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, it remains only to point out the respective shares of the factors in producing the total result; and the way in which the proportions of their respective shares vary as evolution progresses.
 | on each species: affecting |  | its individuals, |  | immediately through their functions; |  | which, partially in the first generation, and completely in the course of generations, are directly equilibrated with the changed agencies. | |||||
Astronomic changes Geologic changes Meteorologic changes |  | alter the incidence of inorganic forces. | which have their direct equilibration with the changed agencies, aided by indirect equilibration, through the more frequent survival of those in which the direct equilibration is most rapid. | |||||||||
| mediately through the aggregate of individuals; |  | positively—by aiding the multiplication of those whose moving equilibria happen to be most congruous with the changed agencies: thus, in the course of generations, indirect equilibrating certain individua with them. | ||||||||||
| Enemies Competitors Co-operators Prey |  | varying in number |  | alter the incidence of organic forces. | negatively—by killing those whose moving equilibria are most incongruous with the changed agencies: thus, in the course of generations, indirectly equilibrating each of its surviving individuals with them. | |||||||
its aggregate of individuals, |  | by acting on it in some parts of the habitat more than in others; and thus differentiating the species into local varieties. | ||||||||||
| Enemies Competitors Co-operators Prey | | varying in kind | by acting differently on slightly-unlike individuals in the same locality; | | and thus causing differentiations of the species into varieties, irrespective of locality. | |||||||
| and thus causing modification of the species as a whole, by abstracting a certain class of its units. | ||||||||||||
At first, changes in the amounts and combinations of inorganic forces, astronomic, geologic, and meteorologic, were the only causes of the successive modifications; and these changes have continued to be causes. But as, through the diffusion of organisms and consequent differential actions of inorganic forces, there arose unlikenesses among them, producing varieties, species, genera, orders, classes, 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: eventually becoming the chief factors.
Passing from the external causes of change to the internal 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 small abilities to coordinate their actions, and adjust them to environing actions, natural selection worked almost alone in moulding and remoulding 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 almost passive, is necessarily effected indirectly, by the action of incident forces on the species as a whole. But along with the evolution of organisms having some activity, there grows up a kind of equilibration which is in part 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 or greater importance. As fast as essential faculties multiply, and as fast as the number of organs which co-operate in any given function increases, indirect equilibration through natural selection becomes less and less capable of producing specific adaptations; and remains capable only of maintaining the general fitness of constitution to conditions. The production of adaptations by direct equilibration then 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 limited to the destruction of those who are constitutionally too feeble to live, even with external aid. As the preservation of incapables is secured by our social arrangements; and as very few save incarcerated criminals are prevented by their inferiorities from leaving the average number of offspring; it results that survival of the fittest can scarcely at all act in such way as to produce specialities of nature, either 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.[[54]]
CHAPTER XIV.
THE CONVERGENCE OF THE EVIDENCES.
§ 171. Of the three classes of evidences that have been assigned in proof of Evolution, the à 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 to have moral implications irreconcilable 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 with the proved truths of modern science, as is the antagonist hypothesis with the proved errors of ancient ignorance. We saw that instead of being a mere pseud-idea, it admits 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 proves to be one actually going on around us. To which add that, morally considered, this hypothesis presents no radical incongruities.