XXI 



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 confined. 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 excluding man) may be more specialized, but the 

 specialization is not as obvious as in the cases referred to, and it is not evi- 

 dent how we are to balance irrelative specializations against each other, 

 or even how we shall subordinate such cases. 1 We are thus compelled by 

 the reductio adabsurdum to the confession that irrelative 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 Elasrao- 

 branchiates 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, Geoffrey St.-Hilaire, Latreille, Agassiz 

 (formerly), Liitken) have combined the two forms in a peculiar group, con- 

 tradistinguished from the other fishes. The most earnest and extended 

 argument, in English, in favor of this combination, has been published by 

 Prof. Agassiz, in his "Lake Superior," 8 but that eminent naturalist subse- 

 quently arrived at the opposite conclusions already indicated. 



The evidences of the closer affinity of the Elasmobranchiates (than of 

 any other Fishes) with the Marsipobranchiates, are furnished by (1) the 

 cartilaginous condition of the skeleton ; (2) the post-cephalic position of the 

 branchiae ; (3) the development of the branchiae, and their restriction to spe- 

 cial chambers; (4) the larger number of branchiae ; (5) the imperfect develop- 



1 It will recur to the reader that in the case referred to, the question is really as to 

 the degree of specialization. 



2 Agassiz (Louis). Lake Superior: its Physical Character, Vegetation, and Animals, 

 159, pp. 249-252. 



