DISPERSAL AND MIGRATIONS 145 



ones. Birds are gifted with very exceptional 

 means of locomotion ; most species can fly ; 

 many can swim ; so that the barriers that might 

 confine or check terrestrial animals would, not 

 a few readers may think, be utterly futile in 

 arresting the colonising movements of such 

 volant creatures. The facts, however, disclosed 

 in the preceding chapter all tend to demon- 

 strate that this is very far from being the case, 

 and to suggest that many more or less per- 

 ceptible influences are at work controlling the 

 dispersal of birds over the globe. In the first 

 place, if we admit that birds have sprung from 

 a reptilian stock, as the details relating to their 

 origin given in our first chapter appear incon- 

 testably to prove, and that the twelve thousand 

 (in round numbers) known species now in ex- 

 istence were not each created in the places 

 they at present occupy, then we must com- 

 mence with the fact that all species in each 

 genus (to go no farther back, this being suffi- 

 cient to illustrate our contention, but the same 

 principles apply to every family and order) have 

 descended from a common ancestor which must 

 at one time have been confined to a continuous 

 area, from which its descendants have spread 

 into other districts, where specific change has 



