114 THE MIGRATION OF BIRDS. 



changes that we have already dwelt upon at some 

 length in an earlier chapter. The two last Glacial 

 Epochs (at the South and North Pole respectively) 

 were vast incentives to emigration, causing it to be 

 undertaken on a scale, so far as the class Aves is 

 concerned, never equalled before or since those 

 periods. And not only were the emigrants dis- 

 persed from these desolated Polar centres, but their 

 influx in lower latitudes must have had such a 

 disturbing influence on avian life in those latitudes, 

 as to lead, through more severe conditions of life 

 (owing to competition wdth invading species), to 

 much emigration amongst southern forms as well. 

 The next great cause of emigration is the vast 

 and almost universal amount of glaciation w^hich 

 has taken place on every continent during periods 

 of high orbital eccentricity. " In the Alps," says 

 Wallace, " the Pyrenees, in the British Isles and 

 Scandinavia, in Spain and the Atlas, in the Cauca- 

 sus and the Himalayas, in Eastern North America 

 and West of the Rocky Mountains, in the Andes, 

 in the mountains of Brazil, in South Africa, and in 

 New Zealand, huge moraines, and other unmistak- 

 able ice-marks, attest the universal descent of the 

 snow-line for several thousand feet below its present 

 level." That such ice action produced much 

 climatal change and direct banishment of organic 

 life is indisputable ; and that the influx of birds from 

 hill districts to plains and valleys necessitated emi- 

 gration on a wdde scale among upland and lowland 

 species alike, can scarcely be doubted. 



