6o THE MIGRATION OF BIRDS. 



separated from the rest of the species by the lofty 

 mountain ranges, the deserts, and other physical 

 barriers, which would effectually arrest a forest or 

 woodland haunting species. More and more severe 

 became the winters, longer and longer ; the glaciers 

 descended lower and lower, exterminating or 

 driving before them all living things. At last the 

 Spotted Flycatcher, or the form which then repre- 

 sented this species, came to be divided into two 

 enormous colonies — an African one and a Chinese 

 one — the individuals of each being completely 

 isolated from each other, summer and winter alike. 

 During the ages that this state of things continued, 

 the Flycatchers became segregated into two species, 

 owing primarily to the absence of any intermarriage ; 

 the eastern race became smaller, the tail shorter, 

 and the breast-streaks broader ; or the western race 

 became larger, with a longer tail and narrow breast- 

 streaks. It is almost impossible to say which form 

 now most closely resembles the ancestral species, but 

 such are the present differences between the two races 

 known to ornithologists respectively as Muscicapa 

 grisola (the Western and British form) and Musci- 

 capa griseisticta (the Eastern form). Such was the 

 state of things at the close of this Inter-Glacial Period. 

 Then came the gradual immigration north again 

 as precession and lower eccentricity initiated a 

 milder climate. Age after age the journey in the 

 spring became longer. Certain routes to and fro 

 became to be recognized highways of passage ; and 

 so imperceptibly did the northern breeding-grounds 

 expand, that the birds became regular migrants, 



