32 THE MIGRATION OF BIRDS 



race ; that the perils of the journey are greater 

 than those occasioned by more sedentary habits. 

 It has even been suggested that migration is a habit 

 specially created to thin down the surplus bird 

 population. Dr W. K. Brooks, however, puts this 

 idea, which is not entirely devoid of truths, in 

 rather a different way. " Adaptations of nature 

 are primarily for the good of the species beneficial 

 to individuals only so far as these individuals are 

 essential to the welfare of the species " (9). The 

 destruction of overabundant young, the thinning 

 down of superfluous numbers, may be an economic 

 advantage. It is one thing to say that migration 

 has been caused to kill off a surplus, and another 

 to show that, once a habit has been originated and 

 become an advantage, it will be conducive to a 

 greater prolificness, and that the natural sequence 

 of an increased birthrate, when food supply and 

 other conditions remain unchanged, must be an 

 increased mortality. Thus the perils of migration 

 may become a boon to the species. 



The theories of C. L. Brehm (7) and Marek that 

 birds are living barometers, foretelling by intuition 

 the changes of barometric pressure, may be dis- 

 missed as purely speculative. That birds begin 

 their journeys during particular barometric con- 

 ditions is certain, but what they know of forth- 

 coming weather conditions is guess-work. 



