized. Fig. 142, representing the living net- work which remains when the shell of an Or- bitolite has been dissolved, shows the continuity that exists among the occupants of its aggregated chambers. Still, the occupant of each chamber may fairly be considered as homo¬ logous with a solitary Foraminifer ; and if so, the Orbitolite is an aggregate of the second order : this indefinite marking- off of its morphological units, being the obverse of the fact that the individualities of their prototypes are feebly pro¬ nounced. Forms of essentially the same kind are aggregated in another manner among the Spongidce. The fibres of a living sponge are clothed with gelatinous substance, which is separable into Amoeba-Mike creatures, capable of moving about by their pseudopodia when detach¬ ed. These nucleated portions of sarcode, which are the morphological units of the sponge, lining all its channels and chambers, subsist on the nutritive particles brought to them by the currents of water that are drawn in through the superficial pores, and sent out through the larger open¬ ings — currents produced by ciliated units, such as are shown m Fig. 143. So that, in the words of Prof. Huxley, “the sponge represents a kind of subaqueous city, where the people are arranged about the streets and roads, in such a manner, that each can easily appropriate his food from the water as it passes along.” In the compound Infusoria, the 82 MORPHOLOGICAL DEVELOPMENT. component units remain quite distinct. Being, as aggre¬ gates of tlie first order, much, more definitely organized, their union into aggregates of the second order does not de¬ stroy their original individualities. Among the Vorticellce , of which two kinds are delineated in Bigs. 144 and 145, there are various illustrations of this : the members of the com¬ munity being sometimes appended to a single stem ; some¬ times attached by long separate stems to a common base ; and sometimes massed together. Thus far, these aggregates of the second order exhibit but indefinite individualities. The integration is physical ; but not physiological. Though, in the Thalassicollce, there is a shape that has some symmetry ; and though, in the Fora - minifera, the formation of successive chambers proceeds in such methodic ways, as to produce quite-regular and tolerably- spe¬ cific shells ; yet no more in these than in the Sponges or the compound Vorticellce, do we find such co-ordination as gives the whole a life predominating over the lives of its parts. We have not yet reached an aggregate of the second order, so individuated as to be capable of serving as a unit in still higher combinations. But in the class Goclenterata, this ad¬ vance is displayed. The com¬ mon Hydra, habitually taken as the type of the lowest division of this class, has specialized parts performing mutually-subservient functions ; and thus exhibiting a total life distinct from the lives of the units. Fig. 146 represents one of these creatures in its contracted state and in its expanded state ; while Fig. 147 is a rude diagram from memory showing the wall of this creature’s sack-like body as seen in section under the microscope : a and b being the outer and inner cellular layers ; while in the central space between them, is that nucleated substance, or sarcode, or protoplasm, in which the cells originate. But this lowly-organized THE MORPHOLOGICAL COMPOSITION OP ANIMALS. 83 tissue of the Hydra, illustrates a phase of integration in which the lives of the minor aggregates are only par¬ tially-subordinated to the life of the major aggregate formed by them. For a Hydra’s substance is separable into AmoebaHkQ portions, capable of moving about independ¬ ently. Prof. Green quotes Ecker, Lewes, and Jager, in proof that “ this animal exhibits, at certain seasons of the year, a tendency to break up into particles of a sarcode aspect, which retain for a long time an independent vitality.” And if we bear in mind how analogous are the extreme extensibility and contractility of a Hydra’s body and tentacles, to the pro¬ perties displayed by the sarcode among Phizopods ; we may infer that probably the movements and other actions of a Hydra, are due to the half-independent co-operation of the Amoeba -like individuals composing it. § 202. A truth which we before saw among plants, we here see repeated among animals — the truth that as soon as the integration of aggregates of the first order into aggregates of the second order, produces compound wholes so specific in their shapes and sizes, and so mutually dependent in their parts, as to have distinct individualities ; there simultaneously arises the tendency in them to produce, by gemmation, other such aggregates of the second order. The approach towards definite limitation in an organism, is, by implication, an ap¬ proach towards a state in which growth passing a certain point, results, not in the increase of the old individual, but in the formation of a new indi¬ vidual. Thus it happens that the common polype buds out other polypes, some of which very shortly do the like, as shown in Fig. 148 : a process paral¬ leled by the fronds of sundry Algce, and by those of the lower Jungermanniacece. And just as, among these last plants, the b4 MORPHOLOGICAL DEVELOPMENT. proliferously-procluced fronds, after growing to certain sizes and developing rootlets, detach themselves from their parent- fronds ; so among these animals, separation of the young ones from the bodies of their parents, ensues when they have acquired tolerably complete organizations. There is reason to think that the parallel holds still fur¬ ther. Within the limits of the Jungermanniaceoe, we found that while some genera exhibit this discontinuous develop¬ ment, other genera exhibit a development that is similar to it in all essential respects, save that it is continuous. And here within the limits of the Hydrozoa, we find, along with this genus in which the gemmiparous individuals are pre¬ sently cast off, other genera in which they are not cast off, but form a permanent aggregate of the third order. Figs. 149 and 150, exemplify these compound Hydrozoa — one of them showing this mode of growth so carried out as to produce a single axis ; and the other showing how, by repetitions ot the process, lateral axes are produced. Integrations character¬ izing certain higher genera of the Hydrozoa , which swim or float instead of being fixed, are indicated by Figs. 151 and 152 : the first of them representing the type of a group in which the polypes growing from an axis, or coenosarc, are drawn through the water by the rhythmical contractions of the organs from which they hang ; and the second of them representing a Physalia the component' polypes of which are united into a cluster, attached to an air-vessel. It should be added that in the Fhizostomes, the integration is carried so far, that the individualities of the polypes are al¬ most lost in that of the aggregate they form. A parallel series of illustrations might be drawn from that second di- THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 85 vision of the Coelenterata , known as tlie Adinozoa. Here, too, we have a group of species — the Sea-anemonies — the individ¬ uals of which are solitary. Here, too, we have agamogenetic multiplication : occasionally by gemmation, but more fre¬ quently by that modified process called spontaneous fission. And here, too, we have compound forms resulting from the arrest of this spontaneous fission before it is complete. To give examples is needless ; since they would but show, in more varied ways, the truth already made sufficiently clear, that the compound Coelenterata are aggregates of the third order, produced by integration of aggregates of the second order such as we have in the Hydra. As before, it is manifest that on the hypothesis of evolution, these higher in¬ tegrations will insensibly arise, if the separation of the gem- miparous polypes is longer and longer postponed ; and that an increasing postponement will result by survival of the fittest, if it profits the group of individuals to remain united instead of dispersing. § 203. The like relations exist, and imply that the like processes have been gone through, among those more highly- organized animals called Molluscoida. We have solitary individuals, and we have variously-integrated groups of indi¬ viduals : the chief difference between the evidence here fur¬ nished, and that furnished in the last case, being the absence of a type obviously linking the solitary state with the aggre¬ gated state. It is now an accepted belief that the creatures named Brack i- opoda, very abundant in the so-called palaeozoic times, but at present comparatively rare, are akin in structure to the JPolyzoa ; widely as they differ from them in size. If we can¬ not fairly say that by union of many Brachiopods there would be produced a compound animal like a Polyzoon; yet we may fairly say that were a small imperfectly-developed Brachiopod united with others like itself, a Polyzoon would result. This in¬ tegration of aggregatesof the second order, is carried on among 86 MORPHOLOGICAL DEVELOPMENT. tlie Polyzoa in divers ways, and with different degrees of com¬ pleteness. The little patches of minute cells, shown as magnified in Fig. 153, so common on the fronds of sea- weeds and the surfaces of rocks at low-water mark, display little beyond me¬ chanical combination. The adjacent individuals, though sever¬ ally originated by gemmation from the same germ, have hut little physiological dependence. In kindred kinds, however, as shown in Figs. 154 and 155, one of which is a magnified portion of the other, the integration is somewhat greater : the co-operation of the united individuals being shown in the production of those tubular branches which form their common support, and establish among them a more decided community of nutrition. Among the Ascidians, another order of the Molluscoida, this general lav/ 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 mentula, is one of the soli¬ tary forms. A type in which the individuals are united by a stolon that gives origin to them by successive buds, is shown in Pcrr/phora, Fig. 157. Among the Botryllidcp, of which one THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 87 kind is drawn on a small scale in Fig. 159, and a portion of 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 of the third degree, analogous to that which we have found so prevalent among the Coelentercitci and the Molluscoida ? 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 types 88 MORPHOLOGICAL DEVELOPMENT. above described, in which the united animals maintain their individualities so distinctly tliat the individuality of tbe aggregate remains vague, are constructed in such ways that tlie united animals carry on tlieir several activities with scarcely any mutual hindrance. Tbe 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. Jn 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. Put 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 among 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 Pyi'osoma, 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 stones and weeds, the several members of the chain become differently conditioned in the way above described ; and may therefore be expected to become unlike while they become united. A clear idea of the contrast between these two 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 arc severally able to use their CO MORPHOLOGICAL DEVELOPMENT. weapons in like ways witli like efficiency ; and 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. Yery little search suffices ; for structures of the kind to be expected are abund¬ ant. In that great division of the animal kingdom called Anmilosa, especially if the Annuhida be regarded as part of TIIE MORPHOLOGICAL COMPOSITION OF ANIMALS. 91 it, we find a variety of types having the looked- for charac¬ ters. Let ns 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¬ sides having like locomotive appendages, like branchice, 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 tlien displayed liis 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 Coelenterata 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 MORFIIULOGICAL COMPOSITION OF ANIMALS. 93 consists of nntransformed 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 /6S 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. ISTow 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 gemmee 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 progressive integration of the kind to be anticipated. Among the simpler Annuloida, as in the Nemertidce 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. Gi. 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. Pig. 166 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¬ mation. 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 echinococcus , represented in Pig. 167, we have a species in which the number of segments thus united does not exceed four. In Echinohothriurn typus there are eight or ten ; and in cestoids THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 95 generally tliey 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 Syllis , 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) ayant acquis tout son developpement, se detache ordinairement de la colonie et continue encore a. croitre dans l’intestin du meme animal ; il change meme sou- vent de forme et semble doue d’une nouvelle vie ; ses angles s’effacent, tout le corps s’arrondit, et il nage comme une Planaire au milieu des muscosites intestinales.’* VOL. II. 5 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 by 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 gemmae separate as soon as produced ; that we have types in which such gemmae 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 gemmae which do not become individually independ¬ ent, but separate into organized groups ; and that from these we advance to forms in which all the gemmae remain Darts 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, kl. Pagenstecher alleges this of the Exogone gemmifera : describing a certain number of the segments of the body as severally bearing on their dorsal THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 97 surfaces a bucl on eacli 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 gemmae 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 Ccelenterata , we see that longitudinal and lateral gemmation do occur to¬ gether, where the longitudinally-united gemmae are demon¬ strably independent individuals. § 207. It would add to the probability of this conclusion could 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 relation¬ ships ; both because it would necessitate a long digression, encumbering too much the general argument, and because, being highly speculative, it would be impolitic to let the general argument be even apparently implicated by it. 13 ut 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 08 MORPHOLOGICAL DEVELOPM ENT. lower type. Tlie 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. As 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 Hyclrozoa. Indirect evidence of much weight has still to be given. Thus far we have considered only the less-developed Annu- 7osa. 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 shall have grounds for suspecting that in the inferior the individualities of the segments are less pronounced than in those lost forms which initiated the annulose sub-kingdom. CHAPTER Y. THE MORPHOLOGICAL COMPOSITION OF ANIMALS, 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, arc aberrant members of 100 MORPIIOLOG ICAL DEVELOPMENT. their sub-kingdom, comparisons between tbe different orders prove that tlie biglrer are strongly distinguished from tbe lower, by tbe 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 deATelop- 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 having shown that the actions and reactions in- vcd b} its mode of locomotion, are possible causes of those ru imentary structures which the simplest vertebrate animal presents, let us return to the region of established fact, and consi er whether such actions and reactions as we actually v ltness, 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 first of these facts shows unmistakably the immediate effect 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 has taken place most rapidly, we may expect, on tracing up T1IE 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 cVappui ; and that these arise first in those places where the strains are greatest. Now this is just what we do find. The myocom- mata 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 Amphioxus. 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 un¬ 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 Lcpidosiren, which by uniting certain piscine and amphibian characters betrays its close alliance with primitive types, the axial part of the vertebral column is unossified, wliile there is ossification of the peripheral parts. Similarly with numerous genera of fishes classed as palaeozoic. The fossil remains of them show that while the neural and hcemal spines consisted of bone, the central parts of the vertebrae 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 he 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 intenser 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 tlie account (since for similar masses moving with, equal velocities tlie resistances increase but little faster than tlie squares of tbe 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 strain 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. 33 ut 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 of the velocity of the body moving through it ; so that to main¬ tain double the speed, a fish has to expend four times the energjE 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. Now 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 tliat 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 *91 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 P If, as facts war¬ rant us in supposing, a formation of denser substance occurs THE SHAPES OF VERTEBRATE SKELETONS. 205 at those parts 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 at 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 Vertebrata 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 Anipliioxus it is not difficult to see that that portion of the body out of which the head 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 must be comparatively short, and much interfered with by the pre-existing orifices. Hence the cephalic part will not 206 MORPHOLOGICAL DEVELOPMENT. partake in any considerable degree of the lateral undula¬ tions ; 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. F or 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, however, these additional bones are accounted for as arising under actions like those that gave origin to the bones adjacent to them. And similarly with TIIE SHAPES OF VERTEBRATE SKELETONS 207 such bones as those called sesamoid; together with others too numerous to name. Again, in the course of evolution, both as displayed in the Vcrtebrata 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 cliondrification and ossification often proceed with but little respect for the pre-existing divisions ; but severally may result in the establishment of two parts where there was before one, or one where there were before two. Now wholly incongruous as these facts are with the hypothe¬ sis of an archetypal skeleton, they are quite congruous with the mechanical hypothesis. This shows us why, in the course of evolution, a feebly-resisting membranous structure came to be replaced by a more-resisting cartilaginous struc¬ ture, and this, again, by a still-more-resisting osseous struc¬ ture ; and why, therefore, these successive stages succeed one another, as it seems so superfluously, in the vertebrate em¬ bryo. And it further shows us why there is irregularity in the succession ; seeing that the varying mechanical ac¬ tions to which the varying habits of the Vcrtebrata have 208 MORPHOLOGICAL DEVELOPMENT. exposed them, have involved variations in the process of solidification. § 259. Of course the foregoing synthesis is to be taken simply as an adumbration of the process by which the verte¬ brate structure may have arisen through the continued actions of known agencies. The motive for attempting it has been two-fold. Having, as before said, given reasons for con¬ cluding that the segments of a vertebrate animal are not homologous in the same sense as those of an annulose animal or a phaenogamic axis, it seemed needful to do something towards showing how they are otherwise to be accounted for ; and having here, for our general subject, the likenesses and differences among the parts of organisms, as determined by incident forces, it seemed out of the question to pass by the problem presented by the vertebrate skeleton. Leaving out all that is hypothetical, the general argument may be briefly presented thus: — The evolution from the simplest known vertebrate animal, of a powerful and active vertebrate animal, implies the development of a stronger internal fulcrum. The internal fulcrum cannot be made stronger without becoming more dense. And it cannot be¬ come more dense while retaining its lateral flexibility, with¬ out becoming divided into segments. Further, in conformity with the general principles thus far traced, these segments must be alike in proportion as the forces to which they are exposed are alike, and unlike in proportion as these forces are unlike ; and so there necessarily results that unity in variety by which the vertebral column is from the beginning characterized. Once more, we see that the explanation ex¬ tends to those innumerable and more-marked divergences from homogeneity, which vertebrae undergo in the various higher animals. Thus, the production of vertebrae, the pro¬ duction of likenesses among vertebrae, and the production of unlikenesses among vertebrae, are interpretable as parts of TIIE SHAPES OF VERTEBRATE SKELETONS. 209 one general process, and as harmonizing with one general principle. Whether sufficient or insufficient, the explanation here given assigns causes of known kinds producing effects such as they are known to produce. It does not, as a solution of one mystery, offer another mystery of which no solution is to be asked. It does not allege a Platonic i§ea, or fictitious entity, which explains the vertebrate skeleton by absorbing into itself all the inexplicability. On the contrary, it assumes nothing beyond agencies bv which structures in general are moulded— agencies by which these particular structures are, indeed, notoriously modifiable. An ascertained cause of certain traits in vertebrce and other bones, it extends to all other traits of vertebrae; and at the time assimilates the morphological phenomena they present to much wider classes of morphological phenomena. CHAPTER XYL THE SHAPES OF ANIMAL CELLS. § 260. Among animals as among plants, tlie laws of mor¬ phological differentiation must be conformed to by the mor¬ phological units, as well as by the larger parts and the wholes formed of them. It remains here to point out that the con¬ formity is traceable where the conditions are simple. In the shapes assumed by those rapidly-multiplying cells out of which each animal is developed, there is a conspicuous subordination to the surrounding actions. Eig. 294 represents the cellular embryonic mass that arises by repeated spontaneous fissions. In it we see how the cells, origin¬ ally spherical, are changed by pressure against one another and against the limit¬ ing membrane ; and how their likenesses and unlikenesses are determined by the likenesses and un¬ likenesses of the forces to which they are exposed. This fact may be thought scarcely worth pointing out. But it is worth pointing out, because what is here so obvious a con¬ sequence of mechanical actions, is in other cases a conse¬ quence of actions composite in their kinds and involved in their distribution. Just as the equalities and inequalities of dimensions among aggregated cells, are here caused by the equalities and inequalities among their mutual pressures in different directions ; so, though less manifestly, the equalities 211 THE SHAPES OP" ANIMAL CELLS. and inequalities of dimensions among other aggregated cells, are caused by the equalities and inequalities of the osmotic, chemical, thermal, and otner forces besides the mechanical, to which their different positions subject them. § 261. This we shall readily see on observing the or¬ dinary structures of limiting membranes internal and ex¬ ternal. In Fig. 295, m shown a much-magnified section of a papilla from the gum. The cells of which it is composed originate in its deeper part ; and are at first approximately spherical. Those of them which, as they develop, are thrust outwards by the new cells that continually take their places, have their shapes gradually changed. As they grow and successively advance to replace the superficial cells, when these exfoliate, they become exposed to forces that are more and more dif¬ ferent in the direction of the surface from what they are in lateral directions ; and their dimensions gradually assume corresponding differences. Another species of limiting membrane, called cylinder- epithelium, is represented in Fig. 296. Though its mode of development is such as to render the shapes of its cells quite unlike those of pavement- epithelium, as the above-described kind is sometimes called, its cells equally exemplify the same general truth. For the chief contrast which each of them presents, is the contrast between its dimension at right angles to the surface of the membrane, and its dimension parallel to that surface. It is needless for our present purpose to examine further 212 MORPHOLOG L CAL DEVELOPMENT. the evidence furnished by Histology ; nor, indeed, would further examination of this evidence be likely to yield de¬ finite results. In the cases given above we have marked differences among the incident forces ; and therefore have a chance of finding, as we do find, relations between these and differences of form. But the cells composing masses of tissue are severally subject to forces that are indeterminate ; and therefore the interpretation of their shapes is imprac¬ ticable. It must suffice that so far as the facts go they are congruous with the hypothesis. CHAPTER XVII. SUMMARY OF MORPHOLOGICAL DEVELOPMENT. § 262. That any formula should be capable of expressing a common character in the shapes of things so unlike as a tree and a cow, a flower and a centipede, is a remarkable fact ; and is a fact which affords strong prirna facie evidence of truth. Eor in proportion to the diversity and multiplicity of the cases to which any statement applies, is the probability that it sets forth the essential relations. Those connexions which remain constant under all varieties of manifestation, are most likely to be the causal connexions. Still higher will appear the likelihood of an alleged law of organic form possessing so great a comprehensiveness, when we remember that on the hypothesis of Evolution, there must exist between all organisms and their environments, certain congruities expressible in terms of their actions and reac¬ tions. The forces being, on this hypothesis, the causes of the forms, it is inferable, a priori , that the forms must admit of generalization in terms of the forces ; and hence, such a generalization arrived at a posteriori, gains the further pro¬ bability due to fulfilment of anticipation. Nearer yet to certainty seems the conclusion thus reached, on finding that it does but assert in their special manifesta¬ tions, the laws of Evolution in general — the laws of that uni¬ versal re-distribution of matter and motion which hold through¬ out the totality of things, as well as in each of its parts. 214 MORPHOLOGICAL DEVELOPMENT. It will be useful to glance back over the various minor inferences arrived at, and contemplate tbem in their ensem¬ ble from these higher points of view. § 263. That process of integration which every plant dis¬ plays during its life, we found reason to think has gone on during the life of the vegetal kingdom as a whole. Proto¬ plasm into cells, cells into folia, folia into axes, axes into branched combinations — such, in brief, are the stages passed through b}^ every shrub ; and such appear to have been the stages through which plants of successively-higher kinds have been evolved from lower kinds. Even among certain groups of plants now existing, we find aggregates of the first order passing through various gradations into aggregates of the second order — here forming small, incoherent, indefinite assemblages, and there forming large, definite, coherent fronds. Similar transitions are traceable through which these integrated aggregates of the second order pass into aggregates of the third order : in one species the unions of parent-fronds with the fronds that bud out from them, being temporary, and in another species such unions being longer continued ; until, in species still higher, by a gemmation that is habitual and regular, there is produced a definitely- integrated aggregate of the third order — an axis bearing fronds or leaves. And even between this type and a type further compounded, a link occurs in the plants which cast off, in the shape of bulbils, some of the young axes they produce. As among plants, so among animals. A like spontaneous fission of cells ends here in separation, there in partial aggregation, while elsewhere, by closer combina¬ tion of the multiplying units, there arises a coherent and tolerably definite individual of the second order. By the budding of individuals of the second order, there are in some cases produced other separate individuals like them ; in some cases temporary aggregates of such like individuals ; and in other cases permanent aggregates of them : certain of which SUMMARY OF MORPHOLOGICAL DEVELOPMENT. 215 become so definitely integrated that the individualities of their component members are almost lost in a tertiary indi¬ viduality. Along with this progressive integration there has gone on a progressive differentiation. Vegetal units of whatever order, originally homogeneous, have become heterogeneous while they have become united. Spherical cells asrsrreo’atino- , # # **• 00^30 into threads, into laminae, into masses, and into special tis¬ sues, lose their sphericity ; and instead of remaining all alike assume innumerable unlikenesses — from uniformity pass into multiformity. Fronds combining to form axes, severally acquire definite differences between their attached ends and their free ends ; while they also diverge from one another in their shapes at different parts of the axes they compose. And axes, uniting into aggregates of a still higher order, be¬ come more or less contrasted in their sizes, curvatures, and arrangements of their appendages. Similarly among animals. Those components of them which, with a certain license, we class as morphological units, while losing their minor individualities in the major individualities formed of them, grow definitely unlike as they grow definitely com¬ bined. And where the aggregates so produced become, by coalescence, segments of aggregates of a still higher order, they, too, diverge from one another in their shapes. The morphological differentiation which thus goes hand in hand with morphological integration, is clearly what the perpetually-complicating conditions would lead us to antici¬ pate. Every addition of a new unit to an aggregate of such units, must affect the circumstances of the other units in all varieties of ways and degrees, according to their relative positions must alter the distribution of mechanical strains throughout the mass, must modify the process of nutrition, must affect the relations of neighbouring parts to surround¬ ing diffused actions ; that is, must initiate a changed inci¬ dence of forces tending ever to produce changed structural arrangements. Vol. II. 10 216 MORPHOLOGICAL DEVELOPMENT. § 264. This broad statement of the correspondence be¬ tween the general facts of Morphological Development and the principles of Evolution at large, may be reduced to state¬ ments of a much more specific kind. The phenomena of symmetry and unsymmetry and asymmetry, which we have traced out among organic forms, are demonstrably in har¬ mony with those laws of the re-distribution of matter and motion to which Evolution conforms. Besides the myriad¬ fold illustrations of the instability of the homogeneous, that are afforded by these aggregates of units of each order, which, at first alike, lapse gradually into unlikeness ; and besides the myriad- fold illustrations of the multiplication of effects, which these ever-complicating differentiations exhibit to us ; we have also myriad-fold illustrations of the definite equal¬ ities and inequalities of structures, produced by definite equalities and inequalities of forces. The proposition arrived at when dealing with the causes of Evolution, “ that in the actions and reactions of force and matter, an unlikeness in either of the factors necessitates an unlikeness in the effects ; and that in the absence of unlike¬ ness in either of the factors the effects must be alike ” ( First Principles, § 129), is the general formula including all these particular likenesses and unlikenesses of parts which we have been tracing. For have we not everywhere seen that the strongest contrasts are between the parts that are most contrasted in their conditions ; while the most similar parts are those most-similarly conditioned ? In every plant the leading difference is between the attached end and the free end ; in every branch it is the same ; in every leaf it is the same. And in every plant the leading likenesses are those between the two sides of the branch, the two sides of the leaf, and the two sides of the flower, where these parts are two-sided in their conditions ; or between all sides of the branch, all sides of the leaf, and all sides of the flower, where these parts are similarly conditioned on all sides. So, too, is it with animals that move about. The most marked contrasts SUMMARY OF MORPHOLOGICAL DEVELOPMENT. 