20, Mr.W. S. Macleay on the Comparative Anatomy 



On reviewing the above five tables, we find that the amount 

 of variation in each order respectively may be expressed by the 

 following numbers : 



T,^ , f Raptores 11 



Jsormai . . . .< ^ 



^ Insessores 24 



fjlasores 42 



Aberrant . . .•( Grallatores .... 25 



LNatatores 59 



Hence we learn, that the whole number of vertebral joints 

 varies most in the three Aberrant groups, and least in the two 

 Normal ; that it varies much less in the Raptores than in all the 

 other orders ; that it varies the most in the Rasores ; and that the 

 degree of variation is nearly alike in the Rasores and Natatores, ' 

 and in the Insessores and Grallatores. We also perceive from the 

 first of the five tables, that the least number of vertebrse occurs 

 among the Insessores, and the greatest among the Natatores; 

 the difference between the maximum in Cygnus and the mini- 

 mum in Loxia being no less than 23 vertebrae. 



Among the Mammalia the minimum number hitherto observed 

 is in the genus Pteropus, and the maximum among the Cetacea; 

 both thus showing a parallelism of analogy with birds. The 

 minimum number is 24, the maximum 73, the difference 49. 



I have entered into this subject at some length, "not merely 

 because it affords us a curious test of the accuracy of Mr. A'^i- 

 gors's general arrangement of the orders, but also in the idea 

 that it might help to solve a problem of great difficulty ; namely, 

 which two of the five orders of Birds lead us to the contiguous 

 classes of Mammalia and Reptilia ? The argument may be thus 

 stated : — The vertebral axis is the great characteristic of the 

 sub-kingdom Vertebrata, to which all these three classes belong. 

 Jn Birds, as a class, moreover, we find the number of vertebrae 



to 



