§ IGl. In plants, organs engaged in nutrition, and exposed to variations in the amounts and proportions of matters and forces utilized in nutrition, may be expected to undergo cor- responding variations. We find evidence that they do this. The " changes of habit '' which are common in plants, when taken to places milike in climate or soil to those before in- habited by them, are changes of parts in which the modified external actions directly produce modified internal actions. The characters of the stem and shoots as woody or succulent, erect or procumbent; of the leaves in respect of their sizes, thicknesses, and textures; of the roots in their degrees of development and modes of growth; are obviously in imme- diate relation to the characters of the environment. A per- manent difibrence in the quantity of light or heat, afiects, day after day, the processes going on in the leaves. Habitual rain or drought, alters all the assimilative actions, and appreciably influences the organs that carry them on. Some particular substance, by its presence in the soil, gives new qualities to some of the tissues; causing greater rigidity or flexibility, and so affecting the general aspect. Here, then, w^e have, in plants, changes tending to bring about in them, modified arrangements of functions and structures, in equi- librium with modified sets of external forces.
But now let us turn to other classes of organs possessed by plants — organs which are not at once affected in their actions by the variations of incident forces. Take first the organs of defence. Many plants are shielded against animals that Avould else devour them, by formidable thorns; and others, like the nettle, by stinging hairs. These must be counted among the appliances by which equilibriimi is maintained 438 THE EVOLUTION OF LIFE.
between the actions in the organism and the actions in its environment; seeing that all other things remaining the same, if these defences were absent, the destruction by herb- ivorous animals would be so increased, that the number of young plants annually produced would not suffice, 'as it now does, to balance the mortality, and the species would there- fore disappear. But these defensive appliances, though they aid in maintaining the balance between inner and outer actions, cannot have been directly called forth by the outer actions which they serve to neutralize; for these outer actions do not continuously affect the functions of the plant even in a general way, still less in the special way required. Suppose a species of nettle bare of poison-hairs, to be habit- ually eaten by some mammal intruding on its habitat; the agency of this mammal would have no direct tendency to develop poison-hairs in the plant; since the individuals devoured could not bequeath changes of structure, even were the actions of a kind to produce them; and since the in- dividuals that perpetuated themselves, would be those on which the new incident force had not fallen. Another class of organs similarly circumstanced, are those of reproduction. Like the organs of defence, these are not, during the life of the individual plant, variably exercised by variable external actions; and therefore do not fulfil those conditions under which structural changes may be directly caused by changes in the environment. The generative apparatus contained in every flower, acts only once during its existence; and even then, the parts subserve their ends in a passive rather than an active way. Functionally-pro- duced modifications are therefore out of the question. If a plant's anthers are so placed, that the insect which most commonly frequents its flowers, is sure to come in contact with the pollen, and to fertilize with it other flowers of the same species; and if this insect, dwindling away or dis- appearing from the locality, leaves behind no insects that have such shapes and habits as cause them to do the same DIRECT EQUILIBRATION. 439 tiling- efficiently, but only some wliicli do it inefficiently; it is clear tliat tlie cliange of its conditions, has no immediate tendency to work in tlie plant any such structural change as shall bring about a new balance with its conditions. For the anthers, which, even when they discharge their functions, do it simply by standing in the way of the insect, are, under the supposed circumstances, left untouched by the insect; and this remaining untouched, cannot have the effect of so modifying the stamens as to bring the anthers into a position to be touched by some other insect. Only those individuals whose parts of fructification so far differed from the average form of the species, that some other insect could serve them as pollen-carrier, would be sufficiently prolific to have good chances of perpetuating themselves. And on their progeny, inheriting the deviation, there would act no external force directly calculated to make the deviation greater, and the adaptation more complete; since the new circumstances to which re- adaptation is required, are such as do not in the least alter the equilibrium of functions constituting the life of the individual plant.
