The writer must confess that he cannot fathom the mentality of those who accept the evolutionary explanation, so far as plant and animal organisms are concerned, but proceed to draw the line when it comes to applying it to the human body. For if one (to borrow Du Bois-Reymond’s expression) “gives so much as his little finger to” the evolutional argument from organic homology, he must end, in so far as he is consistent, in acknowledging as incontestable its obvious application to man. The only choice which sound logic can sanction is between fixism and a thoroughgoing system of transformism, which does not exempt the human body from the scope of the evolutionary explanation. Indeed, the theory of evolution itself stands or falls upon this issue; for, if structures so strikingly similar as the skeletons of a man and an ape, respectively, have originated from two distinct ancestral stocks, then in no case at all is the inference of common descent from structural resemblance a legitimate procedure. In other words, if the homologies existent between the human and simian organisms are explicable on some other basis than that of common ancestry, then all organic homologies are so explicable, and the whole evolutionary argument collapses. § 1. Two Theories of Descent Two theories have been formulated regarding the alleged bestial origin of the human body: (1) the theory of lineal descent from some known species (living or fossil) of ape or monkey; (2) the theory of collateral descent from a hypothetical bestial ancestor common to apes and men. The theory of lineal descent is that to which Darwin himself stands committed. This theory, however, soon fell into disrepute among scientists, who came to prefer the theory of collateral descent, although signs of a return to the older theory are not wanting in our day. At all events, Darwin came out flatly in favor of the monkey origin of man. This, it is true, has been indignantly denied by loyal partisans anxious to exonerate their idol from the reproach of having advanced a crude and now obsolete theory of human descent. But Darwin’s own words speak for themselves: “The Simiadae,” he says, “then branched off into two great stems, the New World and Old World monkeys; and from the latter, at a remote period of time, Man, the wonder and glory of the Universe, proceeded.” (“Descent of Man,” 2nd ed., ch. VI, pp. 220, 221.) Note that he does not say “probably”; his language is not the language of hypothesis, but of categorical affirmation.