"(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 homological 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 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 compelled by the _reductio ad absurdum_ 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 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, Geoffroy, St. Hilaire, Latreille, Agassiz, formerly Lütken) have combined the two forms in a peculiar group, contradistinguished from the other fishes. The most earnest and extended argument in English, in favor of this combination has been published by Professor Agassiz in his 'Lake Superior,' but that eminent naturalist subsequently arrived at the opposite conclusions already indicated.