217 they present are those between the part in advance and the part behind, and between the upper part and the under part ; while there is complete correspondence between the two sides. Externally the likenesses and differences among limbs, and internally the likenesses and differences among vertebras, are expressible in terms of this same law. And here, indeed, we may see clearly that these truths are corollaries from that ultimate truth to which all phenomena of Evolution are referable. It is an inevitable deduction from the persistence of force, that organic forms which have been progressively evolved must present just those funda¬ mental traits of form which we find them present. It cannot but be that during the intercourse between an organism and its environment, equal forces acting under equal conditions must produce equal effects ; for to say otherwise, is, by im¬ plication, to say that some force can produce more or less than its equivalent effect, which is to deny the persistence of force. Hence those parts of an organism which are, by its habits of life, exposed to like amounts and like combinations of actions and reactions, must develop alike ; while unlike¬ nesses of development must as unavoidably follow unlike¬ nesses among these agencies. And this being so, all the specialities cf symmetry and un symmetry and asymmetry which we have traced, are necessary consequences. PAHT y. PHYSIOLOGICAL DEVELOPMENT. CHAPTER T. THE PROBLEMS OF PHYSIOLOGY. § 265. The questions to be treated under the above title are widely different from those which it ordinarily expresses. We have no alternative, however, but to use Physiology in a sense co-extensive with that in which v e have used Morphology. "We must here consider the facts of function in a manner parallel to that in which we have, in the foregoing Part, considered the facts of structure. As, hitherto, we have concerned ourselves with those most general phenomena of organic form which, holding irre¬ spective of class and order and sub-kingdom, illustrate the processes of integration and differentiation characterizing Evolution in general ; so, now, we have to concern ourselves with the evidences of those differentiations and integrations of organic functions which have simultaneously arisen, and which similarly transcend the limits of zoological and botanical divisions. How heterogeneities of action have progressed along with heterogeneities of structure that is the inquiry before us ; and obviously, in pursuing it, all the specialities with which Physiology usually deals can serve us only as materials. Before entering on the study of Morphological Develop¬ ment, it was pointed out that while facts of structure may be empirically generalized apart from facts of function, they 222 PHYSIOLOGICAL DEVELOPMENT. cannot be rationally interpreted apart ; and throughout the foregoing pages this truth has been made abundantly mani¬ fest. Here we are obliged to recognize the inter-dependence still more distinctly ; for the phenomena of function cannot even be conceived without direct and perpetual consciousness of the phenomena of structure. Though the subject-matter of Physiology is as broadly distinguished from the subject- matter of Morphology as motion is from matter; yet, just as the laws of motion cannot be known apart from some matter moved, so there can be no knowledge of function without a knowledge of some structure as performing func¬ tion. Much more than this is obvious. The study of functions, considered from our present point of view as arising by Evolution, must be carried on mainly by the study of the cor¬ relative structures. Doubtless, by experimenting on the organ¬ isms that are growing and moving around us, we may ascertain the connexions existing among certain of their actions, while we have little or no knowledge of the special parts concerned in those actions. In a living animal that can be conveniently kept under observation, we may learn the way in which conspicuous functions vary together — how the rate of a man’s pulse increases with the amount of muscular exertion he is undergoing ; or how a horse’s rapidity of breathing is in part dependent on his speed. But though observations of this order are indispensable — though by accumulation and comparison of such obser¬ vations we learn •which parts perform which functions — though such observations, prosecuted so as to disclose the actions of all parts under all circumstances, constitute, when properly generalized and co-ordinated, what is com¬ monly understood as Physiology ; yet such observations help us but a little way towards learning how functions came to be established and specialized. We have next to no power of tracing up the genesis of a function considered purely as a function — no opportunity of observing the TI1E PROBLEMS OF PHYSIOLOGY, 22re 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 d}re 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 bo 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 INL'ER TISSUES OF PLANTS. 263 elongated. It does tliis for two reasons. That narrowing ol 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 Phasnogams. Omitting all other characteristics of those parts of them through which chiefl}7- 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 P There are various reasons for thinking that it does. In the first place, the simplest and earliest channels, such as we sec in the Liverworts, do not Vol. II. 12 264 PHYSIOLOGICAL DEVELOPMENT. develop in any systematic way, but branch out irregularly, following everywhere the irregular lobes of the frond as 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 O %/ o 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 Phasnogams 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 t race ; 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 I XX Ell TISSUES OF FLANTS. 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 Cactacecv, 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- shoot 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 26G PI1YSIOLOG I CAL D E VELOPM ENT. sections be made through, a growing bud of Opuntia or Cerens, 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 wrays 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 t}Tpes, since, in these, the dehi¬ scences 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 INN Eli TISSUES OF TLANTS. 2G7 surfaces, may consist either of successive rings, or continuous spiral threads, or networks, or structures between spirals and networks, or networks with openings 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 of 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. Were 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 by a layer of deposit. Hence this transformation of cells into sap-channels, is in part 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. It 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 PHYSIOLOGICAL DEVELOPMENT. 2 08 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 has 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 eventually 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 and reactions results in a balancing of the strains by the resistances ; wre 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 IX X Ell 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 these extended outer layers compress 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 it 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. in tlie 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 downwTards; and all which the mechanical actions do is to facilitate the transfer to the places of greatest demand. Hence 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 parts will still cause currents ; and there wrill 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, and, 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 Welwitschia mirabilis, 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. Stili, 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 PHYSIOLOGICAL DEVELOPMENT. 272 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 lavers 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-cliannels, 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 moire 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 arising1 in ways too involved to admit of r.pecific 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 Y. 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. But 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 Algce, 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 Algce, 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 Qther 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. l>ut O O 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 TN PLANTS. 277 dinary strains ; and in those assimilating organs we see elaborate appliances for yielding to the stem and roots the materials enabling them to fulhl 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 Bryophyllum. It is well known that the succulent leaves of B. calycinum, borne on foot-stalks 278 PHYSIOLOGICAL DEVELOPS! LIST so brittle that tliey 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. prolferum. 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. “ Linnmus, 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 Myceiales. This structural indefiniteness is accom- PHYSIOLOGICAL INTEGRATION IN PLANTS. 279 panied by functional indefiniteness. Though, 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. Alga, which are mostly aquatic, include many small forms that frequent the damp places piefeired Lichens and Fungi. Among Lichens, as among Fungi, there are kinds that lead submerged lives like the Algcc. vVhile 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 A/gce 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. Acroo-ens 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 onl}7- will the same species exhibit great diversity in the size, mode of branching, form and nervation of tho leaves, but the characters of even the peristome itself are not constant.” And concerning the classification of the remaining group, Filicales , he says: “Not only is tlicie 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 PHYSIO LOG I CA L DEVE LOPM E N T. of structure and habit and divisibility of groups among the higher plants, appear relatively marked. Though Ph sen ogams 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 arc 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,” says Dr. Ilooker, “and greater restriction of Faunas than Floras, should lead us to anticipate that plant types arc, geologically speaking, more ancient and permanent than the higher animal types are, and so I believe them to be, and I would extend the doctrine even to plants of highly complex structure.” “ Those classes and orders which are the least complex in organization are the most widely distributed.” § 286. Thus that which the general doctrine of evolution leads us to anticipate, we find implied by the facts. The physiological division of labour among parts, can go on only in proportion to the mutual dependence of parts ; and the mutual dependence of parts can progress only as fast as there arise structures by which the parts are efficiently combined, and the mutual utilization of their actions made easy. To say definitely by what process is brought about this co-ordination of functions which accompanies their specializa¬ tion, is hardly practicable. Direct and indirect equilibration doubtless co-operate in establishing it. We may see, for example, that every increase of fitness for function produced in the aerial part of a plant by light, as well as every increase of fitness for function produced in its imbedded part by the PHYSIOLOGICAL INTEGRATION IN PLANTS. 281 direct action of the moist earth, must conduce to an increased current of the liquid evaporated from the one and supplied bv the other — must serve, therefore, to aid the formation of sap-channels in the ways already described 5 that is must serve to develop the structures through which mutual aid of the parts is given the additional differentiation tends imme¬ diately to bring about the additional integration. Con¬ trariwise, it is obvious that an interdependence such as vc see between the secretion of honey and the fertilization of germs, or between the deposit of albumen in the cotyledons of an embryo-plant and the subsequent striking root, is a kind of integration in the actions of the individual or of the species, which no differentiation has a direct tendency to initiate. Hence we must regard the total results as due to a plexus of influences acting simultaneously on the individual and on the species : some chiefly affecting the one and some chiefly affecting the other. CHAPTER YI. DIFFERENTIATIONS BETWEEN THE OUTER AND INNER TISSUES OF ANIMALS. § 287. What was said respecting the primary physiological differentiation in plants, applies with little beyond change of terms to animals. Among Protozoa , as among Protop/ajta, the first definite contrast of parts that arises is that between outside and inside. The speck of jelly or sarcode which appears to constitute the simplest animal, proves, on closer examina¬ tion, to be a mass of substance containing a nucleus — a periplast in the midst of which there is a minute endoplast, consisting of a spherical membrane and its contents. This parallel, only just traceable among these Rhizopods, which are perpetually changing the distribution of their outer substance, becomes at once marked in those higher Protozoa which have fixed shapes, and maintain constant relations between their surfaces and their environments. Indeed the Rhizopods themselves, on passing into a state of quiescence in which the relations of outer and inner parts are fixed, become encysted : there is formed a hardened outer coat different from the matter w7hich it contains. And what is here a temporary character answering to a temporary definiteness of conditions, is in the Infusoria a constant character, answering definite conditions that are constant. Each of these minute creatures, though not coated by a dis¬ tinct membrane, has the outer layer of its sarcode indurated : the indurated substance being not separable from the sub¬ stance inclosed, but passing into it insensibly. the outer and inner tissues of animals. 283 8 288. The early establishment of this primary contrast of tissues answering to this primary contrast of conditions, is no less conspicuous in aggregates of the second order. The feebly-integrated units of a Sponge, with individualities so little merged in that of the whole they form that most of them still retain their separate activities, nevertheless show us, in the unlikeness that arises between the outermost layer and the contained mass, the effect of converse with unlike conditions. This outermost layer is composed of units some¬ what flattened and united into a continuous membrane— a kind of rudimentary skin. Secondary aggregates in which the lives of the units arc more subordinate to the life of the whole, carry this dis¬ tinction further. The leading physiological trait of every ccelenterate animal is the divisibility of its substance into endoderm and ectoderm— the part next the food and the part next the environment. Fig. 147, rudely representing a poi- tion of the body- wall of a Uyclra seen in section, gives some idea of this fundamental differentiation. The creature con¬ sists of a simple sac, the cavity of which is in direct commu¬ nication with the surrounding water ; and hence there is but little unlikeness between the outer and inner layers : indeed they are said to be capable of exchanging their functions. The essential contrast is that between the parts in contact with foreign substances and the parts sheltered from them— between the developed surfaces of the endoderm and ectoderm, and that intermediate stratum of nucleated sarcode fioin which the two grow in opposite directions. Between this case and tne case of the Sponge, w c may readily trace the connexion. Suppose a mass of A-iuoaba-^oriw units, the outermost of which are united into a layei analogous to that by which a living Sponge is covered, to be represented by a lump of plastic clay ; and for convenience of identifica¬ tion, suppose the surface of the clay to be coated by an extensible film, say of caoutchouc. Let this clay, so coated, be moulded into the shape of a cup; the cup be gradually 28-Jt PHYSIOLOGICAL DEVELOPMENT. deepened until it becomes jar-shaped ; and finally, narrow ing its neck, vase-sbaped. And conceive the stretched film to continue everywhere covering the surface during these changes of form. What will finally be the relations of the parts to one another ? The caoutchouc will line the inside of the vase as well as coat its outside. The vase will consist of a stratum of the clay included between the two India-rubber surfaces. We shall have a distribution of layers answering completely to the distribution of tissues in the Hydra. Now if we imagine that this artificial layer which has covered the clay during its changes of form, is produced by transforma¬ tion of the clay, we shall see that when the mass is changed into the vase-shape, the surfaces that have become outer and inner will develop in opposite directions from the substance lying between them ; just as do the Hydra’s ectoderm and endoderm. And if, once more, we conceive these outer and inner surfaces so resulting, to be affected by conditions some¬ what unlike* — the one by matters placed in the jar, and the other by the medium surrounding the jar — we shall have, in the slight difference produced between them, a difference corresponding to that between the surfaces of the Hydra’s stomach and skin. Besides being able thus to understand how an aggregate of Amccba- form units, originally coated by a single layer, may pass into an aggregate composed of a double layer ; we may also understand under what influences the transition takes place. If the habit which some of the primary aggre¬ gates have, of wrapping themselves round masses of nutri¬ ment, is followed by a secondary aggregate, there will naturally arise just that re- differentiation which the Hydra shows us. § 280. These duplicated surfaces which we see in every simple coelenterate animal, are re- duplicated in all animals of higher classes— -the more developed Ccelentcrata themselves showing us the transition. “ Compared with the Hydroid Polypes/' THE OUTER AND INNER TISSUES OF ANIMALS. 285 Bays Prof. Huxley, “ the higher forms are double animals, and a section of their bodies is, morphologically speaking, like a section of two Hydrce , one contained within the other. The relations of the parts may be illustrated thus Cut off the finger of a leather glove that has a lining ; and let the leather and the lining represent the ectoderm and ondodei in of a Hydra. Thrust the point of the glove-finger back into the cavity, until the introverted portion comes out beyond the open end. Cut off the projecting apex of the introverted portion level with the edges of the open end ; and then unite the edges of the introverted portion and the outer portion. The arrangement of structures will then typify that which is common to all animals except the Protozoa and the lower Cce ten ter at a : the introverted part representing the alimentary canal ; the outer part representing the body-wall ; and the closed cavity between the two representing the peri-visceral sac. This, however, is not the whole parallelism. If in the glove-finger, representing in its original form the Hydra, we suppose the leather standing for the ectoderm to be growing outwards, and the lining standing for the endolcrm to be growing inwards, then it in the part that is inti ovci ted the same relations of growth are maintained, it is manifest that of its two layers the one v’hich was outermost and is now innermost, will grow towards the open cavity which stands for the alimentary canal, while the other layer will grow towards the closed cavity standing lor the peri- visceral sac. And these are the directions of growffh actually found in the parts thus symbolized. This simile must not have more meaning given to it than is intended. Though there is reason for suspecting that a re- duplication has taken place in the course of evolution, and that the peri-visceral sac which distinguishes all the higher classes of animals from the lower, has been formed by it ; yet the method of re- duplication cannot have been anything like that described ; and has probably been so different a one as to negative the implied homologies of the layers. The ilius- 28G PHYSIOLOGICAL DEVEL( )P M E N T . tration is here used merely to convey, in a way easy to follow, an idea of the relations between outer and inner tissues, as they exist in the more complex animals. The two facts which we have to note are these : — First that, as Prof. Huxley points out in his essay on “ Tegumentary Organs,” the course of differentiation in the body- wall of the Hydra , is paralleled by the course of differentiation in the skin of every more complex animal up to the highest mammal. Between the epidermis and the derma there is a layer of indifferent tissue corresponding to the layer that lies between the endo- derm and ectoderm of the Hydra ; and from this layer, as from its homologue, the differentiations proceed in opposite directions. Though the resulting two layers, exposed to more unlike conditions than those of the Hydra , are more unlike one another, yet we see in them essentially the same course of metamorphosis and the same subordination of it to the relations of outside and inside. In the second place, we have to note that the wall of the alimentary canal, though it is in one sense internal by contrast with the skin as external, and is correspondingly differentiated from the skin, is in another sense like the skin, in having one surface in contact with foreign substances (presented as food) and the other surface in contact with the living substance of the body ; and that consequently it undergoes, like the skin, a differentia¬ tion into two layers, one growing towards the relatively external or food- containing cavity, and the other towards the rigorously internal cavity — the closed peri- visceral sac. § 290. Whether direct equilibration or indirect equilibra¬ tion has had the greater share in producing this universally- present contrast between the inner and outer tissues of animals, must be left undecided. The two causes have all along co-operated — modification of the individual accumu¬ lated by inheritance predominating in some cases, and in other cases modification of the race by survival of the inci¬ dentally fittest. On the one hand, the action of the medium THE OUTER AND INNER TISSUES OF ANIMALS. 287 on the organism cannot fail to change its surface more than its centre, and so differentiate the two ; while on the other hand, the surfaces of organisms inhabiting the same medium display extreme unlikenesses which cannot be due to the immediate actions of their medium. Let us dwell a moment on the antithesis. We have abundant evidence that animal protoplasm is rapidly modified by light, heat, air, water, and the salts contained in water — coagulated, turned from soluble into in¬ soluble, partially changed into isomeric compounds, or other¬ wise chemically altered. Immediate metamorphoses of this kind are often obviously produced in ova by changes of their media. At the outset, therefore, before yet there existed any such differentiation as that which now usually arises by inheritance, these environing agencies must have tended to originate a protective envelope. For a modification produced by them on the superficial part of the protoplasm, must either have been a decomposition or else the formation of a compound that remained stable under their subsequent action. There would be generated an outer layer of substance that was so molecularly immobile as to be incapable of further metamorphoses, while it would shield the contained proto¬ plasm from that too great action of external forces which, by rapidly changing the unstable equilibrium of its molecules into a relatively stable equilibrium, would arrest development. Evidently organic evolution, whether individual or general, must always and everywhere have been subordinate to these physical necessities. Though natural selection, beginning with minute portions of protoplasm, must all along have tended to establish a molecular composition apt to undergo this differentiation of surface from centre to the most favour¬ able extent ; yet it must all along have done so while con¬ trolled by this process of direct equilibration. Contrariwise, the many and great unlikenesses among the dermal structures of creatures inhabiting the same element, cannot be ascribed to any such cause. The contrasts between Vol. II. 13 2SS PHYSIOLOGICAL DEVELOPMENT. naked and shelled Gastropods, between marine Worms and Crustaceans, between soft-skinned Fish and Fish in armour like the PtcriclhySy must have been produced entirely by natural selection. Environing forces are, as before, the ultimate causes ; but the forces are now not so much those exercised by the medium as those exercised by the other inhabitants of the medium ; and they do not act by modifying the surface of the individual, but by killing off individuals whose surfaces are least fitted to the requirements : thus slowly affecting the species. The dermal skeleton bristling with spines, which protects the Diodon or the Cyclicthys from enemies it could not escape, still comes within the general formula of an outer tissue differentiated from inner tissues by the outer actions to which the creature is exposed — the differentiation having gone on until there is equilibrium between the destructive forces to be met and the protective forces which meet them. If we venture to apportion the respective shares which mediate and immediate actions have had in differentiating outer from inner tissues, we shall probably not be far wrong in ascribing that part of the process which is alike in all animals, mainly to the direct actions of their media ; while we ascribe the multitudinous unlikenesses of the process in various animals, partly to the indirect actions of the media, and partly to the indirect actions of other animals by which the media are inhabited. That is to say, while assigning the specialities of the differentiations to the specialities of con¬ verse with the agencies in the environment, most of them organic, we may assign to the constant and universal con¬ verse with its inorganic agencies, that universal characteristic of tegumentary structures — their development into a double layer separated by undifferentiated substance, from which the outermost grows outwardly and the innermost grows in¬ wardly. Here let me add a piece of evidence which strengthens very greatly the general argument, at the same time that it justifies this apportionment. When ulceration has gone deep THE OUTER AND INNER TISSUES OF ANIMALS. 289 enough to destroy the tegumentary structures, these are never reproduced. The puckered surface formed where an ulcer heals, consists of modified connective tissue, which, as the healing goes on, spreads inwards from the edges of the ulcer — some of it, perhaps, growing from the portions of connective tissue that dip down between the muscular bundles. This connective tissue, mark, out of which is thus constituted the make-shift skin, is normally covered by both the epidermis and that stratum of indifferent tissue from which the growth proceeds in opposite directions — is the inner layer that grows inwardly. AVhat has happened to it P It has now become the outermost layer. And how does it comport itself under its new conditions P It produces a layer that plays the part of epidermis and grows outwardly. For since the surface, subject to friction and exfoliation, has to be continually renewed, there must be a continual reproduction of a super¬ ficial layer from a layer beneath. That is to say, the con¬ tact of this deep-seated tissue with outer agencies, produces in it some approach towards that composition which we find universally characterizes outer-tissue — a protomorphic layer, which differentiates in opposite directions. But while we see under this exposure to the conditions common to all integu¬ ment, a tendency to assume the structure common to all integument, we see no tendency to assume any of the specialities of tegumentary structure : no rudiments of glands or hair sacs make their appearance. This apportionment we shall see the more reason to accept as approximately expressing the truth, on remembering that the mode of differentiation of outer from inner tissues which is common to all animals is common to all plants ; and on observing, further, that the more special interpretation suggested as not improbable in the case of plants, is not improbable in the case of animals. For as it was argued that in plants the forces evolved from within the organism, and the forces falling on it from without, must have some place between centre and surface at which they balance ; and 290 PHYSIOLOGICAL DEVELOPMENT. that at this place will lie the unstable protoplasm that develops outwardly into a substance which is stable in face of outer forces, and inwardly into a substance which is stable in face of inner forces ; so in animals, we may regard this universally-present layer whence epidermis grows outwardly and connective tissue inwardly, as similarly the place of equilibrium between these antagonist forces. And for this a priori interpretation we may indeed, among animals, find a posteriori warrant. We have but to increase the mechanical action or chemical irritation at some part of an animal’s surface, to make this plane of indifferent tissue retreat in¬ wardly ; for to say that the epidermis becomes thicker, is, in mechanical terms, to say that the place of equilibrium between outer and inner forces is further from the surface. CHAPTER VII. DIFFERENTIATIONS AMONG THE OUTER TISSUES OF ANIMALS. § 291. Tbe outer tissues of animals, originally homo¬ geneous over their whole surfaces, pass into a heterogeneity which fits their respective parts to their respective conditions. So numerous and varied are the implied differentiations, that it is impracticable here to deal with them all even in outline. To trace them up through classes of animals of increasing degrees of aggregation, would carry us into undue detail. Did space permit, it would he possible to point out among the Protozoa , various cases analogous to that of the Arcella; which may be described as like a microscopic Limpet, having a sarcode body of which the upper surface has become horny, while the lower surface with its protruding pseudopodia, retains the primitive jelly-like character. That differentia¬ tions of this kind have been gradually established among these minute creatures through the unlike relations of their parts to the environment, is an inference supported by cases like that of Pamphagus — an intermediate form which is like the Amoeba in having no carapace, but “ agrees with Arcella and Difflugia in having the pseudopodia protrusible from one extremity only of the body.” Many parallel specializations of surface among aggregates of the second order might be instanced from the Coelenterata. In the Hydra , the ectoderm presents over its whole area no conspicuous unlikenesses ; but there usually exist in the hydroid polypes of superior t}rpes, decided contrasts between 292 PHYSIOLOGICAL DEVELOPMENT. tlie higher and lower parts. While the higher parts retain their original characters, the lower parts excrete hard outer layers yielding support and protection. Various stages of the differentiation might be followed. “ In Hydractinia” says Prof. Green, this horny layer “ becomes elevated at intervals to form numerous rough processes or spines, while over the general surface of the ectoderm its presence is almost imperceptible.” In other types, as in Corclylophora , it spreads part way up the animal’s sides, ending indefinitely. In Bimeria it “ extends itself so as to enclose the entire body of each polypite, leaving bare only the mouth and tips of the tentacles.” While in Campanularia it has become a partially- detached outer cell, into which the creature can retract its exposed parts. But it is as needless as it would be wearisome to trace through the several sub-kingdoms the rise of these multiform contrasts, with the view of seeking interpretations of them. It will suffice if we take a few groups of the illustrations furnished by the higher animals. § 292. We may begin with those modifications of surface which subserve respiration. Though we ordinarily think of respiration as the quite special function of a quite special organ, yet originally it is not so. Little- developed animals part with their carbonic acid and absorb oxygen, through the general surface of the body. Even in the lower types of the higher classes, the general surface of the body aids largely in aerating the blood ; and the parts that discharge the greater part of this function are substantially nothing more than slightly altered and extended portions of the skin. Such differentiations, marked in various degrees, are to be seen among Mollusca. In the Ptcropoda the only modification which appears to facilitate respiration, is the minute vascularity of one part of the skin. In other types the specialized parts facilitating the exchange of gases, are those simple but numerous expansions of surface constituting the papilla; ; TILE OUTER TISSUES OF ANIMALS. 