152 TROPICAL NATURE, AND OTHER ESSAYS. 



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 pecu- 

 liarities 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 ex- 

 tensible 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, 



