40 THE MIGRATION OF BIRDS. 



the Himalayas, and elsewhere, wherever great ex- 

 tents of high land offered favourable conditions 

 to its initiation. From these localities all living 

 things were driven down the hillsides by the 

 advancing glaciers ; although on the lowlands and 

 in the Polar area, where no vast masses of elevated 

 land occurred, and where other geographical con- 

 ditions were favourable, such a phase of high eccen- 

 tricity produced little change, and these areas 

 remained in the enjoyment of an uninterrupted 

 climate of a warm character. This local glaciation 

 would have the effect of slowly compelling resident 

 birds to adopt migratory habits, or cause consider- 

 able emigration amongst them ; and it is conceivable 

 how birds that visited the uplands every summer 

 gradually modified their movements in various ways 

 to adapt themselves to the changing aspect of their 

 breeding areas. As these glaciers once more 

 retreated, migratory movement would again be 

 excited in many directions ; and one can imagine 

 how birds after becoming residents on the lower 

 grounds, renewed their seasonal visits, or even emi- 

 grated on a large scale to the haunts their ancestors 

 held in past ages. Even during such periods of 

 low eccentricity as prevail at the present day, the 

 influence of precession on the snow-line of moun- 

 tains would be considerable, and its results on the 

 movements of birds important. 



A few words now on the probable future of Migra- 

 tion. That it will undergo many important changes 

 during the next precession of the equinoxes is as 



