vni CASES OF ADAPTATION 153 



to indicate that birds, in the full and varied perfection in 

 which we now find them, are dependent on a correspondingly 

 widespread development of insects ; and more especially of 

 those higher orders of insects, whose exceedingly diverse 

 stages of larva, pupa, and perfect insect, afforded the special 

 food for immature and full-grown birds respectively. We 

 can see how the omnipresence of insects adapted to feed on 

 every kind of vegetable food, as well as on all kinds of 

 animal refuse, has afforded sustenance to the various kinds 

 of small mammalia, reptiles, and birds, which have succes- 

 sively become specialised to capture and feed on them. 

 The early birds with toothed jaws were able to feed upon 

 the cockroaches and ancestral Neuroptera and beetles of 

 the same period. As these early birds became more 

 numerous, so they became successively specialised to feed 

 upon particular kinds of insects or their larvae, however 

 completely these might seem to be concealed or protected. 

 Thus were gradually formed the true fly-catchers (Musci- 

 capidse) and the totally distinct American fly-catchers or 

 tyrant birds (Tyrannidae), which capture all kinds of insects 

 on the wing ; the swallows, and the very distinct swifts, so 

 specialised as almost to live in the air, and to feed on this 

 kind of food exclusively ; the goatsuckers, which capture 

 night-flying insects ; the curious little nuthatches and 

 creepers which hunt over trees for small beetles concealed 

 in crevices of the bark ; while the marvellously specialised 

 woodpeckers discover the larger grubs or caterpillars which 

 burrow deeply into the wood of trees, and dig down to them 

 with their wonderfully constructed hammer-and-chisel-like 

 head and bill, and then pull them out on the tip of their 

 extensile barbed tongue. In the tropics many distinct 

 families of birds have been developed to grapple with the 

 larger and more varied insect-forms of those countries, so 

 that it may be safely concluded that no group of the vast 

 assemblage of insects but what has its more or less dangerous 

 enemies among the birds. Even the great rapacious birds, 

 the hawks, buzzards, and owls, when their special food, 

 the smaller mammals and birds, fails them, will capture 

 almost every kind of ground -feeding insects; while the 

 enormous tribes which feed largely on fruits and seeds often 



