DISTRIBUTION OF BIRDS IN TIME AND SPACE. 191 



intensification : and they must therefore be the 

 descendants of less specialised, or, as we say, 

 more generalised forms, of which, at present, 

 we have no record whatever. Thus, far from 

 supposing that birds had their origin in the 

 Jurassic period, we must conclude that they 

 had already appeared before this epoch. 



The discovery of so extremely specialised a 

 form as Hesperornis is one of deep significance, 

 and shows that the divers must have been 

 already long in existence, possibly extending 

 as far back as Archceopteryx itself. 



Moreover, we must guard against supposing, 

 as some do, that such and such a group of birds 

 must be descended from such and such a fossil. 

 Hesperornis for instance is not the ancestor of 

 the modern divers, which so closely resemble it. 

 A form which had lost the power of flight so 

 completely cannot have begotten offspring in 

 which the wing is fairly well developed. The, 

 present divers are the descendants of flying 

 birds. Hesperornis is a collateral branch of the 

 same family. 



We shall see in the next chapter, wherein we 

 shall discuss pedigrees, how intricate this problem 

 of descent is. 



Closely connected with the question of the 

 distribution of birds in time is their distribution 

 in space, or as we generally call it their " Geo- 

 graphical distribution." 



A moment's reflection will convince us that 

 the distribution of animals — whether birds, or 

 beasts, or fishes — is not likely to be influenced 

 by political boundaries, nor even necessarily 