§ 162. Among animals, adaptation by direct equilibration is similarly traceable, wherever, during the life of the indi- vidual, an external change generates some constant or re- peated change of function. This is conspicuously the case with such parts of an animal as are immediately exposed to diffused influences, like those of climate, and with such parts of an animal as are occupied in its mechanical actions on the environment. Of the one class of cases, the darkening or lightening of the skin, that follows exposure to greater or less heat, may be taken as an instance; and with the other class of cases, we are made familiar by the increase and de- crease which use and disuse cause in the organs of motion and manipulation. It is needless here to exemplify these: they were treated of in the Second Part of this work.
But in animals, as in plants, there are many indispensable 440 THE EVOIATTTOIV' OF TJFE.
offices fulfilled by parts, between which and the external con- ditions they respond to, there is no such action and reaction as can directly produce an equilibrium. This is especially manifest with dermal appendages. Some ground, perhaps, exists for the conclusion that the greater or less development of hairs, is in part immediately due to increase or decrease of demand on their passive function, as non-conductors of heat; but be this as it may, it is impossible that there can exist any such cause for those immense developments of hairs which we see in the quills of the porcupine, or those complex de- velopments of them known as feathers. Such an enamelled armour as is worn by the Lejndosteus, is inexplicable as a direct result of any functionally- worked change. For purposes of defence, such an armour is as needful, or more needful, for hosts of other fishes; and did it result from any direct re- action of the organism against any offensive actions it was subject to, there seems.no reason why other fishes should not have developed similar protective coverings. Of sundry reproductive appliances, the like may be said. The secretion of an egg-shell round the substance of an q^^^ in the ^oviduct of a bird, is quite inexplicable as a consequence of some functionally- wrought modification of structure, im- mediately caused by some modification of external con- ditions. The end fulfilled by the egg-shell, is that of protecting the contained mass against certain slight pressures and collisions, to which it is liable during incubation. How, by any process of direct equilibration, could it come to have the required thickness? or, indeed, how could it come to exist at all? Suppose this protective envelope to be too weak, so that some of the eggs a bird lays are broken or cracked. In the first place, the breakages or crackings are actions of a kind which cannot react on the maternal organ- ism, in such way as to cause the secretion of thicker shells for the future: to suppose that they can, is to suppose that the bird imder stands the cause of the evil, and that the secretion of thicker or thinner shells can be controlled bv its DIRECT EQUILliniATlON. 441 will. In tlio second place, such developing chicks as aro contained in the shells which crack or break, arc. almost certain to die; and cannot, therefore, acquire any appro- priately-modified constitutions: even supposing any con- ceivable relation could be shown, between the impression received and the change required. Meanwhile, such eggs as escape breakage, are not influenced at all by the require- ment; and hence, on the birds developed from them, there cannot have acted any force tending to work the needful adjustment of functions. In no way, therefore, can a direct equilibration between constitution and conditions be here produced. Even in organs that can be modified by certain incident forces into correspondence with such incident forces, there are some re- adjustments which cannot be effected by the direct balancing of inner and outer actions. It is thus with the bones. The majority of the bones have to resist muscular strains; and it is a familiar fact that variations in the muscular strains, call forth, by reaction, variations in the strengths of the bones. Here there is direct equilibration. But though the greater massiveness acquired by bones subject to greater strains, may be ascribed to a counter- acting force evoked by a force brought into action; it is impossible that the acquirement of greater lengths by bones can be thus accounted for. It has been supposed that the elongation of the metatarsals in wading birds, has resulted from direct adaptation to conditions of life. To justify this supposition, however, it must be shown that the mechanical actions and reactions in the legs of a wading bird, difier from those in the legs of other birds; and that the differential actions are equilibrated by the extra lengths. There is not the slightest evidence of this. The metatarsals of a bird, have to bear no appreciable strains but those due to the superincumbent weight. Standing in the water does not appreciably alter these strains; and even if it did, an increase in the lengths of these bones would not fit them any better to meet the altered strains.
