moving equilibrium constituted by a species, an action producing change in a given direction, is followed, in the next generation, by a reaction producing an opposite change. Observe, too, that these effects correspond in their degrees of violence. If the alteration of some external factor is so great that it leaves alive only a few individuals, characterized by extreme endowments of the power required to antagonize it; then, in succeeding generations, there is a rapid multiplication of individuals similarly characterized by extreme endowments of this power — ^the force impressed calls out an equivalent conflicting force. Moreover, the change is temporary where the cause is temporary, and permanent where the cause is permanent.
All that are deficient in the needful attribute having been killed off; and the survivors having the needful attribute in a comparatively high degree; there will descend from them, not only some possessing equal amounts of this attribute with themselves, but also some possessing less amounts of it. If the agency which proves fatal to them has not continued in action, such less-endowed individuals will multiply; and the species, after sundry oscillations, will return to its pre\dous mean state. But if this agency be a persistent one, such less INDIRECT EQUILIBRATION. 4G1 endowed individuals will be continually killed off; and evcntuall}^ none but the highly-endowed individuals will be produced — a new moving equilibrium, adapted to the new environing conditions, will result.
It may be objected that this mode of expressing the facts, does not include the numerous cases in which a species becomes modified in relation to surrounding agencies that do not actively influence it — cases like that of the plant which acquires hooked seed-vessels, by which it lays hold of the skins of passing animals, and makes them the distributors of its seeds — cases in which the outer agency has no direct tendency at first to affect the species, but in which the species so alters itself as to take advantage of the outer agency.
To cases of this kind, however, the same mode of interpretation applies on simply changing the terms. "While, in the aggregate of influences amid which a species exists, there are some which tend to overthrow the moving equilibria of its members, there are others which facilitate the maintenance of their moving equilibria, and some which are capable of giving their moving equilibria increased stability: instance the spread into their habitat of some new kind of prey, which is abundant at seasons when other prey is scarce. I^ow what is the process by which the moving equilibrium in any species, becomes adapted to some additional external factor which furthers its maintenance? Instead of an increased resistance to be met and counter-balanced, there is here a diminished resistance; and the diminished resistance is equilibrated in the same way as the increased resistance. As, in the one case, there is a more frequent survival of those individuals whose peculiarities of constitution enable them best to resist the new adverse factor; so, in the other case, there is a more frequent survival of individuals whose peculiarities of constitution enable them to take advantage of the new favourable factor. In each member of the species, the balance of functions and correlated arrangement of structures, differ slightly from those existing in other members. To say that among all its members, one is better adapted than tbe rest to take advantage of some before-unused agency in the environment, is to say that its moving equilibrium is, in so far, more stably adjusted with respect to the aggregate of surrounding influences. And if, as a consequence, this individual maintains its moving equilibrium when others fail to do so, and produces offspring which do the like — that is, if individuals thus characterized multiply and supplant the rest; there is evidently, as before, a process by which an equilibration between the organism and its environment is effected, not immediately but mediately, through the continuous intercourse between the species as a whole and the environment.
§ 168. Thus we see that indirect equilibration does whatever direct equilibration cannot do. It is scarcely possible too much to emphasize the conclusion, that all these processes by which organisms are re-fitted to their ever-changing environments, must be equilibrations of one kind or other. As authority for this conclusion, we have not simply the universal truth that change of every order is towards equilibrium; but we have also the truth which holds throughout the organic world, that life itself is the maintenanceof a moving equilibrium between inner and outer actions — the continuous adjustment of internal relations to external relations; or the maintenance of a correspondence between the forces to which an organism is subject and the forces which it evolves. For if the preservation of life is the preservation of such a moving equilibrium, it becomes a corollary that those changes which enable a species to live under altered conditions, are changes towards equilibrium with the altered conditions.
Hence, all such changes being equilibrations, their differences can be nothing but differences in the ways through which they result. If they are not effected immediately, they must be effected mediately. A prioriy therefore, we may be certain that all processes of modification which do INDIRECT EQUILIBRATION. 4G3 not come within the class of direct equilibrations, must come within the class of indirect equilibrations.
