484 ^nulytkal Notices <>/ Books. 



within the reach of naturalists at home. To this undertaking he has 

 been chiefly induced by a desire of ascertaining the accuracy of the va- 

 rious positions taken by Mr. Vigors in his general view of Ornithology, 

 published in an earlier part of the Linnean Transactions, wherein, on 

 the principle of the variation of structure, he has developed the arrange- 

 ment of Birds much further than has been done in any other class of 

 animals, pointing out the place of every family w^ith reference to its 

 affinities and analogies, and giving the reasons in detail for this location ; 

 and subsequently carrying the same principle to the extent of assigning a 

 determinate place to every genus hitherto discovered. To try the validity 

 of the arrangement deduced from the variation of external structure, by 

 comparing it with the results obtained from a careful examination of the 

 variation of the internal structure, is to apply to it a test which can be 

 withstood by a natural pystem alone. 



On the principle that in those groups where the variation of an organ 

 is at its maximum, there such an organ is of less consequence as a ground 

 of division characterising large groups, it follows that the number of 

 vertebrce is of much more importance as connected with natural arrange- 

 ment in Birds than it is in the Mammalia, where it is subject to much 

 greater variation, differing occasionally even in closely allied species. From 

 a series of tables exhibiting the maximum and minimum numbers of the 

 vertebrce generally, and of the cervical, dorsal, sacral, and coccygian 

 vertebra respectively, in the five orders of Birds, it is shewn that the 

 variation is least in the typical orders, the Raptores and Insessores, and 

 is remarkably greatest in the two most aberrant orders, the Rasores and 

 the JVatatores: evidencing that in the latter nature is, as it were, looking 

 out for the structure of some other class ; a deduction from facts precisely 

 according with theory. And as it is clear that the JVatatores approximate 

 to the Chelonian Reptiles, it is consequently among the Rasores that the 

 approach to Mammalia must be found. To show the point of nearest 

 approach made by Mammalia to Birds, and that made by Birds to 

 Mammalia, is a branch of the enquiry into which Mr. MacLeay enters 

 fully: premising to it an exposition of the analogies existing between 

 the orders of Birds and those of the Mammalia, and an explanation of 

 the connexion of the various orders of Mammalia, in their own series of 

 affinity. 



The orders of Mammalia were clearly indicated by Aristotle and by 



