of certain Birds of Cuba. 13 
M. Cuvier, that the natural history of an animal is the know- 
ledge of every thing that regards that animal,—then Natural 
History, as a science, is only studied in effect when we are 
engaged in the pursuit of the natural system. It is not, there- 
fore, so much the difference between organs, that we ought to lay 
stress upon, as the mode in which they vary ; from which truth 
arises another, namely, that in those groups where the variation 
of an organ is at its maximum,—or, in other words, where the 
differences between the various states of an organ are the most, 
—there such an organ is of less consequence as a principle of 
division characterizing large groups; for it is the mode of vari- 
ation that we ought to attend to. Thus in the Natatores, the 
number of cervical vertebre is at its maximum of variation ; 
in the Grallatores, the form of the beak is at its maximum of 
variation ; in the Rasores, the number of lumbar vertebre ; in 
the Insessores, the economy and nature of food ;—and so on. 
The primary divisions, therefore, of these several large groups of 
birds, will not depend upon the above respective circumstances, 
although by tracing the variation of them we are enabled to 
apply corrections to the place that may have been assigned 
each species from other considerations, as close to its next of 
kin in natural affinity. 
Generally speaking, Mammalia have more vertebrz in their 
spine than Birds: but this is scarcely to be imagined a mark of 
their superior perfection; for man, undoubtedly the chief of 
Mammalia, has one of the lowest numbers of vertebrz that the 
class presents. The variation of the number of vertebra in 
Mammalia is not at all conducted on the same principles as that 
in birds: in the latter class this number varies in every possible 
way, yet on the whole is tolerably regular in its variation. In 
Mammalia the number in some respects, such as that of the ver- 
tebrz of the cervix, is almost constant; and yet with respect 
to 
