GLACIAL EI'OCTIS AND POLAR CLLMATES. 39 



Now I think from the above evidence we may 

 safely come to the conclusion that vast changes of 

 climate have taken place at irregular intervals, not 

 only throughout Tertiary time, but during the 

 entire progress of the Glacial Epoch in the northern 

 hemisphere. Warm tropical climates have pre- 

 vailed ; cold climates have succeeded them ; the Polar 

 area has been peopled with a rich fauna and flora 

 (birds of course included) ; and just as surely has 

 it been devastated and become the habitude of icy 

 desolation, entailing the utter banishment of life. 

 We have thus seen that the Glacial Epoch was not 

 a primary cause of Migration ; rather has it been a 

 colossal agent of emigration and of banishment ; for 

 many of the avian forms then driven from their 

 Polar homes never returned, but were the founders 

 of innumerable colonies in more southern latitudes. 

 In this banishment many new forms had their origin 

 through Variation, preserved by Isolation and 

 Natural Selection. 



It now becomes necessary to show what effect 

 local glaciation may have had on the vertical Migra- 

 tion of birds. There can be little doubt that 

 during the Tertiary Period many birds although 

 resident in the Northern Hemisphere retired from 

 the lowlands to the mountains to breed, or were 

 even residents on such lofty ranges as occurred 

 within this area. Now it seems probable that 

 during phases of high eccentricity, in Miocene ages, 

 for instance, much local glaciation occurred in the 

 Alps, in Scandinavia, the Pyrenees, the Caucasus, 



