Classification of Fishes 385 



cental mammals are devoid of a placenta, and by the variation 

 (presence or want) among the Elasmobranchiates themselves. 



" (4) The organs of generation in the Elasmobranchiates 

 are certainly more complex than in most other fishes, but as 

 the complexity results from specialization of parts sui generis 

 and different from those of the higher (quadruped) vertebrates, 

 it is not evident what bearing the argument has. If it is claimed 

 simply on the ground of specialization, irrespective of homo- 

 logical agreement with admitted higher forms, then are we 

 equally entitled to claim any specialization of parts as evidence 

 of high rank, or at least we have not been told within what 

 limits we should be confiened. The Cetaceans, for example, 

 are excessively specialized mammals, and, on similar grounds, 

 would rank above the other mammals and man; the aye-aye 

 exhibits in its dentition excessive specialization and deviation 

 from the primitive type (as exhibited in its own milk teeth) of 

 the Primates, and should thus also rank above man. It is 

 true that in other respects the higher primates (even including 

 man) may be more specialized, but the specialization is not as 

 obvious as in the cases referred to, and it is not evident how we 

 are to balance irrelative specializations against each other, or 

 even how we shall subordinate such cases. We are thus com- 

 pelled by the rednctio ad absiirdum to the confession that irrela- 

 tive specialization of single organs is untrustworthy, and are 

 fain to return to that better method of testing affinities by the 

 equation of agreement in whole and after the elimination of 

 special teleological modifications. 



"The question then recurs. What forms are the most nearly 

 allied to the Marsipobranchiates, and what show the closest 

 approach in characteristic features? And in response thereto 

 the evidence is not undecisive. Wide as is the gap between 

 Marsipobranchiates and fishes, and comparatively limited as is 

 the range of the latter among themselves, the Elasmobranchiates 

 are very appreciably more like, and share more characters in 

 common with them, than any other; so much is this the case 

 that some eminent naturalists (e.g., Pallas, Geoft'roy, St. Hilaire, 

 Latreille, Agassiz, formerly Liitken) have combined the two 

 forms in a peculiar group, contradistinguished from the other 

 fishes. The most earnest and extended argument in Enghsh, 