-'Yr^ 442 THE EVOLUTION OF IJFE.
§ 163. The conclusion at which we arrive is, then, that there go on in all organisms, certain changes of function and structure that are directly consequent on changes in the incident forces — inner changes by which 'the outer changes are balanced, and the equilibrium restored. Such re-equi- librations, which are often conspicuously exhibited in in- dividuals, we have reason to believe continue in successive generations; until they are completed by the arrival at structures fitted' to the modified conditions. But, at the same time, we see that the modified conditions to which or- ganisms may be adapted by direct equilibration, are con- ditions of certain classes only. That a new external action may be met by a new internal action, it is needful that it shall either continuously or frequently be borne by the in- dividuals of the species, without killing or seriously injuring them; and shall act in such way as to affect their functions. And we find on examination, that many of the environing changes to which organisms have to be adjusted, are not of these kinds: being changes which either do not immediately affect the functions at all, or else affect them in ways that prove fatal.
Hence there must be at work some other process, which equilibrates the actions of organisms with the actions they are exposed to. Plants and animals that continue to exist, are necessarily plants and animals whose powers balance the powers that act on them; and • as their environments change, the changes which plants and animals undergo, must necessarily be changes towards a re- establishment of the balance. Besides direct equilibration, there must therefore be an indirect equilibration. How this goes on we have now to inquire.
CHAPTER XII.
INDIRECT EQUILIBRATION.
§ 164. Besides those perturbations produced in the moving equilibrium of any organism by special disturbing forces, there are ever going on many other perturbations — some which are the still-reverberating effects of disturbing forces previously experienced by the individual, and others which are the still- reverberating effects of disturbing forces expe- rienced by ancestral individuals; and the multiplied devia- tions of function so caused, imply multiplied deviations of structure. In § 155 there was re-illustrated the truth, set forth at length when treating of Adaptation (§ 69), that an organism in a state of moving equilibrium, cannot have extra fimction thrown on any organ, and extra growth pro- duced in such organ^ without there being entailed correlative changes throughout all other functions, and eventually throughout all other organs. And when treating of Yaria- tion (§ 90), we saw that individuals which have been made, by their different circumstances, to deviate functionally and structurally from the average type in different directions, will bequeath to their joint offspring, compound perturbations of function and compoimd deviations of structure, endlessly varied in their kinds and amounts. That is to say, besides the primary perturbations and deviations directly caused in organisms by altered actions in their environments, there are ever being indirectly caused, secondary and tertiary per- 444 THE EVOLUTION OF LIFE.
turbations and deviations, which, when compounded with one another from generation to generation, work innumerable slight modifications in the moving equilibria and correlative structures throughout the species.
Now if the individuals of a species are thus necessarily made unlike, in countless ways and degrees — if the compli- cated sets of rhythms which we call their functions, though similar in their general characters^ are dissimilar in their details — if in one individual the amount of action in a par- ticular direction is greater than in any other individual, or if here a peculiar combination gives a resulting force which is not found elsewhere; then, among all the individuals, some will be less liable than others to have their equilibria over- thrown by a particular incident force, previously unexperi- enced. Unless the change in the environment is of so vio- lent a kind as to be universally fatal to the species, it must affect more or less differently the slightly different moving equilibria which the members of the sj)ecies present. It cannot but happen that some will be more stable than others, when exposed to this new or altered factor. That is to say, it cannot but happen that those individuals whose functions are most out of equilibrium with the modified aggregate of external forces, will be those to die; and that those will sur- vive whose functions happen to be most nearly in equilibrium with the modified aggregate of external forces.
But this survival of the fittest, implies multiplication of the fittest. Out of the fittest thus multiplied, there will, as before, be an overthrowing of the moving equilibrium wher- ever it presents the least opposing force to the new incident force. And by the continual destruction of the individuals that are the least capable of maintaining their equilibria in presence of this new incident force, there must eventually be arrived at an altered t} pc completely in equilibrium with the altered conditions.
