THE EVOLUTION OF MIGRATIONS 101 



the western environment has greatly increased its 

 survival rate and the extermination of many thou- 

 sands annually at the hand of man has failed to prove 

 an effective check. Whether it is increased culti- 

 vation, as has frequently been suggested, is proble- 

 matical. There is evidence that 35 years ago, be- 

 fore general cultivation had begun, crows were more 

 abundant than 20 years ago. The magpie, we know, 

 has in recent years returned to territory from which 

 it had wholly disappeared some 30 years previously. 

 The Canadian West, incidentally, is now nearly as 

 dry and waterless as it was in the nineties. Ducks, 

 comparatively scarce then, are again greatly re- 

 duced. The environmental factor, even though in 

 our limited state of knowledge we may fail to detect 

 it, that proves beneficial to one species may prove 

 detrimental to another. With ducks getting scarcer, 

 crows and magpies are increasing. The cessation 

 of breeding in the southern range of the longspur 

 may have been due to one or several of many fac- 

 tors but even though it would be the merest guess- 

 work to attempt to consign them, we can rest as- 

 sured that selection has, in one way or another, 

 decreed the situation. Changes on the face of the 

 North American continent have been considerable 

 at different periods. What was luxuriant forest 

 once is now grassy prairie or even desert. What is 



