94 THE RIDDLE OF MIGRATION 



and perhaps depending, like Darwin's classical ex- 

 ample of the clover crop, on a network of other 

 factors. Each species has to face the selection 

 committee of the northern elements to be judged 

 on its own merits. If it fails to meet the require- 

 ments it has no place in the north during the winter 

 season. Every year sees a few individuals of vari- 

 ious species failing, for one reason or another, to go 

 south. Every normal year witnesses their exter- 

 mination. During the exceptional winters they 

 survive for varying periods and their presence is 

 duly commented on. The mild winter of 1930-31, 

 unprecedented within living memory, provided a 

 particularly good example. Several thousand mal- 

 lards survived. Even the odd robin and meadow- 

 lark seem to have got through successfully. But 

 this happens only once in a long run of years and 

 the survivors, if any, count for nothing. They are 

 not even a drop in the bucket of competition. 



As has been mentioned, the selective factor is 

 not necessarily obvious. This remark would apply 

 to ultra-violet radiation. Deficiency would not 

 affect the individual in the same way as food short- 

 age for it would not prove immediately fatal. The 

 bird, other things being favorable, would still be 

 there in the spring. But its general vitality might 

 conceivably be affected seriously enough to produce 