§ 165. This survival of the fittest, v/hich I have here IXDIRECr EQUILIIiRATION. 445 souglit to express in mcclianical terms, ia tliat whicli Mr Dar- win has called " natural selection, or the preservation of favoured races in the struggle for life." That there is going on a process of this kind throughout the organic world, Mr Darwin's great work on the Origui of Species has shown to the satisfaction of nearly all naturalists. Indeed, when once enunciated, the truth of his hypothesis is so obvious as scarcely to need proof. Though evidence may be required to show that natural selection accounts for everything ascribed to it, yet no evidence is required to show that natural selec- tion has always been going on, is going on now, and must ever continue to go on. Recognizing this as an a priori cer- tainty, let us contemplate it under its two distinct aspects.
That organisms which live, thereby prove themselves fit to live, in so far as they have been tried; while organisms which die, thereby prove themselves in some respects unfitted for living; are facts no less manifest, than is the fact that this self-acting purification of a species, must tend ever to insure adaptation between it and its environment. This adaptation may be either so maintained or so produced. Doubtless many who have looked at Nature with philosophic eyes, have observed that death of the worst and multiplication of the best, must result in the maintenance of a constitution in harmony with surrounding circumstances. That the aver- age vigour of any race would be diminished, did the diseased and feeble habitually survive and propagate; and that the destruction of such, through failure to fulfil some of the con- ditions to life, leaves behind those which are able to fulfil the conditions to life, and thus keeps up the average fitness to the conditions of life; are almost self-evident truths. But to recognize '' natural selection " as a means of preserving an already- established balance between the powers of a spe- cies and the forces to which it is subject, is to recognize it only in its simplest and most general mode of action. It is the more special mo^e of action with which we are here con- cerned. This more special mode of action, Mr Dar- 446 THE EVOLUTION OF LIFE.
win has been the first to perceive. To him we owe the dis- covery that natural selection is capable of producing fitness between organisms and their circmnstances; and he, too, has the merit of appreciating the immensely-important conse- quences that follow from this. He has w^orked up an enormous mass of evidence into an elaborate demonstration, that this " preservation of favoured races in the struggle for life," is an ever-acting cause of divergence among organic forms. He has traced out the involved results of the process with marvellous subtlety. He has show^n how hosts of otherwise inexplicable facts, are fully accounted for by it. In brief, he has proved that the cause he alleges is a true cause; that it is a cause which we see habitually in action; and that the results to be inferred from it, are in harmony with the phe- nomena which the Organic Creation presents, both as a whole and in its details. Let us glance at a few of the more im- portant interpretations which the hypothesis furnishes.
