20. Mr.W.S. Mactreay 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 : 
NGaaeT feelbell Riaptores “a6. s it 
Imsessores «<. .-°. 94 
(Rasores’... eee 42 
Aberrant . . Rs Grallatores® . :... [25 
LNatatores.... . 39 
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 vertebrie occurs 
among the Insessores, and the greatest among the Natatores ; 
the difference between the maximum in Cygnus and the mini- 
mum in Loaia being no less than 23 vertebre. 
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. Vi- 
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. 
In Birds, as a class, moreover, we find the number of vertebre 
to 
