74 THE RIDDLE OF MIGRATION 



safe in assuming that the environmental influences 

 to which birds respond today, must have eHcited 

 response twenty milHon years ago. In analysing 

 the present-day factors that might have a bearing 

 on migration we are at the same time, with little 

 question, examining those of the distant past. The 

 ice-fields of today may be restricted to the vicinity 

 of the poles; yesterday they may have stretched 

 south to Kansas. This is merely a shift of particular 

 environmental conditions. The fundamentals re- 

 main constant. We may therefore proceed to ex- 

 amine more precisely the effects of the northern 

 environment on bird life as it now exists and apply 

 our findings to the past with some sense of confi- 

 dence. We may be, and no doubt shall be, wrong 

 in details but there seems no reason to make mys- 

 tery of the past. The essentials could not have 

 been radically different. 



We have noted above that many species of birds 

 could not possibly survive a northern winter. They 

 meet the situation by going south in the fall. To 

 go north, or east or west at the same latitudes, 

 would be as fatal as remaining. Their road south 

 maybe quite indirect, even circuitous. Some species, 

 for example, go south a certain distance, then w^est 

 across the mountains and then further south; others 

 go mainly south-east, to winter in the eastern States; 