Examination of the facts confirms this conclusion. The external factors to which a species is exposed, are of two kinds. They are such as act continuously or frequently on the individuals; or they are such as do not act continuously or frequently on the individuals. To a factor which continuously or frequently acts on the individuals, the functions of the individuals re-adjust themselves — there is direct equilibration. While a factor which does not act continuously or frequently on the individuals, acts continuously on the species as a whole — either destroying such of the members as are least capable of resisting it, or fostering such of the members as are most capable of taking advantage of it. And by the abstraction, generation after generation, of those least in equilibrium with the new factor; or by the extra multiplication, generation after generation, of those most in equilibrium with the new factor; the species as a whole is eventually brought into complete equilibrium with the new factor — there is indirect equilibration.
CHAPTER XIII.
THE CO-OPERATION OF THE FACTOES.
§ 169. Thus the phenomena of organic evolution, may be interpreted in the same way as the phenomena of all other evolution. Those universal laws of the re-distribution of matter and motion, to which things in general conform, are conformed to by all living things; whether considered in their individual histories, in their histories as species, or in their aggregate history. However otherwise they may ordinarily be expressed, the truths of development as exhibited in the animal and vegetal kingdoms, prove to be expressible as manifestations of those abstract truths set forth in First Principles. Fully to see this, it will be needful for us to contemplate in their ensemble, the several processes separately described in the four preceding chapters.
If the forces acting on any aggregate remain the same, the changes produced by them in the aggregate will presently reach a limit, at which the constant outer forces are balanced by the constant inner forces; and thereafter no further metamorphosis will take place. Hence, that there may be continuous changes of structure in organisms, there must be continuous changes in the incident forces. This condition to the evolution of animal and vegetal forms, we find to be fully satisfied. The astronomic, geologic, and meteorologic changes that have been slowly but incessantly going on, and have been increasing in the complexity of their combinations, THE CO-OrERATlON OF THE FACTORS. 465 have been perpetually altering the circumstances of organisms; and organisms, as they have become more numerous in their kinds and higher in their kinds, have been perpetually altering one another's circumstances. Thus, for those progressive modifications upon modifications which organic evolution implies, we find a sufficient cause in the modifications after modifications, which every environment over the Earth^s surface has been undergoing, throughout all geologic and pregeologic times. The progressive inner changes for which we thus find a cause in the continuous outer changes, conform, so far as we can trace them, to that universal law of the instability of the homogeneous, which is manifested throughout evolution in general. We see that in organisms, as in all other things, the exposure of different parts to different kinds and amounts of incident forces, has necessitated their differentiation; and that for the like reason, aggregates of individuals have been lapsing into varieties, and species, and genera, and classes. We also see that in each type of organism, as in the aggregate of types, the multiplication of effects has continually aided this transition from a more homogeneous to a more heterogeneous state. And yet again, we see that that increasing segregation, and concomitant increasing definiteness, which characterizes the growing heterogeneity of organisms, has been insured by the necessary maintenance of them under combinations of forces not greatly unlike preceding combinations — by the continual destruction of those which expose themselves to aggregates of external actions markedly incongruous with the aggregates of their internal actions, and the survival of those subject only to comparatively small incongruities. Finally, we have found that each change of structure, superposed on preceding changes, has been a re-equilibration necessitated by the disturbance of a preceding equilibrium. The maintenance of life being the maintenance of a balanced combination of functions, it follows that individuals and species that have continued to live, are individuals and species in which the balance of functions lias not been gverthrown. Inevitably, therefore, survival through successive changes of conditions, implies successive adjustments of the balance to the new conditions. This deduction we find to be inductively verified. What is ordinarily called adaptation, is, when translated into mechanical terms, direct equilibration. And that process which, under the name of natural selection, Mr Darwin has shown to be an ever-acting means of fitting the structures of organisms to their circumstances, we find, on analysis, to be expressible in mechanical terms as indirect equilibration.
The actions that are here specified in succession, are in reality simultaneous; and they must be so conceived before organic evolution can be rightly understood. Some aid towards so conceiving them, will be given by the annexed table, representing the co-operation of the factors.
