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.
CHAPTER XIVA.
RECENT CRITICISMS AND HYPOTHESES.
§ 174a. Since the first edition of this work was published, and more especially since the death of Mr. Darwin, an active discussion of the Evolution hypothesis has led to some significant results.
That organic evolution has been going on from the dawn of life down to the present time, is now a belief almost universally accepted by zoologists and botanists—"almost universally," I say, because the surviving influence of Cuvier prevents acceptance of it by some of them in France. Omitting the ideas of these, all biological interpretations, speculations, and investigations, tacitly assume that organisms of every kind in every era and in every region have come into existence by the process of descent with modification.
But while concerning the fact of evolution there is agreement, concerning its causes there is disagreement. The ideas of naturalists have, in this respect, undergone a differentiation increasingly pronounced; which has ended in the production of two diametrically opposed beliefs. The cause which Mr. Darwin first made conspicuous has come to be regarded by some as the sole cause; while, on the part of others there has been a growing recognition of the cause which he at first disregarded but afterwards admitted. Prof. Weismann and his supporters contend that natural selection suffices to explain everything. Contrariwise, among many who recognize the inheritance of functionally-produced changes, there are a few, like the Rev. Prof. Henslow, who regard it as the sole factor.
The foregoing chapters imply that the beliefs of neither extreme are here adopted. Agreeing with Mr. Darwin that both factors have been operative, I hold that the inheritance of functionally-caused alterations has played a larger part than he admitted even at the close of his life; and that, coming more to the front as evolution has advanced, it has played the chief part in producing the highest types. I am not now about to discuss afresh these questions, but to deal with certain further questions.