2(Jo which, in the Eolis and kinds allied to it, are distributed in rows or clusters all along the back. Instead of these, the Doris has appendages developed into elaborately-branched forms — small trees of blood-vessels covered by slightly- changed dermal tissues. And these arborescent branchiae are gathered together into a single cluster. Thus there is evidence that large external respiratory organs have arisen by degrees from simple skin : as, indeed, they do arise during the development of each individual having them. Just as gradually as in the embryo the simple bud on the integu¬ ment, with its contained vascular loop, passes by secondary buddings into a tree-like growth penetrated everywhere by dividing and sub-dividing blood-vessels ; so gradually has there probably proceeded the differentiation which has turned part of the outer surface into an organ for excreting carbonic acid and absorbing oxygen. Certain inferior vertebrate animals present us with a like metamorphosis of tissues. These are the Amphibia. The branchiae here developed from the skin are covered with cel¬ lular epidermis, not much thinner than that covering the rest of the body. Like it they have their surfaces speckled with pigment-cells ; and are not even conspicuous by their extra vascularity — where they are temporary at least. They facili¬ tate the exchange of gases in scarcely any other way than by affording a larger area of contact with the water, and inter- posing a rather thinner layer of tissue between the water and the blood-vessels. Those very simple branchim of the larval Amphibia that have them but for a short time, graduate into the more complex ones of those that have them for a long time or permanently ; showing, as before, the small stages by which this heterogeneity of surface accompanying heterogeneity of function may arise. In what way are such differentiations established? Partly, no doubt, by natural selection ; but also to some degree, I think, by the inheritance of direct adaptations. That a por¬ tion of the integument at which aeration is favoured by local 294 PHYSIOLOGICAL DEVELOPMENT. conditions, should thereby he led to "row into a larger surface of aeration, appears improbable : survival of those individuals which happen to have this portion of the integu¬ ment somewhat more developed, seems here the only likely- cause. Nevertheless there is reason for suspecting that respiratory activity itself aids in the development of a re¬ spiratory appendage. The reason is this. Exchange of liquids through membrane depends on some difference, physical or chemical, between the liquids : if they are in all respects alike, and under equal pressures, no exchange will take place ; while, conversely, if they are much unlike there will be a rapid exchange. Now through the walls of capillaries, or through the sides of lacunae not yet developed into capillaries, there continually goes on an oozing both ways — from the blood into the tissues and from the tissues into the blood. By this double movement nutrition and depuration are alike made possible ; and it is obvious both that in the absence of difference it would not occur, and that nothing would be gained if it did occur. Among other differences continually arising between the intra-vascular liquid and the extra- vascular liquid, is that due to their unlike charges of oxygen and carbonic acid. This difference, like other differ¬ ences, will cause exchange — the rapidity of the exchange doubtless being greater where the difference is greater. Hence if any part of an aquatic animal’s skin is nearest to the place where the blood has become most highly carbonized, or if it is so bathed with moving water that the plasma beneath its surface is more oxygenated than elsewhere, or both ; then, other things equal, this part of the skin will be the seat of an osmotic movement greater than goes on in the rest of the skin. But the exchange of ox3’-gen for carbonic acid, proceeding faster here than elsewhere, will have for its accompaniment a more rapid exudation of nutritive matters. The liquid passing out of the blood-vessels to be replaced by the liquid passing into them, is a liquid containing the substances that build up the surrounding tissues. Hence THE OUTER TISSUES OF ANIMALS. 295 these tissues may be expected to grow : the area supplied by the increased currents of blood set up by this exchange, will become protuberant — will bud out ; and the bud so formed will give origin to secondary buds at those parts of its surface which, as before, are most favourably circumstanced for carrying on the aeration. Of course this process will be checked where, though otherwise advantageously placed, the growing branchiae would be specially liable to damage, or would be great hindrances to the creature’s movements. But bearing in mind that functionally-produced adaptation will here, as in other cases, be both aided and controlled by natural selection, we may ascribe to it an important share, if not a leading share, in the differentiation. § 293. Among the conspicuous modifications by which the originally-uniform outer layer is rendered multiform, are the protective structures. Let us look first at the few cases in which the formation of these is ascribable mainly to direct equilibration. Already reference has been more than once made to those thickenings that occur where the skin is exposed to unusual pressure and friction. Are these adaptations inheritable? and may they, by accumulation through many generations, produce permanent dermal structures fitted to permanent or frequently-recurring stress ? Taking, for instance, the cal¬ losities on the knuckles of the Gorilla , which are adapted to its habit of partially supporting itself on its closed hands when moving along the ground — shall we suppose that these defensive thickenings are produced afresh in each individual by the direct actions ; or that they are inherited modifica¬ tions caused by such direct actions ; or that they are wholly due to the natural selection of spontaneous variations ? The last supposition does not seem a probable one ; since it implies that those slight extra thicknesses of skin on the knuckles, with which we must suppose the selection to have commenced, were so advantageous as to cause survivals of the 296 PHYSIOLOGICAL DEVELOPMENT. individuals having them. That survivals so caused, if they ever occurred at all, should have occurred with the frequency requisite to establish and increase the variation, is hardly supposable. And if wre reject, as also unlikely, the repro¬ duction of these callosities de novo in each individual, there remains only the inference that they have arisen by the transmission and accumulation of functional adapta¬ tions. Another case wrhich seems interpretable only in an analogous way, is that of the spurs that are developed on the wings of certain birds — on those of the Chaja screamer for example. These are weapons of offence and defence. It is a familiar fact that many birds strike with their wings, often giving severe blows ; and in the birds named, the blows are made more formidable by the horny, dagger-shaped growths standing out from those points on the wings wrhich deliver them. Are these spurs directly or indirectly adaptive ? To conclude that natural selection of spontaneous variations has caused them, is to conclude that, without any local stimulus, thickenings of the skin occurred symmetrically on the two wdngs at the places required ; that such thickenings, so localized, happened to arise in birds given to using their wings in fight ; and that on their first appearance the thickenings were decided enough to give appreciable advantages to the individuals distinguished by them — advantages in bearing the reactions of the blows if not in inflicting the blows. But to conclude this is, I think, to conclude against probability. Contrariwise, if we assume that the thickening of the epidermis produced by habitual rough usage is inheritable, the development of these struc¬ tures presents no difficulty. The points of impact would become indurated in wings used for striking with unusual frequency. The callosities of surface thus generated, render¬ ing the parts less sensitive, would enable the bird in which, they arose to give, without injury to itself, more violent blows and a greater number of them — so, in some cases, helping it to conquer and survive. Among its descendants, inheriting 297 THE OUTER TISSUES OF ANIMALS. the modification and the accompanying habit, the thickening would be further increased in the same way — survival of the fittest tending ever to accelerate the process. Presently the horny nodes so formed, hitherto defensive only in their effects, would, by their prominence, become offensive — would make the blows given more hurtful. And now natural selection, aiding more actively, would mould the nodes into spurs : the individuals in which the nodes were most pointed would be apt to survive and propagate ; and the pointedness generation after generation thus increased, would end in the well-adapted shape we see. But if in these cases the differentiations which fit particular parts of the outer tissues to bear rough usage, are caused mainly by the direct balancing of external actions by in¬ ternal reactions, then we may suspect that the like is true of other modifications that occur where special strains and abrasions have to be met. Possibly it is true of sundry parts that are formed of hardened epidermis, such as the nails, claws, hoofs, and hollow horns of Mammals ; “ all of which,” says Prof. Huxley, “ are constructed on essentially the same plan, being diverticula of the whole integument, the outer layer of whose eederon has undergone horny metamorphosis.” Leaving open, however, the question what tegumentary structures are due to direct equilibration, furthered and con¬ trolled by indirect equilibration, it is tolerably clear that direct equilibration has been one of the factors. IIow has it produced its effects ? that is to say — by what physical processes do pressure and friction bring about dermal hardenings ? To this inquiry there is an answer similar to that which was given to the inquiry respecting the formation of wood. (§ 280-2.) As in plants we saw that intermittent compressions of sap- canals increase the exudation of sap, and thus cause increased deposits of its contained substances in the surrounding tissues ; so in animals, we have good reason for concluding that intermittent compressions of the capil¬ laries increase the exudation of serum, and by thus supplying 298 PHYSIOLOGICAL DEVELOPMENT. extra nutriment to the structures adjacent, lead, other things equal, to thickening or induration. The data for the con¬ clusion are these : — Through the walls of the capillaries the liquid plasma of the blood continually oozes. The - oozing is partly osmotic and partly mechanical — partly due, that is, to the exchange of the unlike liquids that lie inside and out¬ side the capillaries, and partly to the greater pressure put upon the liquid inside. That this last is one of the causes is proved by the phenomena of dropsy — a disease in which the exudation is unduly rapid. Dropsy in the legs gets worse during the day, when by sitting and standing the weight of the blood to be borne by the vessels of the legs is increased ; and gets better during the night, when by the recumbent attitude these vessels are relieved from this weight. Contrariwise, that cedematous swelling under the eyes which is common in the aged and debilitated, increases during the night and decreases during the day — gravitation serving, when the body is upright, to diminish the pressure of the blood at this part, and not having this effect when the body is horizontal. But if the plasma is to some extent forced through the walls of the capillaries by pressure, then not only will the action of the heart, aided at some parts by gravity, further the exudation, but the exudation will be furthered by external pressures from time to time falling on the capillaries. If the capillaries of the skin be squeezed by the thrust of some object against the surface, part of their contained blood will be driven back into the arteries, more will be driven forwards into the veins, and some will be made to exude. Immediately they are relieved from the pressure they will be refilled from the arteries, again to yield an extra portion of their contents to the tissues around when again squeezed. Thus recurrent thrusts or impacts, acting on the body from without, aid in the nutrition of the parts on which they fall : producing, in some cases, a node upon the subjacent bone, as on the instep where a boot has pinched ; producing, in other cases, growth of the connective tissue, as in a bunion ; THE OUTER TISSUES OF ANIMALS. 299 And producing, more frequently, thickening of the epidermis.* It is no doubt true that the sensation which pressure causes, propagated to the spinal chord, and reflected thence through the vaso-motor nerve going to the spot, aids the process by exciting a wave of contraction along the minute arteries, thereby helping them to refill the capillaries the instant the pressure is taken off ; and doubtless, as alleged, the excessive exudation that forms a blister when the intermittent com¬ pressions are violent and long-continued, is attributable to this reflex nervous action. But it is clear that the nervous action is secondary, and cannot of itself produce the effect ; for in the absence of intermittent pressure no exudation takes place, however acute and persistent the sensation may be. Continued pressure produces absorption instead of exudation. In animals therefore, as in plants, the external mechanical actions to be resisted, are themselves directly instrumental in working in the tissues they fall upon, the changes which fit those tissues to meet them. And it needs but to contemplate the process of thickening described, to see that it will go on until the shield produced suffices to protect the capillaries from excessive pressures — will go on, that is, until there is equilibrium between the outer and inner forces. § 294. Dermal structures of another class are developed mainly, if not wholly, by the actions of external causes on species rather than on individuals. These are the * An inquiry into the causes of these differences of result, brings further evidence to light. The condition under which only the hypertrophy can arise, is that the pressure intermits sufficiently to allow the capillaries to refill frequently. The epidermis thickens where the pressures are habitually taken off so completely, that the capillaries next the surface can refill, as in the hands. If we consider what happens where the instep is pressed by a tight boot, we shall see that the variations of pressure which occur in walking, do not suffice to relieve the quite superficial vessels and allow them to refill ; but in consequence of the slight mobility and elasticity of the tissues, the vessels at some distance beneath the surface are able to refill, and hence the thicken* ing occurs round them. coo PHYSIOLOGICAL DEVELOPMENT. various kinds of clothing — hairs, feathers, quills, scale*, scutes. Headers who are unfamiliar with the extreme modifiability of organic structures, will be startled by the proposition that all of these — certainly all of them but the last, respecting which there may be doubts — are homologous parts. In¬ spection of a few cases makes this seemingly-incredible pro¬ position not simply credible but obviously true. A retrograde metamorphosis from feathers to appendages that are almost scale-like, is well seen in the coat of the Penguin. Carry the eye along the surface of one of these birds, and there is manifest a transition from the bird-like covering to the fish¬ like covering — a transition so gradual that no place can be found where an appreciable break occurs. Less striking perhaps, but scarcely less significant, are the modifications through which we pass from feathers to hairs, on the surfaces of the Ostrich and the Cassowary. The skin of the Porcupine shows us hairs and quills united by a series of intermediate structures, differing from one another inappreciably. Even more remarkable is the extension of this alliance to certain other dermal structures. “It may be taken as certain, I think,” says Prof. Huxley, “ that the scales, plates, and spines of all fishes are homologous organs ; nor as less so that the tegumentary spines of the Plagiostomes are homo¬ logous with their teeth, and thence with the teeth of all vertebrata. Again, it appears to me indubitable that the teeth and the hairs are homologous organs.” o o « The ultimate justification for classing these unlike parts as divergent modifications of the same thing, is the unity in their modes of development. Besides a linking together of them by intermediate structures, as above indicated, there is a linking together by their common origin. To quote again from Prof. Huxley’s essay on “Tegumentary Organs”: — “ The Hairs and Spines of mammals, the Feathers of birds, and the Integumentary Glands , agree in one essential point, that their development is preceded by that of an involution THE OUTER TISSUES OF ANIMALS. 301 of the ecderon, within which they are formed, and by which the former are, at first, entirely enclosed.” And though the scales of fishes and the dermal plates of reptiles present diffi¬ culties, yet Prof. Huxley concludes that the course of their development is at first essentially the same. Some idea of it, and of the relations it proves among these structures, may be given thus : — Suppose a small pit to be formed on the pre¬ viously flat skin ; and suppose that the growth and casting off of horny cells which goes on over the skin in general, continues to go on at the usual rate over the depressed surface of this pit. Clearly the quantity of horny matter produced within this hollow, will be greater than that produced on a level portion of the skin subtending an equal area of the animal’s outside. Suppose such a pit to be deepened until it becomes a small sac. If the exfoliation goes on as before, the result will be that the horny matter, expelled, as it must be, through the mouth of the sac, which now bears a small proportion to the internal surface of the sac, will be large in quantity compared with that exfoliated from a portion of the skin equal in area to the mouth of the sac : there will be a conspicuous thrusting forth of horny matter. Suppose once more that the sac, instead of remaining simple, has its bottom pushed up into its interior, like the bottom of a beer-bottle — the introversion being carried so far that the introverted part reaches nearly to the external opening, and leaves scarcely any space between the introverted part and the walls of the sac. It is easy to see that the exfoliation continuing from the surface of the introverted part as well as from the inside of the sac generally, the horny matter cast off will form a double layer ; and will come out of the sac in the shape of a tube having within its lower end the intro¬ verted part, as the core on which it is moulded, and from the apex of which is cast off the substance filling, less densely, its interior. The structure resulting will be what we know as a hair. Manifestly by progressive enlargement of the sac, and further complication of that introverted part on which 302 PHYSIOLOGICAL DEVELOPMENT. the excreted substance is moulded, the protruding growth may be rendered larger and more involved, as we see it in quills and feathers. So that insensible steps, thus indicated in principle, carry us from the exfoliation of epidermis by a fiat surface, to the exfoliation of it by a hollow simple sac, an introverted sac, and a sac further complicated ; each of which produces its modified kind of tegumentary appendage. § 295. Among many other differentiations of the outer tissues, the most worthy to be noticed in the space that re¬ mains, are those by which organs of sense are formed. We will begin with the simplest and most closely allied to the foregoing. O D Every hair that is not too long or flexible to convey to its rooted end a strain put upon its free end, is a rudimentary tactual organ ; as may be readily proved by touching one of those growing on the back of the hand. If, then, a creature has certain hairs so placed that they are habitually touched by the objects with which it deals, or amid wdiich it moves, an advantage is likely to accrue if these hairs are modified in a way that enables them the better to transmit the im¬ pressions derived. Such modified hairs w7e have in the vibrissce , or, as they are commonly called, the “ whiskers ” possessed by Cats and feline animals generally, as well as by Seals and many Eodents. These hairs are long enough to reach objects at considerable distances ; they are so stiff that forces applied to their free ends, cause movements of their imbedded ends ; and the sacs containing their imbedded ends being well covered with nerve-fibres, these developed hairs serve as instruments of exploration. By constant use of them the animal learns to judge of the relative positions of objects past which, or towards which, it is moving. When stealthily approaching prey or stealthily escaping enemies, such aids to perception are obviously important : indeed their importance has been proved by the diminished power of self- guidance in the dark, that results from cutting them off. These, then, are THE OUTER TISSUES OF ANIMALS. 303 dermal appendages originally serving the purpose of cloth¬ ing, but afterwards differentiated into sense-organs. That eyes are essentially dermal structures seems scarcely conceivable. Yet an examination of their rudimentary types, and of their genesis in creatures that have them well deve¬ loped, shows us that they really arise by successive modifica¬ tions of the double layer composing the integument. They make their first appearance among the simpler animals as specks of pigment, covered by portions of epidermis slightly convex and a little more transparent than that around it. Here their fundamental community of structure with the skin is easy to trace ; and the formation of them by differen¬ tiation of it presents no difficulty. Not so far in advance of these as much to obscure the relationship, are the eyes which the Crustaceans possess. In every fish¬ monger’s shop we may see that the eyes of a Lobster are carried on pedicles ; and when the Lobster casts its shell, the outer coat of each eye, being continuous with the epidermis of its pedicle, is thrown off along with the rest of the exo¬ skeleton. This pedicle, which gives the name of “ stalk¬ eyed ” Crustacea to a large group, is, strange as it may seem, a transformed limb. Otherwise shown by the homologies of the parts, this truth is made manifest by those transitional cases in which the original form of the limb is retained, and tho transparent portion which serves as a visual organ i3 merely a prominent patch on its under surface, somewhat like a blister, spreading a little up the sides of the limb an arrangement almost thrusting upon us the suspicion that an eye is a modified portion of the skin. That which the outer appeal - ance suggests is proved by the structure within. Beneath the transparent epidermic layer, there exists a group of eyes of the kind which we see in an insect ; and these, accoiding to a high authority, are inclosed in the dermal system. De¬ scribing the arrangement of the parts, M. Milne Edwards writes v — u But the most remarkable circumstance is, that the large cavity within which the whole of these parallel 304 rilYSIOLOGICAL DEVELOPMENT. columns, every one of which is itself a perfect eye, are contained, is closed posteriorly by a membrane, which appears to be neither more nor less than the middle tegumentary membrane, pierced for the passage of the optic nerve ; so that the ocular chamber at large results from the separation at a point of the two external layers of the general envelope.” Thus too is it, in the main, even with the highly-developed eyes of the Vertebrata. “ The three pairs of sensory organs apper¬ taining to the higher senses,” says Prof. Huxley — “ the nasal sacs, the eyes, and the ears — arise as simple coecal involutions of the external integument of the head of the embryo. That such is the case, so far as the olfactory sacs are con¬ cerned, is obvious, and it is not difficult to observe that the lens and the anterior chamber of the eye are produced in a perfectly similar manner. It is not so easy to see that the the labyrinth of the ear arises in this way, as the sac resulting from the involution of the integument is small, and remains open but a very short time. But I have so frequently veri¬ fied Huschke’s and Bemak’s statement that it does so arise, that I entertain no doubt whatever of the fact. The outer ends of the olfactory sacs remain open, but those of the ocular and auditory sacs rapidly close up, and shut off their contents from all direct communication with the exterior.” So that, marvellous as the fact appears, all that part of the eye which lies between its outer surface and the back of the crystalline lens, is formed in the same way as an ordinary hair- sac, and is composed of homologous parts. The interior coat is the epidermic layer, originally continuous with the surface of the skin ; and only made discontinuous with it by closure of the sac at the point which is afterwards the centre of the cornea. This cornea, or front wall of the chamber thus shut off, is consequently composed of a doubled epidermic layer and an intermediate layer of the derma included in the fold of the integument. The crystalline lens, lying at the far side of this chamber, is simply a thickening of the epidermic layer THE OUTER TISSUES OF ANIMALS. 305 lining that part of the chamber — is developed from it in the same way that the substance of a hair is developed from the papilla at the bottom of its sac. The iris originates as an annular thrusting-in of the walls of this chamber in front of the crystalline lens ; and between the two layers of the epi¬ dermic lining, thus folded, comes a portion of the derma in which muscular fibres eventually arise. Though the founda¬ tion of the part behind the crystalline lens is laid by a hollow diverticulum from the brain, which grows outwards to meet the inward-growing tegumentary sac, yet here, too, structures be¬ longing to the tegumentary system eventually predominate. For into this cul-de-sac proceeding from the nervous centre, there takes place a lateral growth of dermal tissue, which, in¬ troverting the wall of the sac, and presently filling the whole cavity of it, is at last shut off by the closure of the now doubled walls of the sac ; and out of this intruding mass of dermal tissue the vitreous humour is formed. That is to say, the eye considered as an optical apparatus is wholly produced by metamorphoses of the skin : the only parts of it not thus produced, being the membranes lying between the sclerotic and the vitreous humour, including those retinal structures formed in them. All is tegumentary save that which has to appreciate the impressions which the modified integument concentrates upon it. Thus, as Prof. Huxley has somewhere pointed out, there is a substantial parallelism between all the sensory organs in their modes of development : as there is, too, between their modes of action. A vibrissa may be taken as their common tvpe. Increased impressibility by- an external stimulus, requires an increased peripheral expansion of the nervous system on which the stimulus may fall ; and this is secured by an introvertion of the integument, forming a sac on the walls of which a nerve may ramify. That the more extended sensory area thus constituted may be acted upon, there requires some apparatus conveying to it from without the appropriate stimulus ; and in the case of the vibrissa, this 306 PHYSIOLOGICAL DEVELOPMENT. apparatus is the epidermic growth which, under the form of a hair, protrudes from the sac. And that the greatest sensitiveness may be obtained, the external action must be exaggerated or multiplied by the apparatus which conveys it to the recipient nerve ; as in the case of the vibrissa, it is by the development of a hair into an elastic lever, that trans¬ forms the slight force acting through considerable space on its exposed end, into a greater force acting through a smaller space at its rooted end. Similarly with the organs of the higher senses. In a rudimentary eye, we have but a slight peripheral expansion of a nerve to take cognizance of the impression ; and to concentrate the impression upon it, there is nothing beyond a thickening of the epidermis into a lens- shape. But the developed eye shows us a termination of the nerve greatly expanded and divided to receive the external stimulus. It shows us an introverted portion of the integu¬ ment containing the apparatus by which the external stimulus is conveyed to the recipient nerve. The structure developed in this sac not only conveys the stimulus, but also, like its homologue, concentrates it; and in the one case as in the other, the structure which does this is an epidermic growth from the bottom of the sac. Even with the ear it is the same. Again we have an introverted portion of the integument, on the walls of which the nerve is distributed. The otolithes contained in the sac thus formed, are bodies which are set in motion by the vibrations of the surrounding medium, and convey these vibrations in an exaggerated form to the nerves. And though it is not alleged that these otolithes are developed from the epidermic lining of the chamber, yet as, if not so developed, they are concretions from the contents of an epidermic sac, they must still be regarded as epidermic products. Whether these differentiations are due wholly to indirect equilibration, or whether direct equilibration has had a share in working them, are questions that must be left open. Possibly a short hair so placed on a mammal’s face as to be THE OUTER TISSUES OF ANIMALS. 307 habitually touched, may, by conveying excitations to the nerves and vessels at its root, cause extra growth of the bulb and its appendages, and so the development of a vibrissa may be furthered. Possibly too, the light itself, to which the tissues of some inferior animals are everywhere sensitive, may aid in setting up certain of the modifications by which the nervous parts of visual organs are formed — producing, as it must, the most powerful effects at those points on the surface which the movements of the animal expose to the greatest and most frecpient contrasts of light and shade , and propa¬ gating from those points currents of molecular change through the organism. Put it seems clear that the complexities of the sensory organs are not thus explicable. They must have arisen by the natural selection of favourable variations. § 296. A group of facts, serving to elucidate those put together in the several foregoing sections, has to be added. I have reserved this group to the last, partly because it is transitional — links the differentiations of the literally outer tissues with those of the truly inner tissues. Though physi¬ cally internal, the mucous coat of the alimentary canal has a g«flsi-externality from a physiological point of view. . As was pointed out in the last chapter, the skin and the assimi¬ lating surface have this in common, that they come in direct contact with matters not belonging to the organism ; and we saw that along with this community of relation to alien substances, there is a certain community of structure and de¬ velopment. The like holds with the linings of all internal cavities and canals that have external openings. The transition from the literally outer tissues to those tissues that are intermediate between them and the truly inner tissues, is visible at all the orifices of the body ; where skin and mucous membrane are continuous, and the one passes insensibly into the other. This visible continuity is not simply associated with a great degree of morphological continuity, but also with a great degree of physiological con- 308 PHYSIOLOGICAL DEVELOPMENT. tinuity. That is to sa}r, these literally outer and quasi-outer layers are capable of rapidly assuming one another’s struc¬ tures and functions when subject to one another’s conditions. Mucous surfaces, normally kept covered, become skin-like if exposed to the air ; but resume more or less fully their normal characters when restored to their normal positions. These are truths familiar to pathologists. They continually meet with proofs that permanent eversion of the mucous membrane, even where it is by prolapse of a part deeply seated within the body, is followed by an adaptation eventu¬ ally almost complete : originally moist, tender to the touch, and irritated by the air, the surface gradually becomes covered with a thick, dry cuticle ; and is then scarcely more sensitive than ordinary integument. Whether this equilibration between new outer forces and reactive inner forces, which is thus directly produced in in¬ dividuals, is similarly produced in races, must remain as a question not to be answered in a positive way. On the one hand, we have the fact that among the higher animals there are cases of quasi- outer tissues which are in one species habitually ensheathed, while in another species they are not ensheathed ; and that these two tissues, though unquestion¬ ably homologous, differ as much as skin and mucous mem¬ brane differ. On the other hand, there are certain analogous changes of surface, as on the abdomen of the Hermit-Crab, which give warrant to the supposition that survival of the fittest is the chief agent in establishing such differentiations ; since the abdomen of a Hermit-Crab, bathed by water within the shell it occupies, is not exposed to phjMcal conditions that directly tend to differentiate its surface from the surface of the thorax. But though in cases like this last, we must assign the result to the natural selection of variations arising incidentally ; we may I think legitimately assign the result to the immediate action of changed conditions where, as in cases like the first, we see these producing in the individual, effects of the kinds observed in the race. THE OUTER TISSUES OF ANIMALS. 