Iv HUMMING-BIRDS 833 
we find them all pointing in the same direction. The sternum 
or breast-bone is not notched behind ; and this agrees with the 
swifts, and not with the sun-birds, whose sternum has two deep 
notches behind, as in all the families of the vast order of 
Passeres to which the latter belong. The eggs of both swifts 
and humming-birds are white, only two in number, and 
resembling each other in texture. And in the arrangement 
of the feather-tracts the humming-birds approach more nearly 
to the swifts than they do to any other birds ; and altogether 
differ from the sun-birds, which in this respect, as in so many 
others, resemble the honey-suckers of Australia and other true 
passerine birds. 
Resemblances of Swifts and Humming-birds 
Having this clue to their affinities, we shall find other 
peculiarities common to these two groups, the swifts and the 
humming-birds. They have both ten tail-feathers, while the 
sun-birds have twelve. They have both only sixteen true 
quill-feathers, and they are the only birds which have so 
small a number. The humming-birds are remarkable for 
having, in almost all the species, the first quill the longest of 
all, the only other birds resembling them in this respect 
being a few species of swifts; and, lastly, in both groups 
the plumage is remarkably compact and closely pressed to 
the body. Yet, with all these points of agreement, we find 
an extreme diversity in the bills and tongues of the two 
groups. The swifts have a short, broad, flat bill, with a flat 
horny-tipped tongue of the usual character; while the 
humming-birds have a very long, narrow, almost cylindrical 
bill, containing a tubular and highly extensible tongue. The 
essential point, however, is, that whereas hardly any of the 
- other characters we have adduced are adaptive, or strictly 
correlated with habits and economy, this character is pre- 
eminently so; for the swifts are pure aerial insect-hunters, 
and their short, broad bills and wide gape are essential to 
their mode of life. The humming-birds, on the other hand, 
are floral insect-hunters, and for this purpose their peculiarly 
long bills and extensile tongues are especially adapted ; while 
they are at the same time honey-suckers, and for this purpose 
have acquired the tubular tongue. The formation of such a 
