THE EVOLUTION OF MIGRATIONS 93 



resident Alberta race. The small numbers are 

 annually swamped by the southern migratory 

 hordes coming up each spring from the south. 



The mallard thus comes to be represented by 

 races — not necessarily distinguishable morphologi- 

 cally — which differ in their migration dates and dis- 

 tances and perhaps routes. For reasons considered 

 in detail below, the same applies to all species of 

 wide range. Each community has worked out its 

 own salvation at the dictates of local conditions 

 and the migrations of any given species as a whole 

 thus come to show infinite local variation in detail. 

 Migrations of a single species of wide range cannot 

 be accurately described in a single comprehensive 

 statement. 



The failure of some mallards to go south and their 

 subsequent extinction affords an admirable example 

 of the working of natural selection, a basic principle 

 in the evolution of migrations. With reference to 

 normal winter conditions in Alberta the mallard is 

 unfit; it fails to survive. The principle applies to 

 all birds in the northern hemisphere (as elsewhere) 

 though the food supply is not inevitably the de- 

 termining factor. It may be low temperatures, 

 water supply, ultra-violet radiation, shortage of 

 daylight or something else. It may be obvious or 

 it may be apparently insignificant, difftcult to detect 



