X INTRODUCTION. 



structure ; enough has been said to show how far these two 

 groups are separated from each other in their general organ- 

 ization ; and it needs scarcely to be added that the divers- 

 ity of their habits is not less remarkable. 



The relations of these groups seem almost to set all the 

 established principles of classification at defiance ; nor is there 

 any one system hitherto promulgated which appears to me 

 satisfactorily to solve the difficulty. Those who have made 

 the most philosophical attempts to ascertain the natural sys- 

 tem, the grand and harmonious plan upon which all organic 

 creation is believed to have been formed, have concurred in 

 considering the Reptilia as constituting a group of equal 

 value in the vertebrate division of the animal kingdom, with 

 the Mammalia and birds. It may be safely predicated that, if 

 the system to which I more particularly refer be true, all the 

 groups of equal rank must be founded upon characters of 

 equal value and importance. That if, for instance, the 

 group of Mammalia and that of birds, be equal to each other, 

 each of the other classes, that is to say, every other group 

 of the same rank, must be equal to each other ; and also, 

 that the subordinate groups in each of these classes must ex- 

 hibit the same mutual relations in every case. But if it can 

 be shown that in one class, so called, two ordinal groups 

 exhibit as great a discrepancy in their relative plan of organ- 

 ization as any two classes do, then the relation of the former 

 to either of the latter is not, and cannot be the same, as that 

 which exists between the latter two. Yet in this predica- 

 ment stand the three first classes of the Vertebrata ; the 

 relations of the Mammalia and birds being much stronger and 

 more obvious than those of the Reptilia to either, and the two 

 groups of the latter, which I have just sketched, the Tor- 

 toises and the Serpents, being nearly or quite as far removed 

 by their structure from each other as the birds are from the 

 Mammalia. The mode of reproduction is the sole exception 