309 However this may be, the force of the general argument remains the same. In these exchanges of structure and function between the outer and quasi- outer tissues, we get undeniable proof that they are easily differentiable. And seeing this, we are enabled the more clearly to see how there have, in course of time, arisen those extreme and multi¬ tudinous differentiations of the outer tissues that have been glanced at. CHAPTER VIII. DIFFERENTIATIONS AMONG THE INNER TISSUES OF ANIMALS. § 297. The change from the outside of the lips to their inside, introduces us to a new series of interesting and instructive facts, joining on to those with which the last chapter closed. They concern the differentiations of those coats of the alimentary canal, which, as we have seen, are physiologically outer, though physically inner. These coats are greatly modified at different parts ; and their modifications vary greatly in different animals. In the lower types, where they compose a simple tube, running from end to end of the bodjq they are almost uniform in their histological characters ; but on ascending from these tjqaes, we find them presenting an increasing variety of minute structures between their two ends. The argument will be adequately enforced if we limit ourselves to the leading modifications they display in some of the higher animals. The successive parts of the alimentary canal are so placed with respect to its contents, that the physical and chemical changes undergone by its contents while passing from one end to the other, inevitably tend to transform its originally homogeneous surface into a heterogeneous surface. Clearly, the effect produced on the food at any part of the canal by trituration, by adding a secretion, or by absorbing its nutri¬ tive matters, implies the delivery of the food into the next part of the canal in a state more or less unlike its previous THE INNER TISSUES OF ANIMALS. 311 states — implies that the surface with which it now comes in contact is differently affected by it from the preceding sur¬ faces — implies, that is, a differentiating action. To use con¬ crete language ; — food that is broken down in the mouth acts on the oesophagus and stomach in a way unlike that which it would have done had it been swallowed whole ; the masti¬ cated food, to which certain solvents or ferments are added, becomes to the intestine a different substance from that which it must have otherwise been ; and the altered food, resolved by these additions into its proximate principles, cannot have those proximate principles absorbed in the next part of the intestine, without the remoter parts being affected as they would not have been in the absence of absorption. It is true that in developed alimentary canals, such as the reasoning here tacitly assumes, these marked successive differentiations of the food are themselves the results of pre-established differentiations in the successive parts of the canal. But it is also true that actions and reactions like those here so definitely marked, must go on indefinitely in an undeveloped alimentary canal. If the food is changed at all in the course of its transit, which it must be if the creature is to live by it, then it cannot but act dissimilarly on the successive tracts of the alimentary canal, and cannot but be dissimilarly reacted on by them. Inevitably, therefore, the uniformity of the surface must lapse into greater or less multiformity : the differentia¬ tion of each part tending ever to initiate differentiations of other parts. Not, indeed, that the implied process of direct equilibra¬ tion can be regarded as the sole process. Indirect equilibra¬ tion aids; and, doubtless, there are some of the modifications which only indirect equilibration can accomplish. But we have here one unquestionable cause — a cause that is known to work in individuals, changes of the kind alleged. Where, for instance, cancerous disease of the oesophagus so narrows the passage into the stomach as to prevent easy descent of i he food, the oosophag'us above the obstruction becomes Vol. II. 14 312 PHYSIOLOGICAL DEVELOPMENT. enlarged into a kind of pouch ; and the inner surface of this pouch begins to secrete juices that produce in the food a kind of rude digestion. Again, stricture of the intestine, when it arises gradually, is followed by h}rpertroph}r of the muscular coat of the intestine above the constricted part : the ordinary peristaltic movements being insufficient to force the food forwards, and the lodged food serving as a constant stimulus to contraction, the muscular fibres, habitually more exercised, become more bulky. The deduction from general principles being thus inductively enforced, we cannot, I think, resist the conclusion that the direct actions and reactions between the food and the alimentary canal have been largely instru¬ mental in establishing the contrasts among its parts. And we shall hold this view with the more confidence on observ¬ ing how satisfactorily, in pursuance of it, we are enabled to explain one of the most striking of these differentiations, which we will take as a type of the class. The gizzard of a bird is an expanded portion of the alimen¬ tary canal, specially fitted to give the food that trituration which the toothless mouth of the bird cannot give. Besides having a greatly-developed muscular coat, this grinding- chamber is lined with a thick, hard cuticle, capable of bearing the friction of the pebbles swallowed to serve as grind-stones. This differentiation of the mucous coat into a ridged and tubercled layer of horny matter — a differentiation which, in the analogous organs of certain Molltisca , is carried to the extent of producing from this membrane bony plates, and even teeth — varies in birds of different kinds, according to their food. It is moderate in birds that feed on flesh and fish, and extreme in granivorous birds and others that live on hard substances. How does this immense modification of the alimentary canal originate ? In the stomach of a mammal, the macerating and solvent actions are united with that triturating action which finishes what the teeth have mainly done ; but in the bird, unable to masticate, these internal functions are specialized, and while the crop is the TIIE INNER TISSUES OF ANIMALS. 313 macerating chamber, the gizzard becomes a chamber adapted to triturate more effectually. This adaptation requires simply an exaggeration of certain structures and actions which chaiacteiize stomachs in general, and, in a less degree, alimentary canals throughout their whole lengths. The massive muscles of the gizzard are simply extreme develop¬ ments of the muscular tunic, which is already considerably developed over the stomach, and incloses also the cesophagus and the intestine. The indurated lining of the gizzard, thickened into horny buttons at the places of severest pres¬ sure, is nothing more than a greatly strengthened and modified epithelium. And the grinding action of the gizzard is but a specialized form of that rhythmical contraction bv which an ordinary stomach kneads the contained food, and which in the cesophagus effects the act of swallowing, while in the intestine it becomes the peristaltic motion. Allied as the gizzard thus clearly is in structure and action to the stomach and alimentary canal in general ; and capable of being gradually differentiated from a stomach where a grow¬ ing habit of swallowing food unmasticated entails more trituration to be performed before the food passes the pylorus; the question is — Does this change of structure arise by direct adaptation? There is warrant for the belief that it does. Besides such collateral evidence as that mucous membrane becomes horny on the toothless gums of old people, when subject to continual rough usage, and that the muscular coat of the intestine thickens where unusual activity is demanded of it, we have the direct evidence of experiment. Hunter habituated a sea-gull to feed upon grain, and found that the lining of its gizzard became hardened, while the gizzard- muscles doubled in thickness. A like change in the diet of a kite was followed by like results. Clearly, if differentiations so produced in the individuals of a race under changed habits, are in any degree inheritable, a structure like a gizzard will originate through the direct actions and reactions between the food and the alimentarv canal. 314 PfiYSlOLOGICAL DEVELOPMENT. 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 unmasticatcd food are stored, to he 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 an}7 — 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 fly, 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 stomachful. 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. How 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 storing-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 ot 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 wTith 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 ? The}'- 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 incompletely — will not 816 PHYSIOLOGICAL DEVELOPMENT. have had enough. Those which masticate incompletely and distend their stomachs with food difficult to digest, will be liable to these regurgitations ; 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 Pcuminants 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. THE INXEll TISSUES UF ANIMALS 317 , As the mass of bile- cells becomes greater, there arise se¬ 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. IIow 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 abej^ance,” says Dr. Harley, the most recent authority on the subject, “ biliverdine acciflnu- lates in the blood (until the serum is as it were completely saturated with the pigment), from which it exudes and stains 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 matter 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, extremely little power of diffusing themselves. 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, primarily, the escape of effete matters from the 319 THE INN Ell TISSUES OF ANIMALS. 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. IIow 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 s}7m- 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 w*aste of the tissues, let us take a further 320 PIl YSIOLOGICAL 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 wav out of f 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 undifferentiated 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 bo 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 is formed, by the growth of a hollow bud into the peri- visceral •322 r H 1' SIOLOG ICAL DEVELOPMENT. cavity, or space between the alimentary canal and the wall of 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-sac , 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 Vertebrcita 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 diverticula from it. In other fishes there is a permanent ductus 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 Lcpiclosteus, 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. IIow are the internal air-chambers, respiratory or non- respirator}’', developed P Upwards from the amphibian stage, in which they arc partially refilled at long intervals, there is THE IXttER TISSUES OF AMMALS. 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? IIow 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 ; hut 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 doing 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 (Esophagus, 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 ot 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 wray 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 why 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 stages of pulmonic development when the extra aeration achieved wTas 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- 325 THE INNER TISSUES OF ANIMALS. 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 branchioe 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 Lepidosiren, 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 here 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 327 THE INKER TISSUES OF ANIMALS. with 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 is indeed probable that the growth as well as the differentiation of the pulmonic surface, when once commenced, will be furthered by the direct process. The reasoning before used in the case of branchiae (§ 292) applies in the case of lungs. If exchange between the plasma in the blood¬ vessels and the plasma in the tissues surrounding them, 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 intra-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 328 PHYSIOLOGICAL DEVELOPMENT. forming tlie integument with, its appendages, and the double layer forming the alimentary canal with its diverticula. 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 the 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 by 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, TKE INNER TISSUES OF ANIMALS. 329 and its vessels when it lias 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. This 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 lbs. to J of a lb. 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 bo 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 where 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 is 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 places 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 | of a pound per square inch while the muscles are at rest, rises to 2J lbs. 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 now pass to the facts to be interpreted. Even in such simple types as the Ilydrozoa , 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 lacunae 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 Molluscoida 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. 331 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, 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 lacunoe 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 P 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 streams 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. 333 of a simple animal, made to ooze now in this direction and now in that by osmotic distension and by 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 earlv 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 t}7pical distribution of the developing ova ; and since the increase of an ovuin 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 ; and 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 left 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 ingthe structures which distinguish finished vascular sj^stems § 301. Of the inner tissues which remain let us next take bone. In what manner is differentiated this dense substance serving in most cases for internal support P 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 the tensions. The framework of an 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 Vol. II. 15 836 PHYSIOLOGICAL DEVELOPMENT. to bear a pull in tbe direction of its length ; and these struts and ties are differently formed to adapt them to these 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. Fow the like relation holds with a difference among the bones and muscles — the difference being, that here 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 would 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 those 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 : tbe muscles and ligaments concerned serving here, as in other cases, so to hold the bones that they bear the pressure instead TIIE INNER TISSUES OF ANIMALS. 337 of being displaced by it. Not forgetting that many processes of the bones have to bear tensions, we may then sav 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 of greatest pressures. Now it is not vertebrse 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 falls. Let us consider how such a bone that has to bear a lonmtu- o 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 straightness 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 coin- 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 fact which here specially concerns us, we have to note that the strongest tensions and pressures are 333 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 difficult}7 — cartilage is a non-vascular tissue : this sub¬ stance of which unossified 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 where 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 aud 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 prosenchyma 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 unossiiied 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 P 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 as 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, and 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 epiphyses of bones and the bony masses which they ter¬ minate. But there is the .reply that the places occupied by this cartilage, being places at which the bone lengthens, the 342 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 t}7pe. 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 fiucli 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 ot 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. Bemembering 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 wliat, 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 what, in medical but unscientific language, is called the vis medicatrix naturce , 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 DEVELOFM ENT. 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 assimi¬ 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 la}7er 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 better placed for resisting the strains to be borne THE INNER TISSUES OF ANIMALS. 345 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 are 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 bad been established by 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 arise ? and in what way is it differentiated from protoplasm simultaneously with the other tissues ? These are profoundly interesting questions ; but questions to wdiich positive answers cannot be expected. All that can be done is to indicate answers which seem feasible. Tli at 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 polymerism. 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 any molecular agitation spreading from a' centre, will go furthest along routes that offer least resistance. What routes will these be ? Those along which there lie most molecules that 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 it on. Molecules which are unstably 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 w7hich, 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 vTe 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 not 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 WTOught along a tract traversed by this wave 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 passed 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 adjacent untransformed molecules, but which, impressing themselves on the parts of such untrans* THE 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 isomeric substances P or may it extend to substances that are closely allied P 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 uncombined 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 FHYSIOLOGICAL DEVELOPMENT. going the isomeric -change, predominate, but have scattered through them the other molecules out of which they may 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 at 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 wre call heat, and its molecules aggregate in one way. Let the wrave die away, and its molecules resume their previous mode of aggregation. And obviousty such transformations may be 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 resume their previous form when the wave has passed : the previous form being the most stable under the conditions which then recur. What follows? It follows that these molecules will be readv m/ again to undergo isomeric transformation when there again occurs the stimulus ; will, as before, propagate the transforma- THE INNER TISSUES OF ANIMALS. 351 lion 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 P 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. IIow 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 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 wrater 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 far it agrees with the facts. If the actions are as here supposed, the contracted or more inte- THE INKER 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 large amounts of molecular 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 "VVislicenus, trans¬ lated by Prof. Wanklyn in the Phil. Mag. for May or June, 1866. See also an article by Prof. Frankland in the September number of the same journal. I 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. If 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 ANIMALS. 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 the 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 bv 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 undifferentiated tissue in general, is a view harmonizing with facts which the diffe- 356 PHYSIOLOGICAL DEVELOPM ENT. 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 sirrht, 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 Vertebrcda 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 youmr 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 soh 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 structural 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 say, 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 struthious 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- Vol. II. 16 360 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 growth along with the power of n^ore 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 with 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 ma}^ 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 worn 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 303 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 great 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 must 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 ; that 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 ANIMALS. 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 two chief products, carbonic acid and water, escaping with especial facility. Hence the place becomes a place of least resistance, through which 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 b}7 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 specificall3r 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 and 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 pari passu with the differentiation. Though it is manifest, a 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. § 306. If the lowly-organized Planaria has its body broken up and its gullet detached, this will, for a while, 3f)f> 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, wTe 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 which 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, wTith 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 more 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. 307 indeed, we see the relation under another aspeet. 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, arc 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 wTell 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 are united into a less regular compound rhythm — arc 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 lit tie -differentiated animal 368 ' FHVSIOLOGICAL 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, wThen 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 parts 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 Ilyclra 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 wdiere, 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 this PHYSIOLOGICAL INTEGRATION IN ANIMALS. 369 affects tko 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 completety-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 fatal o70 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. IIow have these special adaptations been made possible P 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 vascular sj^stem, 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 is the condition under which only 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 DEVELOPS! ENT. 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. § 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 rilYSJO LOGICAL INTEGRATION IN 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 of 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 DE YE LOPMEXT. 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 a 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. AVere 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, which we habitually see. The facts show, unmistakably, that PHYSIOLOGICAL INTEGRATION IN ANIMALS. 375 the slight disturbance communicated to a ganglion, causes an overthrow of that liighly-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 further 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. AVhat 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 off 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 I’ . 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 be a sufficient cause in the process of direct equili¬ bration ; and indirect equilibration may be fairly presumed a sufficient cause 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 re-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 frpm 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 parts 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 Bhizopod 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 Proiophyta and Protozoa , is true of the germs of all organ¬ isms up to the highest : tbe differentiation of outer from inner is the first step. When the endochrome of an Alya- 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. 879 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 Algce , 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 the 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, as in a Protococcus or a Volvox , 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 and upper parts of ordinary Fungi; and we see it in 380 PHYSIOLOGICAL DEVELOPMENT. 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 degree 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 Tania, uniformly bathed by the contents of the intestine it inhabits, the parts of the integument do not appreciably differ from one another ; 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 Vcrtehraia . 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 Pleuronectidce, then it is the 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 sur- SUMMARY OF PHYSIOLOGICAL DEVELOPMENT, 381 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, as 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 of 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 Phrenogams, 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 tlieir 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 endoderm 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 appear later in proportion as the parts are more removed from the influence of the food. Thev 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 Ccelenterata , is a feebly-organized parenchyma traversed by lacunae — either simple channels, or canals lined with simple ciliated cells ; and in the lower Mollusca 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 review 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 bear 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 a Vol. II. 17 384 PIIYSIOLOG ICAL DEVELOPS ENT. 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 other 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 all 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, again, 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 onty 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 voung1, has initiated still more anomalous modifications. •j o 7 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 SUMMARY 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 the 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 tbe 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 adapt ed 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-r-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. liaising 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. And what is thus obviously true of every normal change, is equally true of every abnormal change — every 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 tions. 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 part. And just as certain is that indirect equilibration in organisms ,*0 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 a^ain 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 9 O 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 be changed without initiating changes in all other parts — if the limit 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 jin 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 ia 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 tlie 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 tho 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 or 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 its 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 their 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. Ihis formation of new individuals to replace the old, 13, however, inadequately conceived if we contemplate 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. There must be taken into account a further element — the amount of aid given by the parent to each germ in the shape 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 39(j 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 ma}r, 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 constantly 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. § 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 LAWS OF MULTIPLICATION. 398 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 extremes 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. IIow is this done ? Should any one allege, in conformity with the old method of interpretation, that there is in each case 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 devia* A PRIORI PRINCIPLE. 399 tion. 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 species 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 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 carr}?- 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 wrhich 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 an inevitable consequence of the universal process of equilibration. § 322. Is this the sole equilibration that must exist P 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 wTould hold just a3 it now does, were all species equally prolific and all equally short-lived. It yields no A PRIOHI PRINCIPLE. 401 answer to the inquiries — why do their fertilities differ so enormously, or why do their mortalities differ so enormously ? 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 of 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. Hence, 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 adjustment 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. Gfiven 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 _ 492 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 tliis, that what is a temporary relation in the one is a per¬ manent relation in the other. \ § 323. For the moment it does not concern ns 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 there must be established an inverse proportion between the power to sustain individual life and the power to produce new individuals. Hero 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 ? To these questions let us now address ourselves. 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 is 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 any 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 as 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 thus 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 pl^siological 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 mav 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- Yol. II. 18 408 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 displajmd 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 selff conserving actions. Similarly, the total material devoted to the race at the 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, plus 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 the 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 bo 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- 410 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 us 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 use 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 OBVERSE A. PRIORI PRINCIPLE. 