GLACIAL EPOCHS AND J'OLAR CLLMATES. 59 



geographical distribution admirably illustrates the 

 phenomenon of Migration. When the Sub-Polar 

 regions of the Northern Hemisphere last enjoyed 

 a warm, almost semi-tropical climate — one of the 

 mild periods of the Glacial Epoch — the Spotted 

 Flycatcher inhabited in one unbroken area the 

 Arctic woodlands from the Atlantic to the Pacific. 

 Probably it was a resident species becoming 

 partially nocturnal during the Polar night; food 

 was abundant ; its conditions of life were easy, and 

 it multiplied apace, and became a dominant, firmly 

 established species during the thousands of years 

 that it dwelt in this Sub-Polar habitat. So matters 

 continued until the slow precession of the equinoxes, 

 in conjunction with increasing eccentricity of the 

 earth's orbit, began to have a marked influence on 

 the climate, and gradually the fair forests and the 

 verdant plains were devastated by the ever-increas- 

 ing cold. Age after age the Spotted Flycatcher was 

 driven slowly south; summer after summer grew 

 colder and shorter, the periods of Polar darkness 

 more severe. At last matters became so serious 

 that the birds began to leave their northern haunts 

 in autumn, probably because their food became 

 scarce as the various insects either retreated south 

 or began to hibernate. Further and further south- 

 ward these annual journeys had to be taken, until 

 the Flycatcher at last found its way during winter 

 into Africa, Persia, Arabia, India, China, and even 

 the Philippines and the Moluccas. Summer after 

 summer the belt of breeding-ground became wider 

 and wider, and vast numbers of individuals became 



