72 THE RIDDLE OF MIGRATION 



the height of glaciation. The number of birds 

 breeding in northern Alberta and beyond today, 

 that reach their breeding grounds via the mountains 

 from the shores of the Pacific, a curious and other- 

 wise inexpHcable migratory route, may conceivably 

 represent species that never ceased to breed in the 

 far north and that had a trans-Rocky route thrust 

 on them by the exigencies of the \Msconsin or a 

 previous ice-age. But for most species, particularly 

 the huge group of the Passeres, the northern two- 

 thirds of the American continent was probably 

 (though not inevitably) a closed book. 



The southern third of north America must then 

 have presented conditions very similar to those now 

 obtaining over most of Canada except, of course, in 

 the matter of day-length, a crucial distinction. 

 The climate might have been that of the north but 

 the seasons must have been as they are now. 



Whatever the details of climatic change during 

 one period or another, of one thing we can be sure. 

 At no time were the essential conditions on the globe 

 required by living organisms radically different 

 from what they are today. The composition of air 

 and water, for instance, can never have varied ma- 

 terially. Even the present order of terrestrial tem- 

 peratures can not have been greatly different. A 

 drop of but 6"" or 7°C. in the mean temperature of 