§ 170. Respecting this co-operation of these factors, it remains only to point out their respective shares in producing the total result; and the way in which the proportions of their ^-espective shares vary as evolution progresses.
At first, changes in the amounts and combinations of external inorganic forces, astronomic, geologic, and meteorologic, were the only causes of the successive modifications undergone by organisms; and these changes have continued, and must still continue, to be causes of such modifications. As, however, through the difiusion of organisms, and the consequent difierential actions of inorganic forces on them, there arose unlikenesses among organisms, producing varieties, species, genera, orders, classes, &c.; the actions of organisms on one another became new sources of organic modifications. And as fast as types have multiplied, and become more complex; so fast have the mutual actions of organisms come to be more influential factors in their respective evolutions. Until, eventually, as we see exemplified in the human race, they have come to be the chief factors.
Passing from the external causes of change to the internal THE CO-OrERATION OF THE FACTORS.
ass a .-si's ^C -I CO ed ^S 2^ 1^ a.S^ s to S: cd O VJ fee's §g bC'S cSt3 CO ctj ^ Cm tS.2 a JS s a 1) fs fcC to br ^ ho e« a 6C a »H a a o o A^ processes of change entailed by them, we see that these, too, have varied in their proportions — that which was originally the most important and almost the sole process, becoming gradually less important, if not at last the least important. Always there must have been, and always there must continue to be, a survival of the fittest: natural selection must have been in operation at the outset, and can never cease to operate. While yet organisms had comparatively feeble powers of co-ordinating their actions, and adjusting them to environing actions, natural selection worked almost alone in moulding and re-moulding organisms into fitness for their changing environments; and natural selection has remained almost the sole agency by which plants and inferior orders of animals have been modified and developed.
The equilibration of organisms that are comparatively passive, is necessarily effected indirectly, by the action of incident forces on the species as a whole. But along with the gradual evolution of organisms having some activity, there grows up a kind of equilibration that is relatively direct. In proportion as the activity increases, direct equilibration plays a more important part. Until, when the nervo-muscular apparatus becomes greatly developed, and the power of varying the actions to fit the varying requirements becomes considerable, the share taken by direct equilibration rises into co-ordinate importance. We have seen reason to think that as fast as essential faculties multiply, and as fast as the number of organs that co-operate in any given function increases, indirect equilibration through natural selection, becomes less and less capable of producing specific adaptations; and remains fully capable only of maintaining the general fitness of constitution to conditions. Simultaneously, the production of adaptations by direct equilibration, takes the first place — indirect equilibration serving to facilitate it. Until at length, among the civilized human races, the equilibration becomes mainly direct: the action of natural selection being restricted to the destruction of those who are constitutionally too feeble THE CO-OPERATION OF THE FACTORS. 469 to live, even with external aid. As the preservation of incapables is habitually secured by our social arrangements; and as very few except criminals are prevented by their inferiorities from leaving the average number of offspring (indeed the balance of fertility is probably in favour of the inferior); it results that survival of the fittest, can scarcely at all act in such way as to produce specialities of nature, cither bodily or mental. Here the specialities of nature, chiefly mental, which we see produced, and which are so rapidly produced that a few centuries show a considerable change, must be ascribed almost wholly to direct equilibration.* * As having an instructive bearing on the question of the varieties of Man, let me here refer to a paper on " The Origin of the Human Races " read before the Anthropological Society, March 1st, 1864, by Mr Alfred "Wallace — a gentleman well known among naturalists, as having independently thought out the hypothesis of natural selection, though at a later date, and less elaborately, than Mr Darwin. In this paper, Mr Wallace shows, very clearly I think, that along with the attainment of that degree of intelligence implied by the use of implements, clothing, &c., there arises a tendency for modifications of brain to take the place of modifications of body — still, however, regarding the natural selection of spontaneous variations, as the cause of the modifications. But if the foregoing arguments be valid, natural selection here plays but the secondary part of furthering the adaptations otherwise caused. It is true that, as Mr Wallace argues, and as I have myself briefly indicated (see Westminster Review, for April, 1852, pp. 496 — 501), the natural selection of races, leads to the survival of the more cerebrally-developed, while the less cerebrally-developed disappear. But though natural selection acts freely in the struggle of one society with another; yet, among the units of each society, its action is so interfered with, that there remains no adequate cause for the acquirement of mental superiority by one race over another, except the inheritance of functionally -produced modifications. This view, however, agrees equally well with Mr Wallace's conclusion, that at a certain stage of evolution, the brain begins to change much more than the body.
