SYSTEM OF NATURE. 
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whence it was abstracted, it still becomes a question 
whether the reptiles are a single or a double group; whether, 
like placentals, marsupials and birds, they are constructed 
on some uniform plan, the variations in which, accompanied 
by others in their food and economy, will enable us to 
subdivide them into minor divisions somewhat similar to 
those of the three classes already investigated ; or whether, 
having dichotomously divided them by some sterling and 
constant character, each group will admit of the requisite 
subdivisions. 
First then, as to uniformity of structure, it must be evi¬ 
dent that “ There is no other class of vertebrated animals 
the different groups of which are formed upon types of 
structure differing so essentially from each other as these. 
The eagle and the humming-bird, the ostrich and the petrel, 
widely as they appear to be separated from each other, 
not by size only, but by form and habits, still exhibit the 
same general structure of the skeleton, of the organs of 
digestion and motion, of the integument, and, in fact, of 
the whole organization of the body, — the various systems 
of which differ only amongst the different groups by com¬ 
parative degrees of development. Even amongst the 
Mammalia, the whale, with its enormous and almost 
mountainous bulk, paddled through the deepest retreats of 
ocean by its short fins, which are modifications of the 
anterior extremities, and by that broad expanded oar, its 
fleshy tail, is still formed upon the same general plan of 
organization as the little light and aerial bat, which flits 
so rapidly through the regions of air, supported by its thin 
membranous wings, which are expanded upon slight and 
linear fingers, the representatives of the same bones which, 
in the former animal, are contracted into a massive and 
