In the next chapter we shall have occasion to refer again to Dorlodot’s book. For the present, however, his work need not concern us, while in that of Mr. McCann we single out but one point as germane to our subject, namely, the latter’s inadequate rebuttal of the evolutionary argument from homology. The futility of his method, which consists in matching insignificant differences against preponderant resemblances, and in exclaiming with ironic incredulity: “Note extraordinary resemblances!” becomes painfully evident, so soon as proper presentation enables us to appreciate the true force of the argument he is striving to refute. _Functionally_ the foot of a Troglodyte ape may be a “hand,” but _structurally_ it is the homologue of the human foot, and not of the human hand; nor is this homology effectually disposed of by stressing the dissimilarity of the hallux, whilst one remains discreetly reticent concerning the similarity of the calcaneum. For two reasons, therefore, the irrelevance of Mr. McCann’s reply is of special interest here: (1) because it illustrates concretely the danger of rendering a refutation inconsequential and inept by failing to plumb the full depth of the difficulty one is seeking to solve; (2) because it shows that it is vain to attempt to remove man’s body from the scope of this argument by citing the inconsiderable structural differences which distinguish him from the ape, so that, unless the argument from homology proves upon closer scrutiny to be inherently _inconclusive_, its applicability to the human body is a foregone conclusion, and implies with irresistible logic the common ancestry of men and apes. Such are the reflections suggested by the meager measure of justice which Mr. McCann accords to the strongest zoölogical evidence in favor of evolution, and they contain in germ a feasible program for the present chapter, which, accordingly, will address itself: first, to the task of ascertaining the true significance of homology in the abstract as well as the full extent of its application in the concrete; second, to that of determining with critical precision its intrinsic value as an argument for the theory of transmutation. _Homology_ is a technical term used by the systematists of botany, zoölogy and comparative anatomy to signify basic structural similarity as distinguished from superficial functional similarity, the latter being termed _analogy_. Organisms are said to exemplify the phenomenon of homology when, beneath a certain amount of external diversity, they possess in common a group of correlated internal resemblances of such a nature that the organisms possessing them appear to be constructed upon the same fundamental plan. In cases of this kind, the basic similarity is frequently masked by a veneer of unlikeness, and it is only below this shallow surface of divergence that we find evidences of the identical structure or common type.