66 THE WANDERINGS OF ANIMALS [CH. 



means that it requires a pronounced spring or early 

 summer season. The idea that the arctic circle was the 

 original home of the many kinds of birds which breed 

 there, whence they are now annually driven away by 

 stress, has been coupled with the glacial epoch, that 

 panacea of so many difficulties. One had only to 

 assume that the progressive glaciation drove the 

 ancestors away from their circumpolar home, towards 

 the equator, and that when times improved again, 

 the birds returned to their old home. This plausible 

 view implies several monstrous assumptions. The 

 birds are supposed to have inherited such a loving 

 reminiscence of their old home that they returned to 

 it after a banishment of thousands of generations. 

 The enforced sojourn, and of course the breeding of 

 all these generations, was not sufficient to naturalise 

 them, to supplant the love of their prehistoric home ! 



That the last glacial epoch has driven the limit of 

 many kinds of animals and plants further south, is as 

 certain as that many have recovered the lost ground 

 after the recession of the cold spell, but this must 

 have been by very slow and steady process of spread- 

 ing. It probably does account for the present annual 

 visitations of arctic lands, but as a phenomenon which 

 has been evolved de novo, which would have come to 

 pass even if no birds had existed in preglacial times. 



The question how birds manage to find their way 

 thousands of miles across land and water, has been 



