14 THE STORY OF BIRD-LIFE. 



on a time, it was like that of a swift. This gives 

 us an inkling. We turn to the swifts and com- 

 paring their anatomy generally, with that of 

 the humming-birds, we find some extraordinary 

 resemblances. So much so that we feel justified in 

 regarding the humming-birds as having got their 

 characters from the same source ; in other words, 

 we conclude that the humming-bird and the SAvift 

 are closely allied, and regard the former as a 

 highly " specialised " form of the latter. 



We shall often have to refer to specialisation 

 in these pages, and we shall take up again this 

 question of descent. 



Specialisation may be carried to such a degree 

 of perfection that any considerable change in the 

 environment of an animal may cause its extinc- 

 tion. The sabre-toothed tiger, the moa, great 

 auk, and dodo, are illustrations of this. Let us 

 take another from ourselves. The labourer corre- 

 sponds to what we call a generalised type. He 

 can earn his living in many ways ; the watch- 

 maker, or woodcarver, or artist, only in one. 

 If the latter are prevented from carrying on 

 their trade, starvation and death stare them 

 in the face — they are too specialised to turn 

 to some new mode of earning a livelihood. 



Briefly then, the creature which we know 

 to-day as a " bird " is probably the specialised 

 product of some reptilian form of ancestor, the 

 j^articular direction of this specialisation being 

 that of flight. 



Moreover, the bird differs from all other 

 creatures that fly in the method whereby this 

 power of flight is brought about. Apart from 



