142 THE MIGRATION OF BIRDS. 



is not only of intense interest in itself, but of vital 

 importance as an indicating demonstration of 

 physical mutations and biological changes as vast 

 and far-reaching in their results as any that the 

 Northern Hemisphere has experienced. 



The apparently anomalous fact that very few 

 birds breeding in the Southern Hemisphere during 

 summer in the south are known to migrate north 

 of the Equator to winter during summer in the 

 Northern Hemisphere, is a profoundly important 

 one — a fact which in reality is the key to the 

 phenomenon of migration as it is practised during 

 present time. Broadly speaking, every migratory 

 bird throughout the world leaves a warmer climate 

 or zone to breed in a cooler climate, either by 

 ascending mountains until the altitude furnishes 

 the degree of temperature necessary, or by visiting 

 temperate or Arctic regions where similar conditions 

 prevail. A vast number of species then pass from 

 tropical climates to temperate and Arctic latitudes, 

 probably because this area is much more extensive 

 and suitable than the restricted mountain regions 

 in lower latitudes. In these vast northern areas 

 there is no lack of room, and an abundance of food, 

 resulting in easier conditions of life, and consequent 

 decrease in racial struggle for existence. But, as 

 we have already seen, these Polar regions of avian 

 Paradise are by no means eternal ; banishment waits 

 upon the bird world there, in the form of glaciation 

 and the complete reversal of climate at either Pole 

 in the course of equinoctical precession combined 