CHAPTEE XIY.
§ 171. Of the three classes of evidences that have been assigned, the a priori^ which we took first, were partly negative, partly positive.
On considering the " General Aspects of the Special-creation-hypothesis,'' we discovered it to be worthless. Discredited by its origin, and wholly without any basis of observed fact, we found that it was not even a thinkable hypothesis; and while thus intellectually illusive, it turned out on examination to have moral implications quite at variance with the professed beliefs of those who hold it.
Contrariwise, the " General Aspects of the Evolution-hypothesis," begot the stronger faith in it the more nearly they were considered. By its lineage and its kindred, it was found to be as closely allied mth the proved truths of modern science, as is the antagonist hypothesis with the proved errors of ancient ignorance. Instead of being a mere pseud-•idea, we saw that it admitted of elaboration into a definite conception — so showing its legitimacy as an hypothesis. Instead of positing a purely fictitious process, the process which it alleges, we saw to be one that is actually going on around us. To which add, that morally considered, this hypothesis presents no irreconcilable incongruities.
Thus, even were we without further means of judging, there could be no rational hesitation which of the two views should be entertained.
§ 172. Further means of judging, however, we found to be afforded by bringing the two hypotheses face to face with the general truths established by naturalists. These inductive evidences were dealt with in four chapters.
" The Arguments from Classification " were these. Organisms fall into groups within groups; and this is the arrangement which we see results from evolution, where it is known to take place. Of these groups within groups, the great or primar}^ ones are the most unlike, the sub-groups are less unlike, the sub-sub-groups still less unKke, and so on; and this, too, is a characteristic of groups demonstrably produced by evolution. Moreover, indefiniteness of equivalence among the groups^ is common to those which we know have been evolved, and those here supposed to have been evolved. And then there is the further significant fact, that divergent groups are allied through their lowest rather than their highest members — a truth which the hypothesis of evolution implies.
Of " the Arguments from Embryology," the first and most striking is, that when the developments of embryos are traced from their common starting point, and their divergences and re-divergences symbolized by a genealogical tree, there is manifest a general parallelism between the arrangement of its primary, secondary, and tertiary branches, and the arrangement of the divisions and sub-divisions of our classifications— a general parallelism to be anticipated as a result of evolution. Nor do those minor deviations from this general parallelism, which at first sight look like difficulties, fail, on closer observation, to become additional supports; since those traits of a common ancestry which embryology reveals, are, if modifications have resulted from changed conditions, liable to be distorted or disguised in quite different ways and degrees in different lines of descendants We next considered " the Arguments from Morphology.'* Leaving out those kinships among organisms disclosed by their developmental metamorphoses, the kinships which their adult forms show are profoimdly significant. The remarkable unities of type which are found under such different externals, are inexplicable except as results of community of descent mth non-community of modification. Again, each organism analyzed apart, shows us, in the likenesses obscured by unlikenesses of its component parts, a peculiarity of structure that can be ascribed only to the formation of a more heterogeneous organism out of a more homogeneous one. And once more, the habitual existence of rudimentary organs, homologous with organs that are developed in allied animals or plants, while it admits of no other rational interpretation, has a satisfactory interpretation given to it by the hypothesis of evolution.
Last of the inductive evidences, came ^' the Arguments from Distribution. '^ While the phenomena of distribution in Space, prove to be unaccountable as results of designed adaptation of organisms to their habitats, they prove to be accountable as results of the competition of species, and the spread of the superior into the habitats of the inferior, followed by the changes which new conditions induce. Though the phenomena of distribution in Time, are so fragmentary that no positive conclusion can be drawn from them; yet all of them are reconcileable with the hypothesis of evolution, and some of them yield it strong support — especially the near relationship that exists between the living and extinct types of each great geographical area.