411 the requirements considered under each aspect. The two 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 a priori inferences. If they are true, there must be an agreement between them and the observed facts. Bet us see how far such an agreement is traceable. CHAPTER IV. DIFFICULTIES OF INDUCTIVE VERIFICATION. § 329. Were all species subject to the same kinds and amounts of destructive forces, it would be easy, by comparing different species, to test tlie 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 forces 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 larvse feed on an abundant plant, as those of the genus Vanessa on the Nettle, have practically an environment very unlike that ol insects such as Deilephila Eitphorbicey 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. Two 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 multipl}7ing 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 undefinable 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, there are immense variations 414 LAWS OF MULTIPLICATION. in the inner actions required to counter-balance them. Even 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 i3 physiologically more costly. Animals that protect themselves passively, as the Iledge-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 theii 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 fiequency 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 fei- 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 relativcly- 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 A.riust and especially A.) ius Boakeii — a fish some six or seven inches long, which produces ten or a dozen eggs as large as marbles, that are carried by he male in his mouth till they are hatched. Here though 116 LAWS OF MULTIPLICATION. the degrees of fertility, if measured by the numbers of fertilized germs deposited, are extremely unlike, they are 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 -Arius, 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 journey in gs 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 among higher Vcrtebrata 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 Silurus 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 v incubation, and tbe young afterwards. Evidently all these differences affect tbe proportion between tbe total cost of re¬ production and tbe total cost of individuation. Whether tbe 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 one male, the total quantity of assimilated matter devoted by each generation to the production of a new generation, is greater than it 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 Bornbi , 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 for Genesis : the matter which would otherwise have gone to the 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¬ viduation and Genesis vary inversely, is thus an under¬ taking so beset with difficulties, that we might despair of any satisfactory results, were not the relation too marked a one to be hidden even by all these complexities. Species are 418 LAWS OF MULTIPLICATION. bo 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, and vice versa. Certain minor qualifications, together with sundry impor¬ tant corollaries, may 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 down 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 Protococcus 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 AUjoe which, by their enormous numbers, sometimes suddenly change pools to an opaque green. So, too, the Desmidiacece 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 of 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 intervals, 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 Gonium pcdorale : each cell of the cluster resolving itself into a secondary cluster, and the secondary clusters then separating. “ Supposing, which is very probable, that a young Gonium 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 263,435,456 other organisms like itself.” In the Volvocince this continual dissolution of a primary compound individual into secondary compound individuals, is "carried on endogenously— the parent bursting to liberate the 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 older, such, as the higher Thallogens and the lower Acrogens, do not sacrifice their individualities by fission; but nevei- theless, by the kindred process of gemmation, are continually hindered in the increase of their individualities. The gemmae called tetraspores are cast off in great numbers by the mar ine Among those simple Juiicjenu^yinicicecB which consist of single fronds, the young ones that bud out grow for a time in connexion with their parents, send rootlets lrom 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. Aciogcns 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 Phaonogams. Ihe detachment ot 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 agamogenesis 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 Phscnogams, 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 Amceba it is said that “ when a large variable 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 caunot rightly be called a process of fission.” Put 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-aggie- 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 Paramecium 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! Making allowance for exaggeration in these 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. Brightwell on the buds of Zoothamnium. “At nine iu the morning, one of these buds, or ova, was 424 LAWS OF MULT1PLICATIOX. 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 shown in Fig. 148, the budding of young ones from the parent ITydra 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 Medusce , which are the adult or sexual forms of the species. Respecting Coelenterate 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 Adinozoon 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 in 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.’' GTtOWTII AND ASEXUAL GENESIS. 425 general truth with a difference. In the smaller members the ai^amogenesis is incomplete, and in the larger it disappears, liach sub-section of the Molluscoida shows us this. The gemma¬ 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 Bmchiopoda, which immensely exceed them in size, are solitary and not gemmiparous. So, too, is it with the Asciclioida. And then among the true Mollusca, 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 Entozoci; 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 m 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 fissiparous 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 Mrjriapoda 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 asexu&lly after tliis 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 agamogenesis 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¬ sary a 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 tl|^ size of those which thus multiply with less and less rapidity. With¬ out saying that this inverse proportion is regular, which, as we 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 octween 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 YL ANTAGONISM BETWEEN GROWTH AND SEXUAL GENESIS. § 33S. In 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 AJgce, this is not invariably matter supplied by different individuals : certain Diatomacece exhibit within a single 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 DesmicUaccce and in the Palmellce; or they may be two of the adjacent cells forming a thread, as in some Conjugate ce ; or they may be cells belonging to adjacent threads, as in Zygnema. 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 AJgce 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. 430 LAWS OF MULTIPLICATION. Thus, as quoted by Dr. Carpenter, Fries says — “ in a single individual of Iieticularia maxima , I have counted (calculated P) 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 of 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 Algce 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 Macrocystis pgrifera , 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 lower 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 the GROWTH AND SEXUAL GENESIS. 431 further factor which here complicates the result — the age at which sexual genesis commences. The smaller Phaenogams are 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 P 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 relativelv-small fertility becomes still further reduced where, as in Lodoicea Sechellarum , 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 Protophyta exhibit — a coalescence of the substance of two individuals to form a germ-mass. This has been alleged more especially of Adinophrys. 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 Yol. II. ]9 432 LAWS OF MULTIPLICATION. generation. A modified mode, apparently not fatal to the parents, has been 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~ tina 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. 433 fertile ova in the same period. The same individual, pro¬ ducing in ten days forty egg3, 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 lour 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 \ aviation 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, }et as they do. this at comparatively long intervals, their rates of increase fall immensely below that just instanced in the Botifers. 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 which 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 than 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 dowTn under the present head some of those cases in which the activity, being relatively slight, does not greatly interfere with the relation wre 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. How these contrasts GROWTH AND SEXUAL GENESIS. 435 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 is 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 lien 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 436 LAWS OF MULTIPLICATION. fertility winch 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 Phsenogams, will not dwell on the less conspicu- GROWTH A ND 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 hut few pears at first, it is long before it fruits abundantly. A young Mulberry, branch¬ ing out luxuriantly season after season, but covered ^ 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. Put 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 flowers 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 LAWS OF MULTIPLICATION. 138 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 onty 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, •K ' 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. AVhen, on the other hand, taking a microscopic protophyte which has millions of descendants in a few da}rs, 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 by 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 prodigiously-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. I 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 versa.”— Principles of Phy¬ siology, General and Comparative , Third Edition, 1851, p. 592. CHAPTER YII. THE ANTAGONISM BETWEEN DEVELOPMENT AND GENESIS, ASEXUAL AND SEXUAL. § 343. P y 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. Put 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. Pesides 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. 441 other. An insect, passing from its lowty- 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-undifferentiated 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. Put 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 we will now look at as exemplified. § 314. 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. LAWS OF MULTIPLICATION. 142 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 untransforraed 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 wdiolly true. Various marine Algce which propagate asexually, are larger than some Phsenogams 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 lowlv-organized Liverworts that are habitually gemmiparous, exceed in size many flowering plants. And the Perns 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. 443 power to produce young ones from nearly all parts of its bodjr, 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 Hydrazoa , 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 declining 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 Annulosa 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 homologues 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. Phaenogams that have but little supporting tissue may 144 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- ginalty-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 Till. 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 aifect 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 EXPENDITURE AND GENESIS. 447 maintenance of heat diminishes the rapidity of propagation, is difficult to obtain. It is, indeed, obvious that the warm¬ blooded Vertebrata 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 then, 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, hut 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, as milk *48 LAWS OF MULTIPLICATION. is part of the material from which a young one 3g 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 climates, may naturally 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 p, 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. 449 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 creatures of equal weights which go though the smaller exertion of moving about over solid surfaces. Predatory Birds have fewer young ones than predator}?- Mammals of approximately the same sizes. If we compare Books with Bats, 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 Baptorial 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 we 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 due 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 45 0 LAWS OF MULTIPLICATION. effort than motion on the ground, this geometrical progression tells more rapidly on Birds than on Mammals. Be 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 have commonly 2 broods in the season ; while in the Crow family 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 1 5 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 cost * 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 the Water-Bail, 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. Hot 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 voung 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 said 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 up, 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 erro-s lono- before the former brood is able to leave the nest, so OO O that the parent bird has at the same time young birds and e££s to take care of.” O O § 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 between the Hare and the Babbit. 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 tire Hodentia 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 the 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 by a decrease in the number of 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 moderatel}r and well kept, she will fail gradually, and the diminution will be less rapid.” In 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 of 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 outla}rs 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 have 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. 455 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 agamogenesis which we have in proliferous flowers, is habitually accompanied by a general luxuriance, implying an unusual plethora. No less conclusive is the evidence furnished by agamo¬ genesis in animals. Sir John Dalyell, speaking of Hydra tuba> 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 of materials. So, too, is it with the agamic reproduction of the Aphis. As cited by Professor Huxley, Kyber “ states that he raised viviparous broods of both this species (Aphis Dianthi) and A. Iiosce 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. The 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 inier- \rOL. II. 20 456 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 forces ; 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 localfo, the decline of agamic multiplication and commencement of gamic multiplication, along with a lessening rate of nutrition. Now 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 winch 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 egg3 as the wild variety. And the like holds of Pigeons. After remarking of the Columba licia 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 only 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 158 LAWS OF MULTIPLICATION. shall have a generous diet. Respecting Pigeons Macgillivray 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 congeners. The result is that it has several broods in a sea¬ son, while its field-haunting kindred have none of them more than two broods, and some have only one. Equally 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-year. 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 rate 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 wdiere there is the greatest inactivity — where there is plenty to 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 have twins ; but tlie}^ 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 lambs— the triplets balancing the uniparm. . 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 cither 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 AND GENESIS. 461 carbonaceous molecules locally produced by decomposition of the nitrogenous molecules, have not been replaced by other nitrogenous molecules, as they should bave been. This fatty degeneration is, indeed, a kind of local death. Jb or so regard¬ ing it we have not simply the reason that an active substance has its place occupied by an inert substance \ but we ha\ e the reason that the flesh of dead bodies, under ceitain conditions, is transformed into a fatty matter called adipoceie. The infertility that accompanies fatness in domestic animals, has, however, other 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, pio- perly so called, can be drawn 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, is 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 plants are removed from their natural conditions, they are extremely liable to 462 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 order ; 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 normal!}' 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. have 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-nitrogenous matters it eats. CHAPTER X. SPECIALITIES OF THESE RELATIONS. $ 356. Tests of the general doctrines set forth 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 theii powers of propagation will be enormous. If there are classes of creatures that expend very little for self-support in comparison with allied creatuies, 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 Rafflesiacem, carry their para¬ sitism to the extent of living on the juices they absorb fiom 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 m 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 Phocnogams, is, in these Phmnogams, 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 Fpizoa, such an instance as the Nicothce. Belonging to the Fntomostraca, 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 11ELATIONS. 465 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 emeig- ing to breed, lays as many as 8,000,000 eggs in less than a day. Similarly with the larger types that infest the higher animals. It has been calculated by Dr. Eschrioht, as quoted by Professor Owen, that there are " 64,000,000 of ova in the 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. the greatest extent while the nutrition is the highest possible, the degree of fertility reaches its extreme. These Entozoa 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 part grows and develops reproductive structuies, only in some predatory animal to which its first host falls a sacrifice. Occasionally, however, the egg gives origin, to the sexual 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 166 LAWS OF MULTIPLICATION. containing animal to be devoured ; so that the imperfect Tape¬ worm may find its way into tbe intestine of a higher animal. Thus the Bothriocephalus solidus, 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 Bothriocephalus 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 P 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 Annalosa. 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 P Why among them should imperfect females produce, agamically, others like themselves, generation after generation, with great rapidity ? There is the obvious explanation that they get plenty of easily-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 here a retrogression to asexual genesis, and a greatly-increased rate of multiplication. The recently- discovered instance of internal metagenesis in the maggots of certain Elies has a like meaning. In¬ credible as it at first seemed to naturalists, it is now proved that the Cecydomia- larva develops in its interior a brood of larvae SPECIALITIES 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 larvae are found in such habitats as the refuse of beet-root-sugar 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 specialty-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 larvae to multiply agamically, 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 exa£ aerated 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 on which falls an excessive expenditure for self-preservation and preservation of other’s offspring. CHAPTER XL INTERPRETATION AND QUALIFICATION. § 362. Considering the difficulties of inductive verification, we have, I think, as clear a correspondence between the a priori and a posteriori conclusions, as can be expected. The many 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. 471 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.. We 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 must 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 fioin 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 172 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. § 363. Something remains to be said in reply to the in¬ quiry — how is the ratio between Individuation and Genesis established in each case P 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 each 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 conditions 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. 174 LAWS OF MULTIPLICATION. Each increment cf evolution entails a decrement of re¬ 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 inore-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, eludes pursuit by diving instead of, like others of its kind, by swim¬ ming along the surface, escapes by doubling instead of by sheer speed. Clearty, perseverance in the modified habit will, other INTERPRETATION AND QUALIFICATION. 475 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 bo 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 land. 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 stiength, or both, will increase ; while another part will go towards more active genesis. J ust 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. 176 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 egg s, 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 Bat with the Mouse yields a parallel result. Though they differ greatly in size, yet the one is as prolific IM T E RP [FETATION 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 Eat by its greater intel¬ ligence, greater power and courage, greater ability to utilize what it finds. The Eat is notoriously cunning; and its cunning gives success to its foraging expeditions. It w not, like the Mouse, limited mainly to vegetal food ; but while it eats grain and beans like the Mouse, it also eats flesh an; carrion, devours young poultry and eggs. The result is that, without a proportionate increase of expenditure, it gets a ar larger supply of nourishment than the Mouse ; and this 1 da¬ 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 Eat and the Water-Eat. While the common Eat has habitually several broods a-year of from 10 to 12 each, the Water-Eat, though somewhat smaller, has but 5 or 6 in a brood, and but one brood, or sometimes two broods, a-year. But the Water- Eat 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. Eecognizing 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, and 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 urvival, 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- evolved 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. Ilis 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 such 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 Vol. II. 21 480 LAWS OF MULTIPl 1 CATION. 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 thqpi widely different consumptions of matter for maintenance of MULTIPLICATION OF THE HUMAN RACE. 431 temperature. Both in their qualities and quantities, the foods they 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 evidence 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 “ seldom 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. Bich 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 five- and- twenty, and threatened ‘ le vingt-sixieme pour le pretre.i ” Prom 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 hy 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 Individuation 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, and 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 tlieii 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, yery possibly suffices to meet the comparatively- low 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 cair)ing on parental life, is accompanied by a high rate of genesis* * This is exactly the reverse of Mr. Douhleday’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 the 484 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 Hr. 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 deplcthoric 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 first place, he has confounded normal plethora with what I have, iu § 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 as 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 new individuals after another manner — he would have perceived that facts of this class do not tell in his avour. In the law which Mr. Doubleday alleges, he secs a guarantee for the main- 485 MULTIPLICATION OF THE HUMAN RACE. nutrition, we may suspect that it. is in part due to greater muscular expenditure. A. kindred fact, admitting ot 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 li e that the race can he 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 o 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 t lan less numerous. For, by the hypothesis, the unusual fertility due to the deplethoric state, is the cause of undue increase of population. But it. tie 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 other 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, the species will presently die out from absolute barrenness. LAWS OF MULTIPLICATION. better than tliat 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 which produces a serious reaction on the physique. This diminution of reproductive powTer 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 aihletce 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 men 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 an 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 ; hut 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 interior 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 t le 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 underled. sketches of natives contained in the volumes of Livingstone, Baker, and others, yield clear proofs of the extreme depletion common among the uncivilize .. quality as well as in quantity, their feeding is a . 1 fruits, insects, larvse, vermin, &c., which we *e “se diso-ust, often enter largely into their dietary. Much ot this inferior food they eat uncooked ; and they have not our 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 and 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 chase 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 the)7- 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 shall see that a savage who is the reverse of industrious, may nevertheless be subject to a muscular waste not very MULTIPLICATION OF THE HUMAN RACE. 480 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. Cases 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 wliieli 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 cf 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 thS 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 type 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 functionall) 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 fiom 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 temperatuies. 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 LAWS OF MULTIPLICATION . m 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 safetv- 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. 493 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 so entangled with the influences of other factors which are for the present more important, that we cannot recognize it. Here, if we proceed at all, we must proceed deductively. CHAPTER XIII. HUMAN POPULATION IN THE FUTUEE. § 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 hulk, 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, separately HUMAN POPULATION IN THE FUTURE. 495 and jointl}r, of counteracting the separate and 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 cither 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-oi dina¬ tion of actions, there must be organization of still greater heterogeneity and definiteness. | 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 196 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 likelv 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 of 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. Bight conduct is usually come short of more from defect of 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 19S LAWS OF MU LTIPLICATION. 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 its 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 hence resulting would, on the average, rise as 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 manifest^, 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 suh HUMAN POPULATION IN THE FUTURE. 499 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 aits, is the bringing to bear greater intelligence for satisfying oui 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 powers and the powers of his labourers. To meet the requirements of the market, the manufactuier is pei- 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 moie 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 motliei 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 pio- 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 or 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 successfullv, 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 the 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 io-operation with the cause assigned, or with other causes, produce divergences HUMAN POPULATION IN THE FUTURE. 501 § 374. The proposition at whicli we have thus arrived, is, (hen, 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 ellects are 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 it3 convolutions. And further changes like these which 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 HUM AN 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 intenser -strain — a mentally -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 beyond 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. 604 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 the 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-bearin g adults, equal in number to the last 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 flowings 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 aa a constant cause of migrations and of consequent alterations of environment. Now though the human race differs from HUMAN POPULATION IN THE FUTURE. 505 other races in having a power of 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 versa . The geologically-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 less 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 paii 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 physical 506 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- tainment 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 Genesis, 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, 1 lie due peopling of the globe, HUMAN POPULATION IN THE FUTURE. 507 and the raisin" 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 self-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 eveiy 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. § 1 35). 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 wre 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 versa. 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 wrhich 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. «:|n • M . . 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, I 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 c umbellules : some of which are perfect, and others imperfect only in the shortness of the flower-stalks. The annexed Fig. 69, represent- ;ng in a somewhat conventionalized way, a part of the dried sped- 512 men, will give an idea of this Angelica. At a is shown a single flower partially changed ; in the umbellule marked b , 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 Oow-Parsnep, in which a single terminal umbel, besides the transformations already men¬ tioned, exhibits higher degrees of such transformations.* The com¬ ponents 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 cases when in such localities, or whether it is that the sea-air favours that excessive nutrition whence these transformations result, I am unable to say. 513 umbellule is composed of two, three, or four flowers. 7. But tlio 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 / aro 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 G ermans. 