virtue of the fact that it unifies and explains a number of lesser generalizations, themselves for the most part established by direct induction, in several special sciences. When, in these separate sciences, the subsidiary generalizations underlying the theory of the transformation of species were well established, and generally accepted by specialists, the evidence for evolution must be said to have been logically complete. This does not mean that more facts were not subsequently added; it does mean that the argument was adequate without them, and that no one who found the original evidence unconvincing had any logical ground for being convinced by any of the considerations adduced in the "Origin of Species" or in Huxley's earlier evolutionary writings.
3. Argument from the Homologies in Vertebrates.—This argument was, by 1844, already so old and even hackneyed a one, that it may, in a consideration of the status of the evolutionary argument at that special period, be passed over very briefly. The facts upon which the argument rests had been in the possession of zoologists ever since Buffon and Daubenton had laid the foundations of the science of comparative anatomy (1749). These facts chiefly had, before the end of the eighteenth century, made evolutionists of Diderot,[1] of Kant, and (but for perfunctory reservations in favor of religious orthodoxy) of Buffon himself. It can, therefore, scarcely be necessary to cite evidence to show that the argument was familiar a quarter of a century after the whole conception of homologous organs had been clearly elaborated by E. Geoffroy St. Hilaire.[2] Nor, for the purposes of the present paper, is it necessary to estimate the precise logical weight of this argument when it stands alone. At the time with which this inquiry is concerned, it did not stand alone, but had been complemented by a number of considerations more recently brought to light by scientific discovery.
4. Argument from the Variability of Existing Species.—Not less old than the last-mentioned was the argument from the fact that existing—and, especially, domesticated—species have a marked tendency to variation, exhibit an extensive diversity of form, and are capable of transmitting variations to their descendants. It was mainly this group of facts that had caused Maupertuis[3] to embrace the evolutionary hypothesis before 1751. The same argument, with that from the homologies, is set down by Erasmus Darwin in the "Zoonomia," 1794, as among the principal reasons for believing in the transformation of species. We are led to such a belief, wrote the grandfather of the author of the "Origin,"
