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 vertebre, 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 
