146 THE STORY OF THE BIRDS 



arisen, aided by isolation and changed condi- 

 tions of life. Now most naturalists believe that 

 this subsequent dispersal has been entirely for- 

 tuitous, that species have spread this way and 

 that over continuous land-surfaces or narrow 

 seas (in the case of land birds) ; or, in the case 

 of aquatic species, in whichever direction food 

 may have been procurable. Many of those 

 ancient land - surfaces, over which so many 

 species are believed to have emigrated, have 

 long since been replaced by water; as, for in- 

 stance, between Asia and America, and between 

 West Europe and Africa, or even between our 

 own islands and Europe. Another great factor 

 in the dispersal of birds over the globe is gene- 

 rally believed to be glacial epochs, which have 

 driven numberless species from high latitudes 

 into countries farther south. Hence the very 

 generally prevailing beHef in what is termed 

 Polar Dispersal. In fact some authorities go so 

 far as to say that Life originated first at the Poles 

 and gradually spread towards the Equator. But 

 there are many grave difficulties in the way 

 of accepting such views, and we believe the 

 evidence points to a diametrically opposite 

 interpretation of the facts of dispersal, or in 

 other words, that birds were originally evolved 



