346 COMPARATIVE ANATOMY 



a greater variety of points of view, permitting more general con- 

 ceptions. 



When it searches for the laws of organization, comparative ana- 

 tomy, although remaining tributary to zootomy and a little to physi- 

 ology for the facts of which it makes use, becomes more independent 

 because of its aim, but it continues to be a narrow, limited science, 

 since the number of general principles which it strives to discover 

 is very limited. 



In the search for homologies, comparative anatomy allies itself 

 with embryology and paleontology; thus it gains all its amplitude 

 from the importance and the infinite number of problems upon which 

 it borders. In elevating and enlarging itself, however, does it still 

 maintain its solidity? 



Two organs are homologous, we have said, when they are derived 

 from the same organ or rudiment in their common ancestor, but 

 that ancestor is in no case precisely known. When the homology 

 to be sought is easy, one may often determine by induction the an- 

 cestral character from which are derived the characters to be com- 

 pared. Thus, although the common ancestor of the dog and the 

 cat is not known, we may affirm that it had a tail, and that this tail 

 has become the tail of the dog along one line of descent and that of 

 the cat along another; and that, consequently, the tails of the dog 

 and the cat are homologous. This renders no service, however, for 

 the organs are so similar that their homology is evident without 

 any reasoning. 



But if one asks what is in man the homologue of the pineal eye 

 of the lizard, the matter is more delicate. The common ancestor 

 of man and of the lizard is not known; we cannot infer anything 

 certain relative to the first appearance of the pineal eye; so the 

 question remains unanswered. 



And this is almost constantly the case. Thus, when phylogeny 

 furnishes the required response, the question is solved in advance; 

 again, when the question is difficult, phylogeny remains mute. 



We must, then, lower its pretensions, and in place of phylogeny, 

 whose responses would be certain if we only knew them, but which 

 we do not know, we must turn to ontogeny. 



The latter also gives answers, not without value, but which are 

 indirect and necessitate an induction into which error may glide. 

 We admit, in general, that when organs are represented at some 

 stage of ontogenesis by very similar rudiments, these organs are 

 homologous, however dissimilar they may be in the adult; and, 

 inversely, that they are not homologous, however similar they may 

 be in the adult, if they are represented in ontogenesis by dissimilar 

 rudiments. 



If ontogenesis were a faithful copy of phylogenesis, this reasoning 



