124 ON THE CLASSIFICATION OF BIRDS. 
European species, would do well to investigate this subject. 
Returning, however, to the more perfect examples of this 
type, we may observe in the common swift (fig. 64. a), 
that, altliough all the four toes are directed forward, the 
two outer, which are the shortest, have a sidelong incli- 
nation, something in the same way as our thumb, 
which, although placed forward like the other fingers, 
is yet able to grasp an object in a direction very dif- 
ferent from that which can be accomplished by the 
others. Of these two lateral toes, the inner one is the 
true hallux, and it is, like the thumb of the human 
hand, placed much more backward than the other three. 
We have not yet found, in any author, certain inform- 
ation of the manner in which these singular footed 
birds perch, or rest themselves; for although we have 
frequently witnessed their surprisingly rapid evolutions 
on the wing, their retreats are always in dark caverns 
or inaccessible heights. We think, from the peculiarity 
of their feet and their strong prehensile claws, that the 
swift rests in a semi-perpendicular attitude, clinging, 
and supported alone by the feet, all of which are, there- 
fore, anterior, for it is quite obvious they cannot, hke 
ordinary birds, perch upon a branch. The Colien of 
Africa present us with anotlier instance of this struc- 
ture; the toes, indeed, are disposed like those of the 
swifts, but tile feet are obviously intended for walking, 
which those of the swifts are not: the tarsus, in fact 
considering the size of the body, is unusually thick and 
strong, and nearly as long as in the sparrow : the length of 
the toes are proportionate, but the two outer (including 
the hallux) gradually diminish, leaving the inner next 
in length to that which follows it, and this latter cor- 
responds to the middle toe of the true perchers : the 
claws of all the toes, although much more slender than 
those of the swifts, are more curved than in ordinarv 
birds, so that they must be instruments also of protec- 
tion. The economy of these birds, for a knowledge of 
which we are indebted to Le Vaillant*, confirms what we 
• Ois. d’AfrIque. 
