HOMOLOGY AND ITS INTERPRETATION 39 



geologically speaking, all too brief, and characterized by envi- 

 ronmental conditions much too uniform, to afford us a favor- 

 able opportunity for ascertaining the extreme limits to which 

 the genetic process may possibly extend; and, even apart from 

 this consideration, they say, racial development (phylogeny) 

 may be, like embryological development (ontogeny) an irre- 

 versible process, in which case no recurrence whatever of its 

 past phenomena are to be expected in our times. 



Be that as it may, the evolutionist interprets the resem- 

 blances of homology as surviving vestiges of an ancient ances- 

 tral 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 an- 

 cestor 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 ap- 

 plied, 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 neces- 

 sity of admitting a solution of this kind, that they do not 

 hesitate to make it part and parcel of the definition of homol- 

 ogy itself. For instance, on page 130 of Woodruff's "Founda- 

 tions of Biology" (1922) , we are informed that homology signi- 

 fies "a fundamental similarity of structure based on descent 

 from a common antecedent form." The Yale professor, how- 

 ever, has been outdone in this respect by Professor Calkins of 

 Columbia, who discards the anatomical definition altogether 



