SYSTEM OF NATURE. 
57 
of being placed exterior to the thorax they are all of them 
contained within its cavity, and even the bones of the feet 
are only extended beyond the horny box which protects 
them, when the animal is employing them in progression. 
What a contrast to this solid and compact structure is ex¬ 
hibited by the form of the lithe and tortuous serpent! 
Most of the bones of the head are permanently separate, 
those of the upper and lower jaw particularly being capa¬ 
ble of great extension ; there are perfect teeth ; the verte¬ 
brae, which are extremely numerous, are susceptible of the 
most extensive lateral motion; and the ribs, slender and 
but slightly attached, compensate for the absence 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 cu¬ 
rious 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 remark¬ 
able. 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 im- 
