12 THE MIGRATION OF BIRDS 



they do or do not avoid dangers, or any similar 

 problem, which seems to give finality so far as 

 certain cases are concerned, is met by an absolute 

 negation in other instances. The truth seems 

 clear ; more than one factor has influence on most 

 birds, and different species in different places are 

 influenced by different factors. Elliott Coues' 

 sweeping statement, though I strongly disagree 

 with the article in which it occurs, expresses much 

 that is true. " Isepipteses and magnetic meridians, 

 coast-lines and river channels, food-supply and 

 sex-impulses, hunger and love, homing instincts 

 and inherited or acquired memory, thermometer, 

 barometer and hygrometer, may all be factors 

 in the problem, good as far as they function ; but 

 none of them, and not all such together, can satisfy 

 the whole equation." 



Some of the theses may be laws or rules, but 

 there are no rules without exceptions, and these 

 exceptions may become local rules. Laws regulat- 

 ing migration in one area, whether it be the great 

 continent of America, the British Islands or the 

 islet of Heligoland, may have little application in 

 other parts of the world : local evidence alone can 

 never solve the great problems. 