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 phsenogamic 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 * Phsenogam 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 aro 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 wre 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 to 515 bay, 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 by 516 putting anomalies out of sight, may be inclined to think that tho 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 mate¬ 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 which 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 ON PROF. OWEN'S THEORY OF THE VERTEBRATE SKELETON. ' From the British & Foreign Medico-Ciiirurgical Review for Oot., 1858.] I. On the Archetype and Homologies of the Vertebrate Skeleton . By Richard Owen, F.L.S. — London , 1848. pp. 172. II. Principes cTOsteologie Comparce , ou Lecher ches sur V Archetype et les Homologies da Squelette Vertebre. Par Richard Owen. — • Paris . Principles of Comparative Osteology ; or , Lesearches on the A renetype and the Homologies 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 Loyal Institution of Great Britain. By Richard Owen, F.L.S. — London , 1849. 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 many facts unknown to him which disprove it. But it is not dangerous to give a negative verdict when the alleged demonstra- 518 tion 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 believe 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. W e 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 “ 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, 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 Homologies of the Vertebrate Skeleton , he quotes ap¬ provingly the Platonic hypothesis of $ext, “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 in general (see diagram of the Archetypus ), 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.” AVhether Professor Owen holds that the typical vertebra also exists as an “ idea,” is not so certain. Prom 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 Vtia 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 Homo - logics — if he considers that “ the lUa. ” “ 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 vraisemblance. 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 species 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 there 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. 80, 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 l$e% or organizing principle,” or whether the typical vertebra is itself an “ iKx or organizing principle” — there is alike implied the belief that the typical vertebra has an abstract existence apart from actual vertebras. 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 hcemapophyses and a hcemal spine , forming a haemal arch similar to the neural arch above, and enclosing the haemal 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 521 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 hmmapophyses. If, now, these are the constituents of the vertebrate segment “ in its typical completeness ;” and if the vertebrate skeleton consists of 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 of 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 hsemal 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 vertebra?, 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 homologues 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 Vertebrata 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 any transitional forms between the alleged normal position of the scapular arch in fishes, and its extraordinary displacement in the higher Vertebrata ; and that he makes no reference to the embryonic phases of the higher Vertebrata , 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 here Professor Owen instances the percnnibranchiatc Batrachia as 522 exhibiting the typical structure : remarking that “ mammals, birds, and reptiles show the rule of connexion, and fishes the exception.” Thus in the case of the scapular arch, the evidence afforded by fishes 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 bony frames to which the limbs are articulated are modified haemal 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 “ rudimentai limbs — “ future possible or potential arms, legs, wings, or feet,” we may fairly expect them always to bear to the hamial arches a relation such as the limbs do. But they by no means do this. “ As the vertebrae approach the tail, these appendages are often transferred gradually from the pleur apophysis to the parapopliysis, 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 “ rudimentai 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 hgemapophyses. 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. 16 of the Archetype and Homologies , he says : — “ The earlier stages in the development of all locomotive extremities are permanently retained or represented in the paired iins 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 : finally come the humeral and femoral seg¬ ments j but this stage I have not found attained in any fish." 523 That is to say, alike in ascending through the Vertebrate i gene¬ rally, and in tracing up 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 hsemal 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 f 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 : — Has the “ ideal typical vertebra” any essential constituents at all ? It might 524 naturally lie supposed that though some hones are 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 vertebrae” (p. 96), Professor Owen says — “ 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 — “ The pleurapophyses are less constant elements than the neurapo- pliyses.” 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 imper¬ 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 pass beyond the primitive stage of the notochord (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 sera in Geology, though the number of vertebra? is frequently indicated in Devonian and Silurian iclithyolites by the fossilized neur- and hcemapophyses and their spines ” (p. 98). 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 uni- 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 o&e 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 seems 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 Vertebrata generally, and even in Professor Owen’s Archetypus , the vertebrm 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 ? ri 0 this question some parts of the Archetype and Homologies seem to reply, “ Yes while others clearly answer, “ No.” Criticising the opinions of Geoffroy 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 bones 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 Homologies , after giving a number of instances, he says — * These and the like correspondences between the points of ossification ol 520 the human fcetal skeleton, and the separate hones 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 homo- 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 Pelobates fuscus and Pelobcites 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 nearapophyses. We see, then, that these so-called typical elements of vertebrae 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- mars 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, two haemapophyses, one neural spine, and one haemal 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 vertebrae of some fishes ” (p. 98). Again, in the abdominal region of extinct saurians, and in crocodiles, “ the freely-suspended haemapophyses 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 Testuclo elephcintopus 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. 91). 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 neural and haemal spines, the neurapophyses, the pleu« Vol. IT. 28 528 rapophyses, the haemapophyses, may severally consist, of two or more pieces. This is not all : the like is true even of the centrums. “ In Heptanchus ( Squalus cinereus) the vertebral centres are feebly and vegetatively marked out by numerous slender rings of hard cartilage in the notochordal capsule, the number of vertebrae being more definitely indicated by the neurapophyses and parapophyses. ... In the piked dog-fish (Acanthias) and the spotted dog-lish ( 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 ad¬ 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 homocercal 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 haemal spines of birds ; though, sometimes, as e.g., in the struthionidae, ossification extends from the same lateral centre lengthwise — i.e., forwards and, backwards., calcifying the connate cartilaginous homologues of halves of four or five haemal spines, before these finally coalesce with their fellows at the median line ” (p. 101). So that the sternum of the ostrich, which according to the hypo¬ thesis, should, in its cartilaginous stage, have consisted of four or five transverse pieces, answering to the vertebral segments, and should have been ossified from four or five centres, one to each cartilaginous piece, shows not a trace of this structure; but in¬ stead, consists of two longitudinal pieces of cartilage, each ossified from one centre, and finally coalescing on the median line. These four or five haemal spines have at the same time doubled their in¬ dividualities transversely, and entirely lost them longitudinally ! There still remains to be considered the test of relative position. It might be held that, spite of all the foregoing anomalies, if the typical parts of the vertebrae always stood . towards each other in the same relations — always preserved the same connexions, some¬ thing like a case would be made out. Doubtless, relative position 529 is an important point ; and it is one on which Professor Owen mani¬ festly places great dependence. In his discussion of “ moot cases of special homology,” it is the general test to which he appeals. The typical natures of the alisphenoid, the mastoid, the orbito- sphenoid, the prefrontal, the malar, the squamosal, &c., he deter¬ mines almost wholly by reference to the adjacent nerve-perforations and the articulations with neighbouring bones (see pp. 19 to 72) : the general form of the argument being — This bone is to be classed as such or such, because it is connected thus and thus with these others, which are so and so. Moreover, by putting forth an “ ideal typical vertebra,” consisting of a number of elements standing towards each other in certain definite arrangement, this persistency of relative position is manifestly alleged. The essential attribute of this group of bones, considered as a typical group, is the con¬ stancy in the connexions of its parts : change the connexions, and the type is changed. But the constancy of relative position thus tacitly asserted, and appealed to as a conclusive test in “ moot cases of special homology,” is clearly negatived by Professor Owen’s own facts. For instance, in the “ ideal typical vertebra,” the heemal arch is represented as formed by the two hasmapophyses and the hmmal spine ; but at p. 91 we are told that <£ The contracted haemal arch in the caudal region of the body may be formed by different elements of the typical vertebra : e.g by the para- pophyses (fishes generally) ; by the pleurapophyses (lepidosiren) ; by both parapophyses and pleurapophyses [Suclis, Lepidosteus), and by haemapo- physes, shortened and directly articulated with the centrums (reptiles and mammals).” And further, in the thorax of reptiles, birds, and mammals, “ the haemapophyses are removed from the centrum, and are articulated to the distal ends of the pleurapophyses; the bony hoop being com¬ pleted by the intercalation of the haemal spine ” (p. 82). So that there are fine different ways in which the haemal arch may be formed — four modes of attachment of the parts different from* that shown in the typical diagram ! Nor is this all. The pleurapophyses “ may be quite detached from their proper segment, and suspended to the haemal arch of another vertebra ;” as we have already seen, the entire haemal arch may be detached and removed to a distance, sometimes reaching the length of twenty-seven vertebrae ; and, even more remarkable, the ventral fins of some fishes, which theoretically belong to the pelvic arch, are so much advanced forward as to bo articulated to the scapular arch — “ the ischium elongating to join the coracoid.” With these admissions it seems to us that relative position and connexions cannot be appealed to as tests of homology, nor as evidence of any original type of vertebra. In no class of facts, then, do we find a good foundation for the hypothesis of an “ ideal typical vertebra.” There is no one con- ceivable attribute of this archetypal form which is habitually realised by actual vertebrae The alleged group of true vertebral elements is not distinguished in any specified way from bones not included in it. Its members have various degrees of inconstancy ; are rarely all present together ; and no one of them is essential. They are severally developed in no uniform way : each of them may arise either out of a separate piece of cartilage, or out of a piece con¬ tinuous with that of some other element ; and each may be ossified from many independent points, from one, or from none. Not only may their respective individualities be lost by absence, or by con¬ fluence with others ; but they may be doubled, or tripled, or halved, or may be multiplied in one direction and lost in another. The en¬ tire group of typical elements may coalesce into one simple bone representing the whole vertebra ; and even, as in the terminal piece of a bird’s tail, half-a-dozen vertebrae, with all their many elements, may become entirely lost in a single mass. Lastly, the respective elements, when present, have no fixity of relative position : sundry of them are found articulated to various others than those with which they are typically connected ; they are frequently displaced and attached to neighbouring vertebrae ; and they are even removed to quite remote parts of the skeleton. It seems to us that if this want of congruity with the facts does not disprove the hypothesis, no such hypothesis admits of disproof. LTnsatisfactory as is the evidence in the case of the trunk and tail vertebra, to which we have hitherto confined ourselves, it is far worse in the case of the alleged cranial vertebrae. The mere fact that those who have contended for the vertebrate structure of the skirll, have differed so astonishingly in their special interpretations of it, is enough to warrant great doubt as to the general truth of their theory. From Professor Owen’s history of the doctrine of general homology, we gather that Dumeril wrrote upon “ la tete consideree comme une vertebre that Kielmeyer, “ instead of calling the skull a vertebra, said each vertebra might be called a skull;” that Oken recognized in the skull three vertebra and a rudiment ; that Professor Owen himself makes out four vertebras ; that Goethe’s idea, adopted and developed by Carus, was, that the skull is composed of six vertebra ; and that Geoffroy St. Hilaire divided it into seven. Does not the fact that different comparative anatomists have arranged the same group of bones into one , three , four, six, and seven vertebral segments, show that the mode of de¬ termination is arbitrary, and the conclusions arrived at fanciful ? May we not properly entertain great doubts as to any one scheme being more valid than the others ? And if out of these conflicting schemes we are asked to accept one, ought we not to accept it only on the production of some thoroughly conclusive proof — some 531 rigorous test showing irrefragably that the others must be wrong and this alone right ? Evidently where such contradictory opinions have been formed by so many competent judges, we ought, before deciding in favour of one of them, to have a clearness of demon¬ stration much exceeding that required in any ordinary case. Let us see whether Professor Owen supplies us with any such clearness of demonstration. To bring the first or occipital segment of the skull into corre¬ spondence with the “ ideal typical vertebra,” Professor Owen argues, in the case of the fish, that the parapophyses are displaced , and wedged between the neurapophyses and the neural spine — removed from the haemal arch and built into the upper part of the neural arch. Further, he considers that the pleurapophyses are teleologi¬ cally compound. And then, in all the higher vertebrate, he alleges that the haemal arch is separated from its centrum, taken to a dis¬ tance, and transformed into the scapular arch. Add to which, he says that in mammals the displaced parapophyses are mere processes of the neurapophyses (p. 133) : these vertebral elements, typically belonging to the lower part of the centrum, and in nearly all cases confluent with it, are not only removed to the far ends of elements placed above the centrum, but have become exogenous parts of them ! Conformity of the second or parietal segment of the cranium with the pattern-vertebra, is produced thus : — The petrosals are excluded as being partially-ossified sense-capsules, not forming parts of the true vertebral system, but belonging to the “ splanchno-skeleton.” A centrum is artificially obtained by sawing in two the bone which serves in common as centrum to this and the preceding segment ; and this though it is admitted that in fishes, where their individualities ought to be best seen, these two hypothetical centrums are not simply coalescent, but connate. Next, a similar arbitrary bisection is made of certain elements of the haemal arches. And then, “ the principle of vegetative repetition is still more manifest in this arch than in the occipital one each pleurapophysis is double ; each haem apophysis is double ; and the haemal spine consists of six pieces ! The interpretation of the third and fourth segments being of the same general character, need not be detailed. The only point calling for remark being, that in addition to the above various modes of getting over anomalies, we find certain bones referred to the dermo-skeleton. Now it seems to us, that even supposing no antagonist interpre¬ tations had been given, an hypothesis reconcilable with the facts only by the aid of so many questionable devices, could not be con¬ sidered satisfactory ; and that when, as in this case, various com¬ parative anatomists have contended for other interpretations, the character of this one is certainly not of a kind to warrant the re¬ jection of the others in its favour; but rather of a kind to make 532 as doubt the possibility of all such interpretations. The question which naturally arises is, whether by proceeding after this fashion, groups of bones might not be arranged into endless typical forms. If, when a given element was not in its place, we were at liberty to consider it as suppressed , or connate with some neighbouring element, or removed to some more or less distant position ; — if, on finding a bone in excess, we might consider it, now as part of the dermo- sJceleton , now as part of the splanchno- skeleton, now as transplanted from its typical position, now as resulting from vegetative repetition, and now as a bone teleologically compound (for these last two are intrinsically different, though often used by Professor Owen as equivalents); — if, in other cases, a bone might be regarded as spurious (p. 91), or again as having usurped the place of another ; — if, we say, these various liberties were allowed us, we should not despair of reconciling the facts with various diagrammatic types besides that adopted by Professor Owen. When, in 1851, we attended a course of Professor Owen’s lectures on Comparative Osteology, beginning though we did in the attitude of discipleship, our scepticism grew as we listened, and reached its climax when we came to the skull ; the reduction of which to the vertebrate structure, reminded us very much of the interpretation of prophecy. The delivery, at the Royal Society, of the Croonian Lecture for 1858, in which Professor Huxley, confirming the state¬ ments of several German anatomists, has shown that the facts of embryology do not countenance Professor Owen’s views respecting the formation of the cranium, has induced us to reconsider the verte¬ bral theory as a whole. Closer examination of Professor Owen's doctrines, as set forth in his works, has certainly not removed the scepticism generated years ago by his lectures. On the contrary, that scepticism has deepened into disbelief. And we venture to think that the evidence above cited shows this disbelief to be warranted. There remains the question — What general views are we to take respecting the vertebrate structure ? If the hypothesis of an “ ideal typical vertebra” is not justified by the facts, how are we to under¬ stand that degree of similarity which vertebrae display ? We believe the explanation is not far to seek. All that our space will here allow, is a brief indication of what seems to us the natural view of the matter. Professor Owen, in common with other comparative anatomists, regards the divergences of individual vertebrae from the average form, as due to adaptive modifications. If here one vertebral ele ment is largely developed, while elsewhere it is small— if now the form, now the position, now the degree of coalescence, of a given part varies ; it is that the local requirements have involved this change. The entire teaching of comparative osteology implies that 533 differences in the conditions of the respective vertebrae necessitate differences in their structures. Now, it seems to us that the first step towards a right conception of the phenomena, is to recognize this general law in its converse application. If vertebrae are unlike in proportion to the unlikeness of their circumstances, then, by implication, they will be like in pro¬ portion to the likeness of their circumstances. While successive segments of the same skeleton, and of different skeletons, are all in some respects more or less differently acted on by incident forces, and are therefore required to be more or less different; they are all, in other respects, similarly acted on by incident forces, and are therefore required to be more or less similar. It is impossible to deny that if differences in the mechanical functions of the vertebrae involve differences in their forms ; then, community in them mechani¬ cal functions, must involve community in their forms. And as we know that throughout the Vertebrata generally, and in each vertebrate animal, the vertebra}, amid all their varying circumstances, have a certain community of function, it follows necessarily that they will have a certain general resemblance — there will recur that average shape which has suggested the notion of a pattern vertebra. A glance at the facts at once shows their harmony with this conclusion. In an eel or a snake, where the bodily actions are such as to involve great homogeneity in the mechanical conditions of tho vertebrae, the series of them is comparatively homogeneous. On the contrary, in a mammal or a bird, where there is considerable hetero- „ geneity in their circumstances, their similarity is no longer so great. And if, instead of comparing the vertebral columns of different animals, we compare the successive vertebrae of any one animal, we recognize the same law. In the segments of an individual spine, where is there the greatest divergence from the common mechanical conditions ? and where may we therefore expect to find the widest departure from the average form ? Obviously at the two extremities. And accordingly it is at the two extremities that the ordinary struc¬ ture is lost. Still clearer becomes the truth of this view, when we consider the genesis of the vertebral column as displayed throughout the ascend¬ ing grades of the Vertebrata. In its first embryonic stage, the spine is an undivided column of flexible substance. In the early fishes, while some of the peripheral elements of the vertebrm were marked out, the central axis was still a continuous unossified cord. And thus we have good reason for thinking that in the primitive verte¬ brate animal, as in the existing Amphinxus , the notochord was per¬ sistent. The production of a higher, more powerful, more active creature of the same type, by whatever method it is conceived to have taken place, involved a change in the notochordal structure. Greater muscular endowments presupposed a firmer internal fulcrum 534 — a less yielding central axis. On the other hand, for the central axis to have become firmer while remaining continuous, would have entailed a stillness incompatible with the creature’s movements. Hence, increasing density of the central axis necessarily went hand in hand with its segmentation: for strength, ossification was re¬ quired ; for flexibility, division into parts. The production of ver¬ tebrae resulting thus, there obviously would arise among them a general likeness, due to the similarity in their mechanical conditions, and more especially the muscular forces bearing on them. And then observe, lastly, that where, as in the head, the terminal position and the less space for development of muscles, entailed smaller lateral bendings, the segmentation would naturally be less decided, less regular, and would be lost as we approached the front of the head. But, it may be replied, this hypothesis does not explain all the facts. It does not tell us why a bone whose function in a given animal requires it to be solid, is formed not of a single piece, but by the coalescence of several pieces, which in other creatures are sepa¬ rate ; it does not account for the frequent manifestations of unity of plan in defiance of teleological requirements. This is quite true. But it is not true, as Professor Owen argues respecting such cases, that “ if the principle of special adaptation fails to explain them, and we reject the idea that these correspondences are manifestations of some archetypal exemplar, on which it has pleased the Creator to frame certain of his living creatures, there remains only the alterna¬ tive that the organic atoms have concurred fortuitously to produce such harmony.” This is not the only alternative : there is another, which Professor Owen has overlooked. It is a perfectly tenable supposi¬ tion that all higher vertebrate forms have arisen by the superposing of adaptations upon adaptations. Either of the two antagonist cosmo¬ gonies consists with this supposition. If, on the one hand, we con¬ ceive species to have resulted from acts of special creation ; then it is quite a fair assumption that to produce a higher vertebrate animal, the Creator did not begin afresh, but took a lower vertebrate animal, and so far modified its pre-existing parts as to fit them for the new requirements ; in which case the original structure would show itself through the superposed modifications. If, on the other hand, we conceive species to have resulted by gradual differentiations under the influence of changed conditions; then, it would mani¬ festly follow that the higher, heterogeneous forms, would bear traces of the lower and more homogeneous forms from which they were evolved. Thus, besides finding that the hypothesis of an “ ideal typical vertebra ” is irreconcilable with the facts, we find that the facts are interpretable without gratuitous assumptions. The average com¬ munity of form which vertebrae display, is explicable as resulting from natural causes. And those typical similarities which are trace¬ able under adaptive modifications, must obviously exist if, through¬ out creation in general, there has gone on that continuous super¬ posing of modifications upon modifications which goes on in every unfolding organism. L'l might with propriety have added to the foregoing criticisms, the remark that Professor Owen has indirectly conferred a great benefit by the elaborate investigations he has made with the view of establishing his hypothesis. He has himself very conclusively proved that the teleological interpretation is quite irreconcilable with the facts. In gathering together evidence in support of his own con¬ ception of archetypal forms, he has disclosed adverse evidence which I think shows his conception to be untenable. The result is that the field is left clear for the hypothesis of Evolution as the only tenable one.J APPENDIX C. [From the Transactions of the Linnean Society, yol. xxy.] XV. On Circulation and the Formation of Wood in Plants. By Herbert Spencer, Esq. Communicated by George Busk, Esq., E.P.S., Sec. L.S. Read March 1st, 1SG6. Opinions respecting the functions of the vascular tissues in plants appear to make but little progress towards agreement. The suppo¬ sition that these vessels and strings of partially-united cells, lined with spiral, annular, reticulated, or other frameworks, are carriers of the plant-juices, is objected to on the ground that they often contain air : as the presence of air arrests the movement of blood through arteries and veins, its presence in the ducts of stems and petioles is assumed to unfit them as channels for sap. On the other hand, that these structures have a respiratory office, as some have thought, is certainly not more tenable, since, if the presence of air in them negatives the belief that their function is to dis¬ tribute liquid, the presence of liquid in them equally negatives the belief that their function is to distribute air. Nor can any better defence be made for the hypothesis which I find propounded, that these parts serve “ to give strength to the parenchyma.” Tubes with fenestrated and reticulated internal skeletons have, indeed, some power of supporting the tissue through which they pass ; but lubes lined with spiral threads can yield extremely little support, while tubes lined with annuli, or spirals alternating with annuli, can yield no support whatever. Though all these types of internal framework are more or less efficient for preventing closure by lateral pressure, they are some of them quite useless for holding up the mass through which the vessels pass ; and the best of them are for this purpose mechanically inferior to the simple cylinder. The same quantity of matter made into a continuous tube would be more effective in giving stiffness to the cellular tissue around it. In the absence of any feasible alternative, the hypothesis that these vessels are distributors of sap claims reconsideration. The objections are not, I think, so serious as they seem. The habitual presence of air in the ducts that traverse wood, can scarcely be held anomalous if when the wood is formed their function ceases. The canals which ramify through a Stag’s horn, contain air after the Stag’s horn is fully developed ; but it is not thereby rendered doubtful whether it is the function of arteries to convey blood. Again, that air should frequently be found even in the vessels of petioles and leaves, will not appear remarkable when we call to mind the conditions to which a leaf is subject. Evaporation is going on from it. The thinner liquids pass by osmose out of the vessels into the tissues containing the liquids thickened by evapora¬ tion. And as the vessels are thus continually drained, a draught is made upon the liquid contained in the stem and roots. Suppose that this draught is unusually great, or suppose that around the roots there exists no adequate supply of moisture. A state of capillary tension must result — a tendency of the liquid to pass into the leaves resisted below by liquid cohesion. Now, had the vessels impermeable coats, only their upper extremities would under these conditions be slowly emptied. But their coats, in common with all the surrounding tissues, are permeable by air. Hence, under this state of capillary tension, air will enter ; and as the upper ends of the tubes, being both smaller in diameter and less porous than the •lower, will retain the liquids with greater tenacity, the air will enter the wider and more porous tubes below — the ducts of the stem and branches. Thus the entrance of air no more proves that these ducts are not sap-carriers, than does the emptiness of tropical river-beds in the dry season prove that they are not channels for water. There is, however, a difficulty which seems more serious. It is said that air, when present in these minute canals, must be a great obstacle to the movement of sap through them. The investi¬ gations of Jamin have shown that bubbles in a capillary tube resist the passage of liquid, and that their resistance becomes very great when the bubbles are numerous — reaching, in some experiments, as much as three atmospheres. Nevertheless the inference that any such resistance is offered by the air-bubbles in the vessels of a plant, is, I think, an erroneous one. What happens in a capillary tube having impervious sides, with which these experiments were made, will by no means happen in a capillary tube having pervious sides. . Any pressure brought to bear on the column of liquid con¬ tained in the porous duct of a plant, must quickly cause the expul¬ sion of a contained air-bubble through the minute openings in the coats of the duct. The greater molecular mobility of gases than liquids, implies that air will pass out far more readily than sap. IV hilst, therefore, a slight tension on the column of sap will cause it to part and the air to enter, a slight pressure upon it will force out the air and reunite the divided parts of the column. To obtain data for an opinion on this vexed question, I have 538 lately been experimenting on the absorption of dyes by plants. So far as I can learn, experiments of this kind have most, if not all of them, been made on stems, and, as it would seem from the results, on stems so far developed as to contain all their characteristic structures. The first experiments I made myself were on such parts, and yielded evidence that served but little to elucidate matters. It was only after trying like experiments with leaves of different ages and different characters, and with undeveloped axes, as well as with axes of special kinds, that comprehensible results were reached ; and it then became manifest that the appearances presented by ordinary stems when thus tested, are in a great degree misleading. Let me briefly indicate the differences. If an adult shoot of a tree or shrub be cut off, and have its lower end placed in an alumed decoction of logwood or a dilute solution of magenta,* the dye will, in the course of a few hours, ascend to a distance varying according to the rate of evaporation from the leaves. On making longitudinal sections of the part traversed by it, the dye is found to have penetrated extensive tracts of the woody tissue ; and on making transverse sections, the openings of the ducts appear as empty spaces in the midst of a deeply-coloured prosenchyma. It would thus seem that the liquid is carried up the denser parts of the vascular bundles ; neglecting the cambium layer, neglecting the central pith, and neglecting the spiral vessels of the medullary sheath. Apparently the substance of the wood has afforded the readiest channel. When, however, we examine these appearances critically, we find reasons for doubting this conclusion. If a transverse section of the lower part, into which the dye passed first and has remained longest, be compared with a transverse sec¬ tion of the part which the dye has but just reached, a marked difference is visible. In the one case the whole of the dense tissue is stained ; in the other case it is not. This uneven distribution of stain in the part which the dye has incompletely permeated is not at random ; it admits of definite description. A tolerably regular continuous ring of colour distinguishes the outer part of the wood from the inner mass, implying a passage of liquid up the elongated cells next the cambium layer. And the inner mass is coloured more round the mouths of the pitted ducts than elsewhere : the dense tissue is darkest close to the edges of these ducts ; the colour fades away gradually on receding from their edges ; there is most colour where there are several ducts together ; and the dense tissue which * These two dyes have affinities for different components of the tissues, and may be advantageously used in different cases. Magenta is rapidly taken up by woody matter and other secondary deposits ; while logwood colours the cell-membranes, and takes but reluctantly to the substances seized by magenta. By trying both of them on the same structure, we may guard ourselves against any error arising from selective combination. 