130 ESSAY ON CLASSIFICATION. 



removed to the class of Ecliinoderms by Cuvier before 

 the metamorphoses of Comatula were known, 1 the dis- 

 covery of their pedunculated young furnished a direct 

 proof that this was their true position. 



Embryology, further, affords a test of homologies in 

 contradistinction to analogies. It shows that true ho- 

 mologies are limited respectively within the natural 

 boundaries of the great branches of the animal king- 

 dom. 



The distinction between homologies and analogies, 

 upon which the English naturalists first insisted, 2 has re- 

 moved much doubt respecting the real affinities of animals 

 which could hardly have been so distinctly appreciated 

 before. It has taught us to distinguish between real affi- 

 nity based upon structural conformity, and similarity 

 based upon mere external resemblance in form and 

 habits. But, even after this distinction had been fairly 

 established, it remained to determine within what limits 

 homologies may be traced. The works of Oken, Spix, 

 Geoffroy, and Cams, 3 show to what extravagant compa- 

 risons a preconceived idea of unity may lead. It was not 

 until Baer had shown that the development of the four 

 great branches of the animal kingdom is essentially dif- 

 ferent, 4 that it could even be suspected that organs per- 

 forming identical functions may be different in their 

 essential relations to one another ; and not until Eathke 5 

 had demonstrated that the yolk is in open communication 

 with the main cavity of the Articulata, on the dorsal side 



1 THOMPSON and FORBES, q. a., p. vol.i. p. 160 and 224. The extent of 

 1 19. Baer's information, and the compre- 



2 SWAINSON'S Geography and Clas- hensiveness of his views, nowhere 

 sification, etc. See above, Sect. V., appear so strikingly as in this part of 

 p. 26. his work. 



3 See above, Sect. IV., notes land 2. 5 RATnKE'sUnters. iiber Bild.,etc., 



4 BAER'S Eutwickelungsgeschichte, see above, p. 119, n. 2, 



