X INTRODUCTION. 



pent ! Most of the bones of the head are permanently 

 separate ; those of the upper and lower jaw particularly 

 being capable of great extension ; there are perfect teeth ; 

 the vertebra?, which are extremely numerous, are sus- 

 ceptible of the most extensive lateral motion ; and the ribs, 

 slender and but slightly attached, compensate for the ab- 

 sence of both anterior and posterior extremities, by being 

 themselves the instruments of the animal's progression. It 

 is unnecessary here to enter more particularly into the 

 detail of these curious diversities of structure ; enough has 

 been said to show how far these two groups are separated 

 from each other in their general organization ; and it needs 

 scarcely to be added that the diversity 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 system, 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 exhibit the same mutual relations 

 in every case. But if it can be shown that in one class, so 



