Vol. XXI 

 iqo4 



1 Taverner, The Origin of Migration. ^^7 



attachment, a bird can never be as firmly fixed to its station as 

 trees and plants are to theirs. 



The cases of extermination cited are where the changes had 

 come too suddenly, or where the species had become stereotyped 

 or inflexible in habits and structure by too long and great success 

 under peculiar conditions, and so lacked the elasticity of nature 

 necessary to modify itself and its life to slight changes of 

 environment. 



A. R. Wallace has outlined another idea on the subject. He 

 suggests, in ' Island Life,' that the migrants are in search of soft- 

 bodied insects suitable for nestlings ; that, as the season advances 

 in the tropics, it becomes dryer and dryer, and such insects soon 

 disappear. According to this view, it seems at first sight to be a 

 seeking after food of a certain qualit}^ Reduced to its lowest 

 terms however, it appears as a very different matter, namely, a 

 question of quantity. It is admitted that, even in the tropics, 

 there is at least a short season when there are insects of a suitable 

 quality for nestlings. That this season is long enough to raise 

 birds, is evident, for many species closely related to our migrants 

 successfully nest and raise their broods there. If all birds bred 

 there at this same period, there would be suitable food there and 

 migrations would be unnecessary. That they do not, is an indica- 

 tion that some other factor enters into the question, and it seems 

 very probable that all birds breeding contemporaneously would 

 exhaust the supply of such food. The question, then, is one of 

 quantity more than quality. 



It may be objected that each species requires its own special 

 food at the critical nesting period, which may not be obtainable 

 everywhere. Now, if there is any truth in our present evolution- 

 ary theory, great changes in food habits have occurred in all our 

 species. But the new food supply must, in each and every case, 

 have occurred before the habits and structure for utilizing it 

 appeared. Therefore, food habits could never have originated 

 migrations, though migration undoubtedly has had a great influ- 

 ence in modifying food habits. 



It must be remembered also, that migration is a dangerous 

 undertaking to a race. A journey covering thousands of miles, 

 to be performed against innumerable enemies, both personal and. 



