Be that as it may, the evolutionist interprets the resemblances of homology as surviving vestiges of an ancient ancestral type, which have managed to persist in the descendants notwithstanding the transformations wrought in the latter by the process of progressive divergence. Moreover, just as the existence of a common ancestor is inferred from the _fact_ of resemblance, so the relative position in time of the common ancestor is inferred from the _degree_ of resemblance. The common ancestor of forms closely allied is assumed to have been proximate, that of forms but distantly resembling each other is thought to have been remote. Thus the common ancestor of species grouped under the same genus is supposed to have been less remote than the common ancestor of all the genera grouped under one family. The same reasoning is applied, _mutatis mutandis_, to the ancestry of families, orders and classes. The logic of such inferences may be questioned, but there is no blinking the fact that, in practice, the genetic explanation of homology is assumed by scientists to be the only reasonable one possible. In fact, so strong is their confidence in the necessity of admitting a solution of this kind, that they do not hesitate to make it part and parcel of the definition of homology itself. For instance, on page 130 of Woodruff’s “Foundations of Biology” (1922), we are informed that homology signifies “a fundamental similarity of structure based on descent from a common antecedent form.” The Yale professor, however, has been outdone in this respect by Professor Calkins of Columbia, who discards the anatomical definition altogether and substitutes, in lieu thereof, its evolutionary interpretation. “When organs have the same ancestry,” he says, “that is, when they come from some common part of an ancestral type, they are said to be homologous.” (“Biology,” p. 165.) In short, F. A. Bather is using a consecrated formula culled from the modern biological creed when he says: “The old form of diagnosis was _per genus et differentiam_. The new form is _per proavum et modificationem_.” (_Science_, Sept. 17, 1920, p. 259.)