252 FROM NORTH POLE TO EQUATOR. 



without the regularity characteristic of all true migrations. The 

 largest bats, the flying foxes, fly long distances every evening in 

 search of the fruits on which they chiefly subsist; they do not hesi- 

 tate to cross an arm of the sea fifty or sixty miles in breadth, and 

 they must even have traversed the distance between Southern Asia 

 and the East of Africa, as certain species occur in both these regions. 

 The bats proper accomplish at least as much. Following the reappear- 

 ance of the insects, which occurs at different times in regions of 

 diff'erent altitude, they ascend from the plains to the mountain heights, 

 and descend in autumn to the low grounds again ; they pursue the 

 numerous flies which congregate about the wandering cattle-herds 

 of Central Africa, and they migrate also from the south towards the 

 north and return southwards again, or in reverse order. The boreal 

 bat appears at the beginning of the bright nights in the north of 

 Scandinavia and Russia, and leaves these districts, which may be 

 considered its head-quarters, towards the end of summer, to spend 

 the winter among the mountains of Central Germany and the Alps. 

 The pond-bat is regularly seen on the plains of North Germany 

 during summer, but only exceptionally at that period among the 

 mountains of Central Germany, in whose caverns it spends the 

 winter. That other species of bat occurring in Germany change 

 their place of abode in a similar manner can scarcely be 

 doubted. 



In the cases cited, which have been selected from a mass of 

 available material, I have given examples of those migrations of 

 mammals which we may call voluntary, because of their regularity; 

 but in so doing I have by no means completed my task. Hunger 

 and thirst, the poverty and temporary inhospitableness of a parti- 

 cular region, sometimes press so severely on certain mammals that 

 they endeavour, as if despairing, to save themselves by flight. 

 Abundant nourishment and good weather favour the increase of 

 all animals, and affect that of a few plant-eating mammals to such 

 an extraordinary degree, that, even under propitious conditions, their 

 habitat must be extended. But if one or more rich years — in some 

 cases a few favourable months — be followed by a sudden reverse, 

 the famine soon passes all bounds, and robs the creatures not only 