539 Is fully dyed for some space, is that which lies between two or more ducts. These are indications that while the layer of pitted cells next the cambium has served as a channel for part of the liquid, the rest has ascended the pitted ducts, and oozed out of these into the prosenchyma around. And this conclusion is confirmed by the contrast between the appearances of the lowest part of a shoot under different conditions. Tor if, instead of allowing the dye time for oozing through the prosenchyma, the end of the shoot be just dipped into the dye and taken out again, we find, on making transverse sections of the part into which the dye has been rapidly taken up, that, though it has diffused to some distance round the ducts, it has left tracts of wood between the ducts uncoloured — a difference which would not exist had the ascent been through the substance of the wood. Even still stronger is the confirmation obtained by using one dye after another. If a shoot that has ab¬ sorbed magenta for an hour be placed for five minutes in the log¬ wood decoction, transverse sections of it taken at a short distance from its end show the mouths of the ducts surrounded by dark stains in the midst of the much wider red stains. Based on these comparisons only, the inference pointed out has little weight ; but its ' weight is increased by the results of experi¬ ments on quite young shoots, and shoots that develope very little wood. The behaviour of these corresponds perfectly with the ex¬ pectation that a liquid will ascend capillary tubes in preference to simple cellular tissue or tissue not differentiated into continuous canals. The vascular bundles of the medullary sheath are here the only channels which the coloured liquid takes. In sections of the parts up to which the dye has but just reached, the spiral, fenestrated, scalariform, or other vessels contained in these bundles are alone coloured ; and lower down it is only after some hours that such an exudation of dye takes place as suffices partially to colour the other substances of the bundle. Further, it is to be noted that at the terminations of shoots, where the vessels are but incompletely formed out of irregularly- joined fibrous cells which still retain their original shapes, the dye runs up the incipient vessels and does not colour in the smallest degree the surrounding tissue. Experiments with leaves bring out parallel facts. On placing in a dye a petiole of an adult leaf of a tree, and putting it before tho fire to accelate evaporation, the dye will be found to ascend tho midrib and veins at various rates, up even to a foot per hour. At first it is confined to the vessels ; but by the time it has reached the point of the leaf, it will commonly be seen that at the lower part it has diffused itself into the sheaths of the vessels. In a quite young leaf from the same shoot, we find a much more rigorous restriction of the dye to the vessels. On making oblique sections of its petiole, midrib, and veins, the vessels have the appearance of groups of 540 sharply defined coloured rods imbedded in the green prosenchyma ; and this marked contrast continues with scarcely an appreciable change after plenty of time has been allowed for exudation. The facts thus grouped and thus contrasted seem, at first sight, to imply that while they are young the coats of these ramifying canals lined with spiral or allied structures are not readily perme¬ able, but that, becoming porous as they grow old, they allow the liquids they carry to escape with increasing facility ; and hence a possible interpretation of the fact that, in the older parts, the stain¬ ing of the tissue around the vessels is so rapid as to suggest that the dye has ascended directly through this tissue, whereas in the younger parts the reverse appearance necessitates the reverse conclusion. But now, is this difference determined by difference of age, or is it otherwise determined ? The evidence as presented in ordinary stems and leaves shows us that the parts of the vascular system at which there is a rapid escape of dye are not simply older parts, but are parts where a deposit of woody matter is taking place. Is it, then, that the increasing permeability of the ducts, instead of being directly associated with their increasing age, is directly associated with the increasing deposit of dense substance around them ? To get proof that this last connexion is the true one, we have but to take a class of cases in which wood is formed only to a small extent. In such cases experiments show us a far more general and continued limitation of the dye to the vessels. Ordinary herbs and vegetables, when contrasted with shrubs and trees, illustrate this ; as instance the petioles of Celery, or of the common Dock, and the leaves of Cabbages or Turnips. And then in very succulent plants, such as Bryophyllum calycinum , Kalanchoe rotundifolia, the various species of Crassula , Cotyledon, Kleinia , and others of like habit, the ducts of old and young leaves alike retain the dye very persistently : the concomitant in these cases being the small amount of prosen¬ chyma around the ducts, or the small amount of deposit in it, or both. More conclusive yet is the evidence which meets us when we turn from very succulent leaves to very succulent axes. The tender young shoots of Kleinia ante-euphorbium, or Euphorbia Mauritanica, which for many inches of their lengths have scarcely any ligneous fibres, show us scarcely any escape of the coloured liquid from the vessels of the medullary sheath. So, too, is it with Stapelia Buffonia , a plant of another order, having soft swollen axes. Ana then we have a repetition of the like connexion of facts throughout the Cactacece : the most succulent showing us the smallest perme¬ ability of the vessels. In two species of Rhipsalis , in two species of Cereus , and in two species of Mammillaria, which I have tried, I have found this so. Mammillaria gracilis may be named as ex¬ emplifying the relation under its extreme form. Into one of these • small spheroidal masses, the dye ascends through the large bundles 541 of spiral or annular ducts, or cells partially united into such ducts, colouring them deeply, and leaving the feebly-marked sheath of prosenchyma, together with the surrounding watery cellular tissue, perfectly uncoloured. The most conclusive evidence, however, is furnished by those Caciaceo! in which the transition from succulent to dense tissue takes place variably, according as local circumstances determine. Opuntia yields good examples. If a piece of it including one of the joints at which wood is beginning to form, be allowed to absorb a coloured liquid, the liquid, running up the irregular bundles of vessels and into many of their minute ramifications, is restricted to these where they pass through the parenchyma forming the mass of the stem; but near the joints the hardened tissue around the vessels is coloured. In one of these fleshy growths we get clear evidence that the escape of the dye has no immediate dependence on the age of the vessels, since, in parts of the stem that are alike in age, some of the vessels retain their contents while others do not. Nay, we even find that the younger vessels are more pervious than the older ones, if round the younger ones there is a formation of wood. Thus, then, is confirmed the inference before drawn, that in ordi¬ nary stems the staining of the wood by an ascending coloured liquid is due, not to the passage of the coloured liquid up the substance of the wood, but to the permeability of its ducts and such of its pitted cells as are united into irregular canals. And the facts showing this, at the same indicate with tolerable clearness the process by which wood is formed. What in these cases is seen to take place with a dye, may be fairly presumed to take place with sap. Where the dye exudes but slowly, we may infer that the sap exudes but slowly ; and it is a fair inference that where the dye leaks rapidly out of the vessels, the sap does the same. Inferring, thus, that where- ever there is a considerable formation of wood there is a considerable escape of the sap, we see in the one the result of the other. The thickening of the prosenchyma is proportionate to the quantity of nutritive liquid passing into it ; and this nutritive liquid passes into it from the vessels, ducts, and irregular canals it surrounds. But an objection is made to such experiments as the foregoing, and to all the inferences drawn from them. It is said that portions of plants cut off and thus treated, have their physiological actions arrested, or so changed as may render the results misleading ; and it is said that when detached shoots and leaves have their cut ends placed in solutions, the open mouths of their vessels and ducts are directly presented with the liquids to be absorbed, which does not happen in their natural states. Further, making these objections look serious, it is alleged that when solutions are absorbed through the roots, quit£ different results are obtained : the absorbed matters are found in the tissues and not in the vessels. Clearly, were the ex- 512 periments yielding these adverse results conducted in unobjectionable ways, the conclusion implied by them would negative the conclusions above drawn. But these experiments are no less objectionable than those to which they are opposed. Such mineral matters as salts of iron, solutions of which have in some cases been supplied to the roots for their absorption, are obviously so unlike the matters ordinarily absorbed, that they are likely to interfere fatally with the physiological actions. If experiments of this kind are made by immersing the roots in a dye, there is, besides the difficulty that the mineral mordant contained by the dye is injurious to the plant, the further difficulty that the colouring matter, being seized by the substances for which it has an affinity, is left behind in the first layers of root tissues passed through, and that the decolorized water passing up into the plant is not trace¬ able. To be conclusive, then, an experiment on absorption through roots must be made with some solution which will not seriously in¬ terfere with the plant’s vital processes, and which will not have its distinctive element left behind. To fulfil these requirements I adopted the following method. Having imbedded a well-soaked broad-bean in moist sand, contained in an inverted cone of card¬ board with its apex cut off for the radicle to come through — having placed this in a wide-mouthed dwarf bottle, partly filled with water, so that the protruding radicle dipped into the water — and having waited until the young bean had a shoot some three or more inches high, and a cluster of secondary rootlets from an inch to an inch and a-half long — I supplied for its absorption a simple decoction of logwood, which, being a vegetal matter, was not likely to do it much harm, and which, being without a mordant, would not leave its sus¬ pended colour in the first tissues passed through. To avoid any possible injury, I did not remove the plant from the bottle, but slightly raising the cone out of its neck, I poured away the water through the crevice and then poured in the logwood decoction ; so that there could have been no broken end or abraded surface of a rootlet through which the decoction might enter. Being prepared with some chloride of tin as a mordant, I cut off, after some three hours, one of the lowest leaves, expecting that the application of the mordant to the cut surface would bring out the characteristic colour if the logwood decoction had risen to that height. I got no re¬ action, however. But after eight hours I found, on cutting off another leaf, that the vessels of its petiole were made visible as dark streaks by the colour with which they were charged — a colour differ¬ ing, as was to be expected, from that of the logwood decoction, which spontaneously changes even by simple exposure. It was then too late in the day to pursue the observations ; but next morning the vessels of the whole plant, as far as the petioles of its highest un¬ folded leaves, were full of the colouring-matter ; and on applying chloride of tin to the cut surfaces, the vessels assumed that purplish 543 red which this mordant produces when directly mixed with the Io^ wood decoction. Subsequently, when one of the cotyledons was cut open by Prof. Oliver, to whom, in company with Dr. Hooker, I showed the specimen, we found that the whole of its vascular system was filled with the decoction, which everywhere gave the characteristic reaction. And it became manifest that the liquid absorbed through the rootlets, in the central vessels of which it was similarly traceable, had part of it passed directly up the vessels of the axis, while part of it had passed through other vessels into the cotyledon, out of which, no doubt, the liquid ordinarily so carried returns charged with a supply of the stored nutriment. I have since obtained a verification by varying the method. Digging up some young plants (Marigolds happened to afford the best choice) with large masses of soil round them, placing them in water, so as gradually to detach the soil with¬ out injuring the rootlets, planting them afresh in a flower-pot full of washed sand,, and then, after a few days, watering them with a 1o 111 conformity with ordinary embryological laws, tne cells of the parts exposed to such actions assume these special structures irrespective of the actions— the actions, however, still serving to aid and complete the assumption of the inherited type ® Vol IL 24^ 552 Another side of the general question may now be considered. We have seen how, by intermittent pressures on capillary vessels and ducts and inosculating canals, there must be produced a draught of sap towards the point of compression to replace the sap squeezed out. But we have still to inquire what will be the effect on the distribu¬ tion of sap throughout the plant as a whole. It was concluded that out of the compressed vessels the greater part of the liquid would escape longitudinally — the longitudinal resistance to movement being least. In every case the probabilities are infinity to one against the resistances being equal upwards and downwards. Always, then, more sap will be expelled in one direction than in the other. But in whichever direction least sap is expelled, from that same direction most sap will return when the vessels are relieved from pressure — the force which is powerful in arresting the back current in that direction being the same force which is powerful in producing a forward cur¬ rent. Ordinarily, the more abundant supply of liquid being from below, there will result an upward current. At each bend a portion of the con¬ tents will be squeezed out through the sides of the vessels — a portion will be squeezed downwards, reversing the current ascending from the roots, but soon stopped by its resistance ; while a larger portion will be squeezed upwards towards the extremities of the vessels, where consumption and loss are most rapid. At each recoil the vessels will be replenished, chiefly by the repressed upward current ; and at the next bend more of it will be thrust onwards than backwards. Hence we have everywhere in action a kind of rude force-pump, worked by the wind ; and we see how sap may thus be raised to a height far beyond that to which it could be raised by capillary action, aided by osmose and evaporation. Thus far, however, the argument proceeds on the asumption that there is liquid enough to replenish every time the vessels subject to this process. But suppose the supply fails — suppose the roots have exhausted the surrounding stock of moisture. Evidently the vessels thus repeatedly having their contents squeezed out into the surrounding tissue, cannot go on refilling themselves from other vessels without tending to empty the vascular system. On the one hand, evaporation from the leaves causing a draught on the capillary tubes that end in them, continually generates a capillary tension up¬ wards ; while, on the other hand, the vessels below, expanding after their sap has been squeezed out, produce a tension both upwards and downwards towards the point of loss. Were the limiting mem¬ branes of the vessels impermeable, the movement of sap would, under these conditions, soon be arrested. But these membranes are perme¬ able ; and the surrounding tissues readily permit the passage of ah. This state of tension, then, will cause an entrance of air into the tubes : the columns of liquid they contain will be interrupted by bubbles. It seems, indeed, not improbable that this entrance of air may take place even when there is a good supply of liquid, if the mechanical strains are so violent and the exudation so rapid that the currents cannot refil the half-emptied vessels with sufficient rapidity. And in this case the intruding air may possibly play the same part as that contained in the air-chamber of a force-pump — tending, by moderat¬ ing the violence of the jets, and by equalizing the strains, to prevent rupture of the apparatus. Of course when the supply of liquid becomes adequate, and the strains not too violent, these bubbles will be expelled as readily as they entered. . Here, as before, let me add the conclusive proof furnished by a direct experiment. To ascertain the amount of this propulsive action, I took from the same tree, a Laurel, two equal shoots, and placing them in the same dye, subjected them to conditions that were alike in all respects save that of motion: while one remained at rest, the other was bent backwards and forwards, now by switch¬ ing and now by straining with the fingers. After the lapse of an hour, I found that the clye had ascended the oscillating shoot three times as far as it had ascended the stationary shoot — this result being an average from several trials. Similar trials brought out similar effects in other structures. The various petioles and herba¬ ceous shoots experimented upon for the purpose of ascertaining the amount of exudation produced by transverse strains, showed also the amount of longitudinal movement. It was observable that the height ascended by the dye was in all cases greater where there had been oscillation than where there had been rest— the difference, however, being much less marked in succulent structures than in woody ones. It need scarcely be said that this mechanical action is not here assigned as the sole cause of circulation, but as a cause co-operating with others, and helping others to produce effects that could not otherwise be produced. Trees growing in conservatories afford us abundant proof that sap is raised to considerable heights by other forces. Though it is notorious that trees so circumstanced do not thrive unless, through open sashes, they are frequently subject to breezes sufficient to make their parts oscillate, yet there is evidently a circulation that goes on without mechanical aid. The causes of circulation are those actions only which disturb the liquid equilibrium in a plant, by permanently abstracting water or sap from some part of it ; and of these the first is the absorption of materials for the for¬ mation of new . tissue in growing parts ; the second is the loss bv evaporation, mainly through adult leaves ; and the third is the loss by extravasation, through compressed vessels. Only so far as it pro¬ duces this last, can mechanical strain be regarded as truly a cause of circulation. All the other actions concerned must be classed as aids to circulation — as facilitating that redistribution of liquid that con¬ tinually restores the equilibrium continually disturbed ; and of these. 554 capillary action may be named as the first, osmose as the second, and the propulsive effect of mechanical strains as the third. The first two of these aids are doubtless capable by themselves of producing a large part of the observed result — more of the observed result than is at first sight manifest ; for there is an important indirect effect of osmotic action which appears to be overlooked. Osmose does not aid circulation only by setting up, within the plant, exchange currents between the more dense and the less dense solutions in different parts of it ; but it aids circulation much more by producing distention of the plant as a whole. In consequence of the average contrast in density between the water outside of the plant and the sap inside of it, the constant tendency is for the plant to absorb a quantity in excess of its capacity, and so to produce distention and erection of its tissues. It is because of this that the drooping plant raises itself when watered ; for capillary action alone could only refill its tissues without changing their attitudes. And it is because of this that juicy plants with collapsible structures bleed so rapidly when cut, not only from the cut surface of the rooted part, but from the cut sur¬ face of the detached part — the elastic tissues tending to press out the liquid which distends them. And manifestly if osmose serves thus to maintain a state of distention throughout a plant, it indirectly fur¬ thers circulation ; since immediately evaporation or growth at any part, by abstracting liquid from the neighbouring tissues, begins to diminish the liquid pressure within such tissues, the distended struc¬ tures throughout the rest of the plant thrust their liquid contents to¬ wards the place of diminished pressure. This, indeed, may very pos¬ sibly be the most efficient of the agencies at work. Remembering how great is the distention producible by osmotic absorption — great enough to burst a bladder — it is clear that the force with which the distended tissues of a plant urge forward the sap to places of con¬ sumption, is probably very great. W e must therefore regard the aid which mechanical strains give as being one of several. Oscillations help directly to restore any disturbed liquid equilibrium ; and they also help indirectly, by facilitating the redistribution caused by capil¬ lary action and the process just described ; but in the absence of oscillations the equilibrium may still be restored, though less rapidly and within narrower limits of distance. One half of the problem of the circulation, however, has been left out of sight. Thus far our inquiry has been, how the ascending cur¬ rent of sap is produced. There remains the rationale of the descend¬ ing current. What forces cause it, and through what tissues it takes place, are questions to which no satisfactory answers have been given. That the descent is due to gravitation, as some allege, is difficult to conceive, since, as gravitation acts equally on all liquid columns contained in the stem, it is not easy to see why it should produce downward movements in some while per- 555 mitting upward movements in others — unless, indeed, there existed descending tubes too wide to admit of much capillary action, which there do not. Moreover, gravitation is clearly inadequate to cause currents towards the roots out of branches that droop to the ground. Here the gravitation of the contained liquid columns must nearly balance that of the connected columns in the stem, leaving no appreciable force to cause motion. Nor does there seem much probability in the assumption that the route of the descending sap is through the cambium layer, since experiments on the absorp¬ tion of dyes prove that simple cellular tissue is a very bad conductor of liquids : their movement through it does not take place with one- fiftieth of the rapidity with which it takes place through vessels.* Of course the defence for these hypotheses is, that there must be a downward current, which must have a course and a cause ; and the very natural assumption has been that the course and the cause must be other than those which produce the ascending current. Never¬ theless there is an alternative supposition, to which the foregoing considerations introduce us. It is quite possible for the same vascular system to serve as a channel for movement in opposite directions at different times. We have among animals well-known cases in which the blood-vessels carry a current first in one direction and then, after a brief pause, in the reverse direction. And there seems an ct, priori probability that, lowly organized as they are, plants are more likely to have distributing appliances of this imperfect kind than to have two sets of channels for two simultaneous currents. If, led by this suspicion, we inquire whether among the forces which unite to produce movements of sap, there are any variations or inter¬ missions capable of determining the currents in different directions, we quickly discover that there are such, and that the hypothesis of an alternating motion of the sap, now 'centrifugal and now centri¬ petal, through the same vessels, has good warrant. What are the several forces at work? First may be set down that tendency existing in every part of a plant to expand into its typical form, and to absorb nutritive liquids in doing this. The resulting competition * Some exceptions to this occur in plants that have retrograded in the character of their tissues towards the simpler vegetal types. Certain very succulent leaves, such as those of Sempervivum, in which the cellular tissue is immensely developed in comparison with the vascular tissue, seem to have resumed to a considerable extent what we must regard as the primitive form of vegetal circulation — simple absorption from cell to cell. Theso, when they have lost much of their water, will take up the dye to some dis¬ tance through their general substance, or rather through its interstices, even neglecting the vessels. At other times, in the same leaves, the vessels will become charged while comparatively little absorption takes place through the cellular tissue. Even in these exceptional cases, however, the movement through cellular tissue is nothing like as fast; as the movement through vessels. ° for sap will, other things being equal, cause currents towards the most rapidly-growing parts — towards unfolding shoots and leaves, but not towards adult leaves. Next we have evaporation, acting more on the adult leaves than on those which are in the bud, or but partially developed. This evaporation is both regularly and irregularly intermittent. Depending chiefly on the action of the sun, it is, in fine weather, greatly checked or wholly arrested every evening ; and in cloudy weather must be much retarded during the day. Further, every hygrometric variation, as well as every variation in the movement of the air, must vary the evaporation. This chief action, therefore, which, by con¬ tinually emptying the ends of the capillary tubes, makes upward currents possible, is one which intermits every night, and every day is strong or feeble as circumstances determine. Then, in the third place, we have this rude pumping process above described, going on with greater vigour when the wind is violent, and with less vigour when it is gentle — drawing liquid towards different parts according to their degrees of oscillation, and from diffe¬ rent parts according as they can most readily furnish it. And now let us ask what must result under changing conditions from these variously-conflicting and conspiring forces. When a warm sunshine, causing rapid evaporation, is emptying the vessels of the leaves, the osmotic and capillary actions that refill them will be continually aided by the pumping action of the swaying petioles, twigs, and branches, provided their oscillations are moderate. Under these conditions the current of sap, moving in the direction of least resistance, will set towards the leaves. But what wall happen when the sun sets? There is now nothing to determine currents either upwards or downwards, except the relative rates of growth in the parts and the relative demands set up by the oscillations ; and the oscillations acting alone, will draw sap to the oscillating parts as much from above as from below. If the resistance to be overcome by a current setting back from the leaves is less than the resistance to be overcome by a current setting up from the roots, then a current will set back from the leaves. Now it is, I think, tolerably manifest that in the swaying twigs and minor branches, less force will be required to overcome the inertia of the short columns of liquid between them and the leaves than to overcome the inertia of the long columns between them and the roots. Hence during the night, as also at other times when evaporation is not going on, the sap will be drawn out of the leaves into the adjacent supporting parts ; and them nutrition will be increased. If the wind is strong enough to produce a swaying of the thicker branches, the back current will extend to them also ; and a further strengthening will result from their absorption of the elaborated sap. And when the great branches and the stem are bent backwards and forwards by a m? gale, they too will share in the nutrition. It may at first sight seem that tlmse parts, being nearer to the roots than to the leaves, will draw tneir supplies from the roots only. But the quantity which the roots can furnish is insufficient to meet so great a demand. Under the conditions described, the exudation of sap from the vessels will be very great, and the draught of liquid required to refill them, not satisfied by that which the root-fibres can take in, will extend to the leaves. Thus sap will flow to the several parts according to their respective degrees of activity — to the leaves while light and heat enable them to discharge their functions, and back to the twigs, branches, stem, and roots when these become active and the leaves inactive, or when their activity dominates over that of the leaves. And this distribution of nutriment, varying with the varying activities of the parts, is just such a distribution as we know must be required to keep up the organic balance. To this explanation it may be objected that it does not account for the downward current of sap in plants that are sheltered. The stem and roots of a drawing-room Geranium display a thickening which implies that nutritive matters have descended from the leaves, although there are none of those oscillations by which the sap is said to be drawn downwards as well as upwards. The reply is, that the stem and roots tend to repeat their typical structures, and that the absorption of sap for the formation of their respective dense tissues, is here the force which determines the descent. Indeed it must be borne in mind that the mechanical strains and the pumping process which they keep up, as well as the distention caused by osmose, do not in themselves produce a current either upwards or downwards : they simply help to move the sap towards that place where there is the most rapid abstraction of it — the place towards which its motion is least resisted. Whether there is oscillation or whether there is not, the physiological demands of the different parts of the plant determine the direction of the current ; and all which the oscillations and the distention do is to facilitate the supply of these demands. Just as much, therefore, in a plant at rest as in a plant in motion, the current will set downwards when the function of the leaves is arrested, and when there is nothing to resist that abstraction of sap caused by the tendency of the stem- and root-tissues to assume their typical structures. To which admission, however, it must be added- that since this typical structure assumed, though imperfectly assumed, by the hot-house plant, is itself interpretable as the inherited effect of external mechanical actions on its ancestors, we may still consider the current set up by the assumption of the typical structure to be indirectly due to such actions. Interesting evidence of another order here demands notice. In the • course of experiments on the absorption of dyes by leaves, it happened that in making sections parallel to the