A soil possessing some ingredient in unusual quantity, may supply to a plant an excess of the matter required for a certain class of its tissues; and may cause all the parts formed of such tissues to be abnormally developed. Suppose that among these are the hairs clothing its surfaces, including those which grow on its seeds. Thus furnished Avith some- what longer fibres, its seeds, when shed, are carried a little further by the wind before they fall to the groimd. The young plants growdng up from them, being rather more widely dispersed than those produced by other individuals of the same species, will be less liable to smother one another; and a greater number may therefore reach maturity and fructify. Supposing the next generation subject to the same peculiarity of nutrition, some of the seeds borne by its mem- bers will not simply inherit this increased development of hairs, but will carry it further; and these, still more advan- taged in the same way as before, will, on the average, have still more numerous chances of continuing the race. Thus, by the survival, generation after generation, of those possess- INDIRECT EQUIUHRATION. 447 ing these longer hairs, and the inheritance of successive incre- ments of growth in the hairs, there may result a seed deviat- ing greatly from the original. Other individuals of the same species, subject to the different physical conditions of other localities, may develop somewhat thicker or harder coatings to their seeds: so rendering their seeds less digest- ible by the birds that devour them. Such thicker-coated seeds, by escaping undigested more frequently than thinner- coated ones, will have additional chances of growing up and leaving offspring; and this process, acting in a cumulative manner through successive years, will produce a seed diverg- ing in another direction from the ancestral type. Again, elsewhere, some modification in the physiologic actions of the plant, may lead to an unusual secretion of an essential oil in the seeds; which rendering them unpalatable to crea- tures that would otherwise feed on them, may diminish the destruction of the seeds, so giving an advantage to the variety in its rate of multiplication; and this incidental peculiarity proving a preservative, will, as before, be gradually increased by natural selection, until it constitutes another divergence. Now in these and countless analogous cases, we see that plants may become better adapted, or re-adapted, to the aggregate of surrounding agencies, not through any direct action of such agencies upon them, but through their indirect action — through the destruction by them of the individuals which are least congruous with them, and the survival of those which are most congruous with them. All these slight variations of function and structure, arising among the members of a species, serve as so many experiments; the great majority of which fail, but a few of which succeed. Just as we see that each plant bears a multitude of seeds, out of which some two or three happen to fulfil all the conditions required for reaching maturity, and continuing the race; so we see that each species is perpetually producing numerous slightly-modified forms, deviating in all directions from the average, out of which most fit the surrounding conditions no better than their pa- 448 THE EVOLUTION OF LIFE.
rents, or not so well, but some few of which, fit the conditions better; and doing so, are enabled the better to preserve them- selves, and to produce offspring similarly capable of preserv- ing themselves. Among animals the like process re- sults in the like development of various structures which cannot have been affected by the performance of functions — their functions being purely passive. The thick shell of a moUusk, is inexplicable as a result of direct reactions of the organism against the external actions to which it is exposed; but it is quite explicable as a result of the survival, genera- tion after generation, of individuals whose thicker coverings protected them against enemies. Similarly with such a dermal structure as that of the tortoise. Though we have evi- dence that the skin where it is continually exposed to pres- sure and friction may thicken, and so re-establish the equi- librium, by opposing a greater inner force to a greater outer force; yet we have no evidence that a coat of armour like that of the tortoise can be so produced. 'Nor, indeed, are the conditions under which only its production in such a man- ner could be accounted for, fulfilled; since the surface of the tortoise is not exposed to greater pressure and friction than the surfaces of other creatures. This massive carapace, and the strangely- adapted osseous frame- work which supports it, are unaccountable as results of evolution, unless through the process of natural selection. Thus, too, is it with the pro- duction of colours in birds and in insects; the formation of odoriferous glands in mammals; the growth of such excres- cences as those of the camel. Thus, in short, is it with all those organs of animals, which do not play active parts in the compound rhythms of their functions.
Besides giving us explanations of structural characters that are otherwise unaccountable, Mr Darwin shows how natural selection explains peculiar relations between indi- viduals in certain species. Such facts as the dimorphism of the primrose and other fiowers, he proves to be quite in har- mony with his hypothesis, though stumbling-blocks to all INDIRECT EQUILIBRATION. 449 other hypotheses. While the production of neuters among bees and ants, is inexplicable as a result of direct adaptation, natural selection affords a feasible solution of it. Tlie various differences that accompany difference of sex, sometimes slight, sometimes very great, are similarly accounted for. As before suggested (§ 79), natural selection appears capa- ble of producing and maintaining the right proportion of the sexes in each species; and it requires but to contemplate the bearings of the argiunent, to see that the formation of different sexes may itself have been determined in the same way.
To convey here an adequate idea of Mr Darwin's doctrine, in the immense range of its applications, is of course impos- sible. The few illustrations just given, serving but dimly to indicate the many classes of phenomena interpreted by it, are set down simply to remind the reader what Mr Darwin's hypothesis is, and what are the else insoluble problems which it solves for us.