190 THE STORY OF BIRD-LIFE. 



ments of flight. Even the very folds of skin 

 which invest these wings agree with those of 

 birds that fly, and tell the same tale. 



We may conclude then that though the wing 

 in the moa had completely vanished, or that of 

 the Hesperornis was worse than useless, both 

 almost certainly were not only descended from 

 birds that fly, but once possessed this power 

 themselves. A conclusion fraught with consider- 

 able importance when considered in relation to 

 the distribution of birds in time as we shall see 

 presently. 



For the purpose of illustrating the fact that 

 one organ may be developed at the expense of 

 another, more clearly we may instance the swift 

 and the frigate-bird. These two, though not in 

 the most remote degree related one to another, 

 agree in this, that the wing has reached a com- 

 paratively high grade of specialisation — though 

 taking a different form in each — whilst the leg 

 has in both become so much reduced in size as 

 to render walking a difficult process. In the 

 swift they are so short that the bird can only 

 rise from the ground, should it alight thereon, 

 with extreme difficulty. The powers of flight 

 possessed by these two birds are too well known 

 to need description. 



But what connection can all this have with the 

 distribution of birds in time 1 Briefly this : the 

 discovery of the fossils just enumerated shows 

 us that birds existed as far back as the Jurassic 

 epoch ; but these same fossils are those of birds 

 which have reached a considerable degree of 

 specialisation, or, as we sometimes call it, of 



