GLACIAL EPOC US AND POLAR CLIMATES. 35 



the acquirement of a migratory habit from more 

 local causes. It will thus be seen that it is quite 

 impossible to explain the origin of Migration in 

 every species, or in an individual degree, but only 

 to suggest its cause in the broadest sense. With 

 Polar groups it is comparatively easy, as, for 

 instance, in the ChvVRadrud.e. But in all, broadly 

 speaking, the habit had its origin either in direct 

 climatic change, or in the exposure to a different 

 and more severe climate due to Emigration. Again, 

 some species must have acquired the habit sooner 

 than others, just as we know at the present time 

 the migratory flight of some birds is earlier or later 

 than that of others, according to specific or even 

 individual requirements. From this we must infer 

 that Migration did not have a simultaneous origin ; 

 some species probably remained stationary ages 

 after others had acquired the habit, gradually 

 acquiring it as the causes became more intensified. 

 Broadly speaking, it is birds that subsist on an 

 insectivorous or an animal diet that are the greatest 

 and most regular migrants, and they would be the 

 first to acquire the habit. But even from the great 

 variety of their food many would be compelled to 

 adopt migratory habits before the others. Birds 

 subsisting on a vegetable diet would persist in their 

 sedentary habits the longest, and remain resident so 

 long as their food was not buried in the snow. 

 Many ages, however, must have elapsed between 

 the period when the winters became too severe for 

 insect life to remain active, and the ages when 