In each of these four groups, we thus found several arguments which point to the same conclusion; and the conclusion pointed to by the arguments of any one group, is that pointed to by the arguments of all the other groups. This coincidence of coincidences, would give to the induction a very high degree of probability, even were it not enforced by deduction.
§ 173. But the conclusion deductively reached, is in harmony with the inductive conclusion. Passing from the evidence that evolution has taken place, to the question — How has it taken place? we find in known agencies and known processes,, adequate causes of its phenomena.
In astronomic, geologic, and meteorologic changes, ever in progress, ever combining in new and more involved ways, we have a set of inorganic factors to which all organisms are exposed; and in the varying and complicating actions of organisms on one another, we have a set of organic factors that alter with increasing rapidity. Thus, speaking generally, all members of the Earth's Flora and Fauna are continually passing into new environments — experience perpetual re-arrangements of external forces.
Each organic aggregate, whether considered individually or as a continuously- existing species, is modified afresh by each fresh distribution of external forces. To its pre-existing differentiations, new differentiations are added; and thus that lapse from a more homogeneous to a more heterogeneous state, which would have a fixed limit were the circumstances fixed, has its limit perpetually removed by the perpetual change of the circumstances. Meanwhile, that growing complexity of structure thus produced, must, in the average of cases, be accompanied by an increasing definiteness of structm^e; since only those organisms can survive which subject themselves to aggregates of forces that are not, in their essentials, greatly unlike those with which their structures correspond. And at the same time that progression is thus necessitated as a general result; yet, as change of structure arises only where there is change in the distribution of forces, it will not take place in organisms which elude changes in the distribution of forces, by migration or otherwise.
These modifications upon modifications which result in evolution structurally considered, are the accompaniments of those functional alterations continually required to reequilibrate inner with outer actions. That moving equi- Librium of inner actions corresponding with outer actions, which constitutes the life of an organism, must either be overthrown by a change in the outer actions, or must undergo perturbations that cannot end until there is a re-adjusted balance of functions and correlative adaptation of structures. Wherever the external changes are such as to be continuously or frequently operative on individuals, this direct equilibration must go on.
But where the external changes are either such as are fatal when experienced by the individuals, or such as act on the individuals in ways that do not affect the equilibrium of their functions; then the re- adjustment results through the effects produced on the species as a whole — there is indirect equilibration. By natural selection or survival of the fittest — by the preservation in successive generations of those whose moving equilibria happen to be least at variance with the requirements, there is eventually produced a changed equilibrium completely in harmony with the requirements.
And thus it results that those universal laws of the re-distribution of matter and motion, which are conformed to by evolution in general, are conformed to by organic evolution.
§ 174. Even were this the whole of the evidence assignable for the belief that organisms of all orders have been gradually evolved, this belief would have a warrant much higher than that of very many beliefs that are regarded as established. When we see that there are strong a priori probabilities in its favour, and wholly adverse to the antagonist hypothesis — when an examination of the facts which naturalists have accumulated, leads us to several groups of inductions which unite in supporting it — and when the characteristics which conspire to show that organic evolution has been going on, prove to be deducible from those universal actions known to work evolution of all other kinds; we have a- combination of proofs which might suffice were there no more to be said.
But the evidence is far from exhausted. At the outset of the argument, it was remarked that the ensemble of vital phenomena presented by the organic world as a whole, cannot be properly dealt with apart from the ensemble of vital phenomena presented by each organism, in the course of its growth, development, and decay. The interpretation of either implies interpretation of the other; since the two are in reality parts of one process. Hence, the validity of any hypothesis respecting the one class of phenomena, may be tested by its congruity with phenomena of the other class. We are now about to pass to the more special phenomena of development, as displayed in the structures and functions of individual organisms. If the hypothesis that plants and animals have been progressively evolved, be true, it must furnish us with keys to these phenomena. We shall find that it does this; and by doing it, gives numberless additional vouchers for its truth.
JOHN CHILDS AND SON, PRINTERS.
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