972 STUDY OF NATURAL HISTORY. 
are retractable, and are employed to seize their prey ; 
or they are arboreal, that is, living for the most 
part among trees. Where the curvature, on the 
contrary, is but slight, it is a sure and certain in- 
dication that such birds chiefly live upon the ground. 
This may be considered a general law of nature ; 
and perhaps a more familiar example cannot be 
cited than that of the rook, which seeks its food on 
the ground, when opposed to the jay, whose arboreal 
habits lead it to live and feed among trees. It 
will be perceived that the hind claw of the crow, 
when the foot is placed upon a level surface, is so 
much raised, that the tip or end is perfectly free ; so 
that the bird can not only walk without uninten- 
tionally scratching up in his progress any loose 
‘stones or earth, but the ends of his claws, not coming 
into perpetual contact with other substances, are 
preserved sharp and uninjured. Let us now look to 
the foot of the jay: the hind claw is at once seen to 
be much more curved, and its point, when the toe is 
on a level, touches that level, and would obviously 
be injured by such constant friction, besides im- 
peding the free walk of the bird whenever it moved 
upon the ground. By this simple character, there- 
fore, we arrive at a knowledge of the different 
habits of these two birds; and by verifying this in- 
duction through all the other groups of ornithology, 
we get a general law of structure, which may be 
employed also as an essential character for any rank 
of groups. On the different modifications observed 
among the feet of the rasorial, wading, and swimming 
orders, we cannot here dilate: it will be sufficient 
to state, that, like those just noticed in the perching 
