104 THE RIDDLE OF MIGRATION 



possess an inherent ability to migrate; by far the 

 greater number of those that have tapped the re- 

 sources of the north have been compelled to make 

 use of that ability. Some time during past ages the 

 custom has been acquired. Whether or not it is 

 now hereditary is a topic of paramount importance 

 to which we shall return later. 



Migrations of the northern hemisphere only have 

 been thus far dealt with. Given an animal like a 

 bird with its great powers of flight and a persistent 

 tendency to wander, natural selection alone can 

 completely account for the facts, provided that an 

 acquired habit ca7i become inherited custom. As has 

 already been remarked, the same laws, although 

 conditions may be entirely different, must apply 

 elsewhere, in which case the results will also differ. 



Let us examine the circumstances of a subtropical 

 species that winters on the plains and migrates into 

 the mountains to breed. Starting with random 

 wandering as before, if selection establishes a higher 

 rate of winter survival on the plains and a more 

 successful summer rate of reproduction in the 

 mountains, providing there is a periodic wandering 

 from the one zone into the other, migrations from 

 the plains to the mountains and back again will in 

 time become established. Such migrations will 

 have no particular relationship to the four points of 