plane of a leaf, with 558 the view of separating its middle layer containing the vessels, I came upon some structures that were new to me. These structures, where they are present, form the terminations of the vascular system. They are masses of irregular and imperfectly united fibrous cells, such as those out of which vessels are developed ; and they are sometimes slender, sometimes bulky — usually, however, being more or less club- shaped. In transverse sections of leaves their distinctive characters are not shown : they are taken for the smaller veins. It is only by carefully slicing away the surface of a leaf until we come down to that part which contains them, that we get any idea of their nature. Fig. 1 represents a specimen taken from a- leaf of Euphorbia neriifolia. Occupying one of the interspaces of the ulti¬ mate venous network, it consists of a spirally-lined duct or set of ducts, which connects with the neighbouring vein a cluster of half- reticulated, half-scalariform cells. These cells have projections, many of them tapering, that insert themselves into the adjacent intercellular spaces, thus producing an extensive surface of contact between the organ and the imbedding tissues. A further trait is, that the en- sheathing prosenchyma is either but little developed or wholly ab¬ sent ; and consequently this expanded vascular structure, especially at its end, comes immediately in contact with the tissues concerned in assimilation. The leaf of Euphorbia neriifolia is a very fleshy one ; and in it these organs are distributed through a compact, though watery, cellular mass. But in any leaf of the ordinary type which possesses them, they lie in the network parenchyma composing its lower layer ; and wherever they occur in this layer its cells unite to enclose them. This arrangement is shown in fig. 2, representing a sample from the Caoutchouc-leaf, as seen with the upper part of its envelope removed ; and it is shown still more clearly in a sample from the leaf of Panax Lessonii , fig. 3. Figures 4 and 5 represent, without their sheaths, other such organs from the leaves of Panax Lessonii and Clusia flava. Some relation seems to exist between their forms and the thicknesses of the layers in which they lie. Certain very thick leaves, such as those of Clusia flava , have them less abundantly distributed than is usual, but more massive. Where the parenchyma is developed not to so great an extreme, though still largely, as in the leaves of Holly, Aucuba , Camellia , they are not so bulky ; and in thinner leaves, like those of Privet, Elder, &c., they become longer and less conspicuously club-shaped. Some adaptations to their respective positions seem implied by these modi¬ fications ; and we may naturally expect that in many thin leaves these free ends, becoming still narrower, lose the distinctive and suggestive characters possessed by those shown in the diagrams. Relations of this kind are not regular, however. In various other genera, members of which I have examined, as Rhus, Viburnum , Griselinia , Rrexia, Botryodendron , Pereskia, the variations in the 55$ bulk and form of these structures are not directly determined by the spaces which the leaves allow : obviously there are other modi¬ fying causes. It should be added that while hese expanded free extremities graduate into tapering free extremities, not differing from ordinary vessels, they also pass insensibly into the ordinary in¬ osculations. Occasionally, along with numerous free endings, there occur loops ; and from such loops there are transitions to the ulti¬ mate meshes of the veins. These organs are by no means common to all leaves. In many that afford ample spaces for them they are not to be found. So far as I have observed, they are absent from the thick leaves of plants which form very little wood. In Sempervivum , in Echzveria , in Bryophyllum, , they do not appear to exist ; and I have been unable to discover them in Kcilanchoe rotundifolia , in Kleinici ante-euphorbium and Jicoides, in the several species of Crassula , and in other succulent plants. It may be added that they are not absolutely confined to leaves, but occur in stems that have assumed the functions of leaves. At least I have found, in the green parenchyma of Opuntia , organs that are analogous though much more rudely and irregularly formed. In other parts, too, that have usurped the leaf-function, they occur, as in the phyllodes of the Australian Acacias. These have them abundantly developed ; and it is interesting to observe that here, where the two vertically-placed surfaces of the flattened-out petiole are equally adapted to the assimilative function, there exist two layers of these expanded vascular terminations, one applied to the inner surface of each layer of parenchyma. Considering the structures and positions of these organs, as well as the natures of the plants possessing them, may we not form a shrewd suspicion respecting their function ? Is it not probable that they facilitate absorption of the juices carried back from the leaf for the nutrition of the stem and roots ? They are admirably adapted for performing this office. Their component fibrous cells, having angles insinuated between the cells of the parenchyma, are shaped just as they should be for taking up its contents ; and the absence of sheathing tissue between them and the parenchyma facilitates the passage of the elaborated liquids. Moreover there is the fact that they are allied to organs which obviously have absorbent functions. I am indebted to Dr. Hooker for pointing out the figures of two such organs in the “ leones Anatomicse” of Link. One of them is from the end of a dicotyledonous root-fibre, and the other is from the prothallus of a young Fern. In each case a cluster of fibrous cells, seated at a place from which liquid has to be drawn, is con¬ nected by vessels with the parts to which liquid has to be carried. There can scarcely be a doubt, then, that in both cases absorption is effected through them. I have met with another such organ, more elaborately constructed, but evidently adapted to the same office, in the common Turnip-root. As shown by the end view and longitudinal section in figs. 6 and 7, this organ consists of rings of fenestrated cells, arranged with varying degrees of regu¬ larity into a funnel, ordinarily having its apex directed towards the central mass of the Turnip, with which it has, in some cases at least, a traceable connexion by a canal. Presenting as it does an external porous surface terminating one of the branches of the vascular sys¬ tem, each of these organs is well fitted for taking up with rapidity the nutriment laid by in the Turnip-root, and used by the plant when it sends up its flower-stalk. Nor does even this exhaust the analogies. The cotyledons of the young bean, experimented upon as before described, furnished other examples of such structures, exactly in the places where, if they are absorbents, we might expect to find them. Amid the branchings and inosculations of the vascular layer running through the mass of nutriment deposited in each cotyledon, there are conspicuous free terminations that are club- shaped, and prove to be composed, like those in leaves, of irregularly formed and clustered fibrous cells; and some of them, diverging from the plane of the vascular layer, dip down into the mass of starch and albumen which the young plant has to utilize, and which these structures can have no other function but to take up. Besides being so well fitted for absorption, and besides being similar to organs which we cannot doubt are absorbents, these vascular terminations in leaves afford us yet another evidence of their functions. They are seated in a tissue so arranged as specially to facilitate the abstraction of liquid. The centripetal movement of the sap must be set up by a force that is comparatively feeble, since, the paretes of the ducts being porous, air will enter if the tension on the contained columns becomes considerable. Hence it is needful that the exit of sap from the leaves should meet with very little resistance. Now were it not for an adjustment presently to be described, it would meet with great resistance, notwithstanding the peculiar fitness of these organs to take it in. Liquid cannot be drawn out of any closed cavity without producing a collapse of the cavity’s sides ; and if its sides are not readily collapsible, there must be a corre¬ sponding resistance to the abstraction of liquid from it. Clearly the like must happen if the liquid is to be drawn out of a tissue which cannot either diminish in bulk bodily or allow its components indi¬ vidually to diminish in bulk. In an ordinary leaf, the upper layer of parenchyma, formed as it is of closely-packed cells that are without interspaces, and are everywhere held fast within their framework of veins, can neither contract easily as a mass, nor allow its separate cells to do so. Quite otherwise is it with the network-parenchyma below. The long cells of this, united merely by their ends and having their flexible sides surrounded by air, may severally have their contents considerably increased and decreased without offering 561 appreciable resistances; and. the net work- tissue which they form will, at the same time, be capable of undergoing slight expansions and con¬ tractions of its thickness. In this layer occur these organs that are so obviously fitted for absorption. Here we find them in direct communica¬ tion with its system of collapsible cells. The probability appears to be, that when the current sets into the leaf, it passes through the vessels and their sheaths chiefly into the upper layer of cells (this upper layer having a larger surface of contact with the veins than the lower layer, and being the seat of more active processes) ; and that the juices of the upper layer, enriched by the assimilated matters, pass into the network parenchyma, which serves as a reservoir from which they are from time to time drawn for the nutrition of the rest of the plant, when the actions determine the downward current. Should it be asked what happens where the absorbents, instead of being inserted in a network parenchyma, are, as in the leaves of Euphorbia neriifolia , inserted in a solid parenchyma, the reply is, that such a parenchyma, though not furnished with systematically arranged air-chambers, nevertheless contains air in its intercellular spaces ; and that when there occurs a draught upon its contents, the expansion of this air and the entrance of more from without, quickly supply the place of the abstracted liquid. If then, returning to the general argument, we conclude that these expanded terminations of the vascular system in leaves are absorbent organs, we find a further confirmation of the views set forth respect¬ ing the alternating movement of the sap along the same channels. These spongioles of the leaves, like the spongioles of the roots, being appliances by which liquid is taken up to be carried into the mass of the plant, we are obliged to regard the vessels that end in these spongioles of the leaves as being the channels of the down current whenever it is produced. If the elaborated sap is abstracted from the leaves by these absorbents, then we have no alternative but to suppose that, having entered the vascular system, the elaborated sap . descends through it. And seeing how, by the help of these special terminations, it becomes possible for the same vessels to carry back a quality of sap unlike that which they bring up, we are enabled to understand tolerably well how this rhythmical movement produces a downward transfer of materials for growth. The several lines of argument may now be brought together ; and along with them may be woven up such evidences as remain. Let me first point out the variety of questions to which the hypothesis supplies answers. It is required to account for the ascent of sap to a height beyond that to which capillary action can raise it. This ascent is accounted for by the propulsive action of transverse strains, joined with that of osmotic distention. A cause has to be assigned for that rise of sap 582 which, in the spring, while jet there is no considerable evaporation to aid it, goes on with a power which capillarity does not explain. The co-operation of the same two agencies is assignable for this result also.* The circumstance that vessels and ducts here contain sap and there contain air, and at the same place contain at diiferent seasons now air and now sap is a fact calling for explanation. An explana¬ tion is furnished by these mechanical actions which involve the en¬ trance or expulsion of air according to the supply of liquid. Thai vessels and ducts which were originally active sap-carriers go com¬ pletely out of use, and have their function discharged by other vessels or ducts, is an anomaly that has to be solved. Again, we are supplied with a solution : these deserted vessels and ducts are those which, by the formation of dense tissue outside of them, be¬ come so circumstanced that they cannot be compressed as they originally were. A channel has to be found for the downward current of sap, which, on any other hypothesis than the foregoing, must be a channel separate from that taken by the upward current ; and yet no good evidence of a separate channel has been pointed out. Here, however, the difficulty disappears, since one channel suffices for the current alternating upwards and downwards according to the conditions. Moreover there has to be found a force producing or facilitating the downward current, capable even of drawing sap out of drooping branches ; and no such force is forthcoming. The hypothesis set forth dispenses with this necessity : under the recurring change of conditions, the same distention and oscillation which before raised the sap to the places of consumption, now bring it down to the places of consumption. A physical process has to be pointed out by which the material that forms dense tissue is deposited at the places where it is wanted, rather than at other places. This physical process the hypothesis in¬ dicates. It is requisite to find an explanation of the fact that, when plants ordinarily swayed about by the wind are grown indoors, the formation of wood is so much diminished that they become abnor¬ mally slender. Of this an explanation is supplied. Yet a further * It seems probable, however, that osmotic distention is here, especially, the more important of the two factors. The rising of the sap in spring may indirectly result, like the sprouting of the seed, from the transformation of starch into sugar. During germination, this change of an oxy-hydro-carbon from an insoluble into a soluble form, leads to rapid endosmose ; con¬ sequently to great distention of the seed ; and therefore to a force which thrusts the contained liquids into the plumule and radicle, and gives them power to displace the soil in their way : it sets up an active internal move¬ ment when neither evaporation nor the change which light produces can be operative. And similarly, if, in the spring, the starch stored up in the roots of a tree passes into the form of sugar, the unusual osmotic absorption that arises will cause an unusual distention — a distention which, being resisted by the tough bark of the roots and stem, will result in a powerful upward thrust of the contained liquid. 563 fact to be interpreted is, that in the same individual plant homologous parts, which, according to the type of the plant, should be equally woody, become much thicker one than another if subject to greater mechanical stress. And of this too an interpretation is similarly afforded. Now the sufficiency of the assigned actions to account for so many phenomena not otherwise explained, would be strong evidence that the rationale is the true one, even were it of a purely hypothetical kind. How strong, then, becomes the reason for believing it the true one when we remember that the actions alleged demonstrably go on in the way asserted. They are ever operating before our eyes; and that they produce the effects in question is a conclusion dedu- cible from mechanical principles, a conclusion established by induction, and a conclusion verified by experiment. These three orders of proof may be briefly summed up as follows. That plants which have to raise themselves above the earth’s sur¬ face, and to withstand the actions of the wind, must have a power of developing supporting structure, is an a priori conclusion which may be safely drawn. It is an equally safe ct priori conclusion, that it the supporting structure, either as a whole or in any of its parts, has to adapt itself to the particular strains which the individual plant is subject to by its particular circumstances, there must be at work some process by which the strength of the supporting structure is everywhere brought into equilibrium with the forces it has to bear. Though the typical distribution of supporting structure in each kind of plant may be explained teleologically by those whom teleological explanations satisfy ; and though otherwise this typical distribution may be ascribed to natural selection acting apart from any directly adaptive process ; yet it is manifest that those departures from the typical distribution which fit the parts of each plant to their special conditions are explicable neither teleologically nor by natural selec¬ tion. We are, therefore, compelled to admit that, if in each plant there goes on a balancing of the particular strains by the particular strengths, there must be a physical or physico-chemical process by which the adjustments of the two are effected. Meanwhile we are equally compelled to admit, a priori, that the mechanical actions to be resisted, themselves affect the internal tissues in such ways as to fur¬ ther the increase of that dense substance by which they are resisted. It is demonstrable that bending the petioles, shoots, and stems must compress the vessels beneath their surfaces, and increase the exuda¬ tion of nutritive matters from them, and must do this actively in pro¬ portion as the bends are great and frequent ; so that while, on the one hand, it is a necessary deduction that, if the parts of each plant are to be severally strengthened according to the several strains, there must be some direct connexion between strains and strengths, it is, on the other hand, a necessary deduction from mechanical prin- ciples that the strains do act in such ways as to aid the increase of the strengths. How a like correspondence between two cl 'priori arguments holds in the case of the circulation, needs not to be shown in detail. It will suffice to remind the reader that while the raising of sap to heights beyond the limit of capillarity implies some force to effect it, we have in the osmotic distention and the intermit¬ tent compressions caused by transverse strains, forces which, under the conditions, cannot but tend to effect it ; and similarly with the re¬ quirement for a downward current, and the production of a down¬ ward current. Among the inductive proofs we find a kindred agreement. Diffe¬ rent individuals of the same species, and different parts of the same individual, do strengthen in different degrees ; and there is a clearly traceable connexion between their strengthenings and the intermittent strains they are exposed to. This evidence, derived from contrasts betweeu growths on the same plant or on plants of the same type, is enforced by evidence derived from contrasts between plants of diffe¬ rent types. The deficiency of woody tissue which we see in plants called succulent, is accompanied by a bulkiness of the parts which prevents any considerable oscillations ; and this character is also habi¬ tually accompanied by a dwarfed growth. When, leaving these rela¬ tions as displayed externally, we examine them internally, we find the facts uniting to show, by their agreements and differences, that between the compression of the sap-canals and the production ol wood there is a direct relation. We have the facts, that in each plant, and in every new part of each plant, the formation of sap- canals 'precedes the formation of wood; that the deposit of wood} matter, when it begins, takes place around these sap-canals, anj afterwards around the new sap-canals successively developed ; that this formation of wood around the sap-canals takes place where the coats of the canals are demonstrably permeable, and that the amount of wood-formation is proportionate to the permeability. And then that the permeability and extravasation of sap occur wherever, in the individual or in the type, there are intermittent compressions, is proved alike by ordinary cases and by exceptional cases. In the one class of cases we see that the deposit of wood round the vessels begins to take place when they come into positions that subject them to intermittent compressions, while it ceases when they become shielded from compressions. And in the other class of cases, where, from the beginning, the vessels are shielded from compression by sur¬ rounding fleshy tissue, there is a permanent absence of wrood-forma- tion. To which complete agreement between the deductive and induc¬ tive inferences has to be added the direct proof supplied by experi¬ ments. It is put beyond doubt by experiment that the liquids ab¬ sorbed by plants are distributed to their different parts through their 5S5 vessels — at first by the spiral or allied vessels originally developed, . and then by the better-placed ducts formed later. By experiment it is demonstrated that the intermittent compressions caused by os¬ cillations 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 Ficus 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 leaf of Clusia jlava. Fig. 6. End view of an absorbent organ from the root of a Turnip. It is taken from the outermost layer of vessels. Its funneh shaped interior is drawn as it presents itself when looked at from the outside of this layer, its narrow end being directed towards tbs centre' of the Turnip. Fig. 7 A longitudinal section through the axis of another such organ, showing its annuli of reticulated cells when cut through. The cellular tissue which fills the interior 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 nmet. 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 bo 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 the vertebrate skeleton , there 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 teas unsupported by develop¬ mental evidence. The developmental evidence recently discovered ', how¬ ever, has led Professor Haeckel and others to analogous vieivs respecting the affiliation of the Vertebrata 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 of 1865, at which time the chapter on “ The Shapes of Vertebrate Skeletons ” was written. — December, I860.] The general argument contained in Chap. XVI. of Part IY, I 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 wdien fixation of the larva renders it useless. This disappearance of the tail, however, is not without exception. The Appendicularia is an Ascidian 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. Prom 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 563 APPENDIX. this micl-rib, aro 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 the 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, wTould be perpetuated. What 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 wTould 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 w^ould 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 Appendicidaria , 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 wray 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 same increasing dominance of the organs of external relation, which tho hypothesis itself implies. APPENDIX. 509 Of course, if the rudimentary vertebrate apparatus thus grew into, and spread over, a molluscoid 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 wTould in this case have a different history ; yet the segmentation of them would be eventually determined by the assigned causes. For 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. • ' . ■ . - SPENCER'S SYSTEM OF PHILOSOPHY. THE PHILOSOPHY OF EVOLUTION. By HERBERT SPENCER. Thie great system of scientific thought, the most original and important mes sal undertaking of the age, to which Mr. Spencer has devoted his life, is now well advanced, the published volumes being : First Principles, The Principles of Bi- otogy , two volumes, and The Principles of Psychology , vol. i., which wil\ b« shortly printed. This philosophical system differs from all its predecessors in being solidly based on the sciences of observation and induction ; in representing the order and course of Nature ; in bringing Nature and man, life, mind, and society, under one great law of action ; and in developing a method of thought which may serve for practical guidance in dealing with the affairs of life. That Mr. Spencer is the man for this great work will be evident from the following statements : “ The only complete and systematic statement of the doctrine of Evolution with which I am acquainted is that contained in Mr. Herbert Spencer’s ‘ System Df Philosophy ; 1 a work which should be carefully studied by all who desire to know whither scientific thought is tending.” — T. H. Huxley. “ Of all our thinkers, he is the one who has formed to himself the largest new BClieme of a systematic philosophy.1’— Prof. Masson. “If any individual influence is visibly encroaching on Mills in this country, it is his.” — Ibid. “ Mr. Spencer is one of the most vigorous as well as boldest thinkers that English speculation has yet produced.”— John Stuart Mill. “ One of the acutest metaphysicians of modem times.” — Ibid. “ One of our deepest thinkers.”— Dr. Joseph D. Hooker. It is questionable if any thinker of finer calibre has appeared in our coun¬ try.” — George Henry Lewes. “He alone, of all British thinkers, has organized a philosophy.”— Ibid. “He is as keen an analyst as is known in the history of philosophy ; I do not except either Aristotle or Kant.” — George Ripley. 41 if we were to give our own judgment, we should say that, since Newton, there has not in England been a philosopher of more remarkable speculative and systematizing talent than (in spite of some errors and some narrowness) Mr. He*- Dert Spencer.” — London Saturday Review. « We cannot refrain from offering our tribute of respect to one who, whether for the extent of his positive knowledge, or for the profundity of his speculate insight, has already achieved a name second to none in the whole range of Eng lish philosophy, and whose works will worthily sustain the credit of Rugliali UaoagM La the present generation.”- Westminster Review. D. APPLETON A CO.'S PUBLICATIONS. THE Correlation and Conservation of Forces, WITH AN HTROPUCTION iND BRIEF BIOGEAPIIICAL NOTIOIB By EDWARD H YOUMANS, M.D. 12mo, 490 pages. ■ > CONTENTS L By W. R. Grove. Tlie Correlation of Physical Porces. II. By Prof. Helmholtz. The Interaction of Natural Forces. III. By J. R. Mayer. 1. Remarks on the Forces of Inorganic Nature. 2. On Celestial Dynamics. 3. On the Mechanical Equivalent of Heat. IV. By Dr. Faraday. Some Thoughts on the Conservation of Forces. V. By Prof. Liebig. The Connection and Equivalence of Forces. VI. By Dr. Carpenter. The Correlation of the Physical and Vital Forces “This work is a very welcome addition to our scientific literature, and will b* particularly acceptable to those who wish to obtain a popular, but at the same time precise and clear view of what Faraday justly calls the highest law in physical science, the principle of the conservation of force. Sufficient attention has not been paid to the publication of collected monographs or memoirs upon special subjects. Dr. Youmans’ work exhibits the value of such collections in a very striking mannes, and we earnestly hope his excellent example may be followed in other branches of science.” — American Journal of Science. “It was a happy thought which suggested the publication of this volume. The question is often asked, and not so easily answered, What are the new doctrines of the Correlation and Conservation of Forces? In this volume we have the answer, and with the reasons of its chief expounders ; those who are ignorant on that theme, can thus question the original authorities.” — New Englander. “ We here have the original expositions of the new Philosophy of Forces, accompa¬ nied by an excellent exposition of both the expositions and the expositors; the whole will be a rare treat to the lovers of advancing scientific thought.”— Methodist Quarterly Review. “This is, perhaps, the most remarkable book of the age. We have here the latest discoveries, and the highest results of thought concerning the nature, laws, and con¬ nections of the forces of the universe. No higher or more sublime problem can engage the intellect of man than is discussed by these doctors of science intent alone on an lv »t the truth.” — Detroit Free Press. ‘This work presents a praiseworthy specimen of complete and faithful authorship its publication at this time will form an epoch in the experience of many thin ii in® atindfl.” — {buns, Works published by D. Appleton dt Co. HEAT, CONSIDERED AS A MODE OF MOTION, Being a Course of Twelve Lectures delivered before tLe Royal Institution of Great Britain. BY JOKY TYNDALL, F. E. S., r\jf EfiSGL Or NATURAL PHILOSOPHY IN THE ROYAL INSTITUTION —AUTRO* *&* “GLACIERS OF TILE ALPS,” ETO. Willi One Hundred Illustrations. Svo, 480 pages. Price, $2. From the American Journal of Science. — With all the skill which has made Faraday the master of experimental science in Great Britain, Professor Tyndall enjoys the advantage of a superior general culture, and is thus enabled to set forth his philosophy with all the graces of eloquence and the finish of superior diction. With a simplicity, and absence of technicalities, which render his explanations lucid to un¬ scientific minds, and at the same time a thoroughness and originality by which he in¬ structs the most learned, he unfolds all the modern philosophy of heat Ilis work takes rank at once as a classic upon the subject. New York Times.— Professor Tyndall’s course of lectures on heat is one of the most beautiful illustrations of a mode of handling scientific subjects, which is com¬ paratively new, and which promises the best results, both to science and to literature generally ; we mean the treatment of subjects in a style at once profound and popu¬ lar. The title of Professor Tyndall’s work indicates the theory of heat held by him, and indeed the only one now held by scientific men — it is a mode of motion. Boston Journal.— He exhibits the curious and beautiful workings of nature in a most delightful manner. Before the reader particles of water lock themselves or fly asunder with a movement regulated like a dance. They form themselves into liquid (lowers with fine serrated petals, or into rosettes of frozen gauze; they bound upward In boiling fountains, or creep slowfly onward in stupendous glaciers. Flames burst into music and sing, or cease to sing, as the experimenter pleases, and metals paint them- celves upon a screen in dazzling hues as the painter touches his canvas. New York Tribune.— The most original and important contribution that h&i yet been made to the theory and literature of thermotics. Scientific American.— The wrork is written in a charming style, and is tha most valuable contribution to scientific literature that b is been published in man. years. It is the most popular exposition of the dynamical theory of heat that has ye q pearod. The old material theory of heat may be said to be defunct. Louisville Democrat.— This is one of the most delightful scientific works w« kiYe ever met. The lectures are so full of life and spirit that we can almost imaginfl the lecturer before us, and see his brilliant experiments in every stage of their progress. The theory is 60 carefully and thoroughly explained that no one can fail to understand It. Such books as these create a love for science. Independent. — Professor Tyndall’s expositions and experiments are remarkably Biougntful, ingenious, clear, and convincing; portions of the book have almost tb« fcaterest of a romance, so startling are the descriptions and elucidations. Works of Herbert Spencer published by D. A pphlon & Co. ILLUSTRATIONS OF UNIVERSAL PROGRESS. A SERIES OF DISCUSSIONS. 1 Vol Largra 12 mo. 470 Fagres. Price $2.50. CONTENTS : American Notice of Spencer’s New System of Philosophy. L Progress : its Law and Cause. 11 Manners and Fashion. III. The Genesis of Science. IV. The Physiology of Laughter. V. The Origin and Function of Music. VI. The Nebular Hypothesis. VII. Bain on the Emotions and the Will. VIII. Illogical Geology. IX. The Development Hypothesis. X. The Social Organism. XI. Use and Beauty. XII. The Sources of Architectural Types. XIII. The Use of Anthropomorphism. These Essays constitute a body of massive and original thought upon a large variety of important topics, and will be read with pleasure by all who appreciate a bold and powerful treatment of fundamental themes. The general thought which pervades this book is beyond doubt the most impor¬ tant that the human mind has yet reached. — N. Y. Independent. Those who have read the work on Education, will remember the ana¬ lytic tendency of the author’s mind — his clear perception and admirable ex¬ position of hrst principles — his wide grasp of facts — his lucid and vigorous style, and the constant and controlling bearing of the discussion on practical results. These traits characterize all Mr. Spencer’s writings, and mark, in an eminent degree, the present volume. — N. Y. Tribune. We regard the distinguishing feature of this work to be the peculiarly Interesting character of its matter to the general reader. This is a greal literary as weLl as philosophic triumph. In the evolution of a system of Philosophy which demands serious attention, and a keen exercise of the in¬ tellect to fathom and appreciate, he has mingled much that is really popul&i ind entertaining. — Rochester Democrat. *•; i ;