Animal Morphology. 37 



velopment does not in itself afford at present any 

 absolute criterion whatever for the determination of 

 homology". Similar structures arise in different ways: 

 "The stomodaeum of Lopadorhynchus (an annelid worm) 

 is undoubtedly homologous with that of the earth-worm, 

 though the one appears as a paired, the other as a 

 single median structure. The ventral nerve- cord of 

 Polygordius (a primitive annelid) is certainly homolo- 

 gous with that of the earth-worm, though the former 

 appears as a median unpaired thickening of ectoderm, 

 while the latter arises by the concrescence of two widely 

 separated halves." There is an extraordinary contra- 

 diction between the bud-development and the ovum- 

 development in Tunicates, though the same results may 

 be reached by the two methods. In fact, though it is 

 a hard saying, " homology is not established through 

 precise equivalence of origin, nor is it excluded by total 

 divergence ". 



Thus we understand the reaction to the standard of 

 Owen, which defines homology in reference to the struc- 

 ture and structural relations of the developed organ. 

 As Prof. Wilson says: "We must primarily take 

 anatomy as the key to embryology, and not the reverse. 

 Comparative anatomy, not comparative embryology, is 

 the primary standard for the study of homologies, and 

 hence of genealogical descent. . . . It is the prospec- 

 tive and not the retrospective aspect of development 

 that is decisive." 



Gegenbaur, although in great part an embryologist, 

 has been a consistent upholder of the position that 

 comparative anatomy furnishes the secure basis of 

 homologies. Prof. E. B. Wilson translates the follow- 

 ing passage, which expresses Prof. Gegenbaur's posi- 

 tion : 



" If we are compelled to admit that kainogenetic 

 characters are intermingled with palingenetic, then we 

 cannot regard ontogeny as a pure source of evidence 

 regarding phyletic relationships. Ontogeny, accord- 

 ingly, becomes a field in which an active imagination 

 may have full scope for its dangerous play, but in which 

 positive results are by no means everywhere to be 



