# The Principles of Biology Vol. I/Chapter III.14

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.
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 primary ones are the most unlike, the sub-groups are less unlike, the sub-sub-groups still less unlike, 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.
Of "the Arguments from Embryology," the first is that when developing 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. Nor do the minor deviations from this general parallelism, which look like difficulties, fail, on closer observation, to furnish additional evidence; since those traits of a common ancestry which embryology reveals, are, if modifications have resulted from changed conditions, liable to be disguised in different ways and degrees in different lines of descendants.
We next considered "the Arguments from Morphology." Apart from those kinships among organisms disclosed by their developmental changes, the kinships which their adult ​forms show are profoundly significant. The unities of type found under such different externals, are inexplicable except as results of community of descent with non-community of modification. Again, each organism analyzed apart, shows, in the likenesses obscured by unlikenesses of its component parts, a peculiarity which can be ascribed only to the formation of a more heterogeneous organism out of a more homogeneous one. And once more, the existence of rudimentary organs, homologous with organs that are developed in allied animals or plants, while it admits of no other rational interpretation, is satisfactorily interpreted by the hypothesis of evolution.
Last of the inductive evidences, came "the Arguments from Distribution." While the facts of distribution in Space are unaccountable as results of designed adaptation of organisms to their habitats, they are accountable as results of the competition of species, and the spread of the more fit into the habitats of the less fit, followed by the changes which new conditions induce. Though the facts of distribution in Time are so fragmentary that no positive conclusion can be drawn, yet all of them are reconcilable with the hypothesis of evolution, and some of them yield it strong support: especially the near relationship existing between the living and extinct types in each great geographical area.
Thus of these four groups, each furnished 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 every other group. This coincidence of coincidences would give to the induction a very high degree of probability, even were it not enforced by deduction. But the conclusion deductively reached, is in harmony with the inductive conclusion.
§ 173. 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 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 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.
These modifications upon modifications which result in evolution structurally considered, are the accompaniments of those functional alterations continually required to re-equilibrate inner with outer actions. That moving equilibrium 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.
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 the preservation in successive generations of those whose moving equilibria are least at variance with the requirements, there is produced a changed equilibrium completely in harmony with the requirements.
§ 174. Even were this the whole of the evidence assignable for the belief that organisms have been gradually evolved, ​it would have a warrant higher than that of many beliefs which are regarded as established. But the evidence is far from exhausted.
At the outset it was remarked that the phenomena presented by the organic world as a whole, cannot be properly dealt with apart from the 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.