THE EVOLUTION OF MIGRATIONS 95 



unfitness in its progeny, or like my captive juncos, 

 it might fail to produce young. One or more genera- 

 tions might thus result in a race quite unequal to 

 the stringencies of a northern winter and so the 

 species would again, in final analysis, be represented 

 only by individuals that went far enough south to 

 meet their ultra-violet requirements. Ultimate 

 elimination from the north during winter would 

 be as certain as though immediate starvation of the 

 individual were its fate. 



Winter life in the north is impossible for most 

 birds. If they stay, their end is death. But this 

 statement offers no explanation as to why or how 

 they have ever found their way south . They cannot 

 sum up their environment and decide to leave after 

 the manner of men. They cannot tell that the 

 south, and the south alone, spells salvation. The 

 d'iscovery must have been entirely fortuitous. 



Before attempting to investigate this phase of the 

 question, however, we may profitably turn to a con- 

 sideration of southern conditions to see if there is 

 any incentive for birds of the south to vacate this 

 region in the spring. To take a concrete example, 

 let us imagine that the present wintering head- 

 quarters of the Lapland longspur (Calcarius lap- 

 ponicus), the middle States and Texas, represented 

 the original breeding area of the species in the 



