776 Mr. W. P. Pycraft on the 



land-bridge be disallowed, then we are left but one other 

 interpretation to account for their occurrence in Central 

 and South America^ which is to suppose that they travelled 

 north-eastwards through Asia, and entered the western hemi- 

 sphere by way of North America, whence they made their 

 way southwards, leaving no trace of their journey. This 

 seems incredible. Hence, then, the study of the avifauna 

 of Central America yields results of a most far-reaching 

 character. 



Whenever the problems of Geographical Distribution are 

 discussed, the evidence afforded by birds is commonly ruled 

 out of court, or is, at any rate, relegated to a subsidiary 

 position, on the assumption that from their powers of flight, 

 and proneness to migration, they can afford no reliable data. 

 This assumption is by no means justified, and from its general 

 acceptance much valuable evidence has been missed. It has 

 been overlooked that migratory habits obtain only among 

 birds inhabiting temperate regions subject to marked 

 seasonal changes — that is to say, to a considerable lowering 

 of the temperature during the winter months. But even 

 here only such species are affected as require an optimum 

 temperature all the year round, and especially during the 

 winter and reproductive periods. 



This view I ventured to put forward some time ago ^ in 

 opposition to the commonly accepted view that these periodic 

 movements were entirely due to the inability of the migrants 

 to find food during the winter months in the areas which, 

 during the summer, afforded an abundance. This is only 

 partly true, and in the case of insectivorous birds only. If 

 food were the only controlling factor there would be no need 

 for the return journey of such as were driven south, for in 

 so doing they are leaving a land of plenty to face possible 

 famine, from late frosts and innumerable perils on their 

 passage. That they are driven north with the awakening of 

 the procreative instincts can be demonstrated with some 

 show of certainty by the movements of the migrants of 



* Pycraft, W. P., ' The Infancy of Animals,' p. 93. 



