PURPOSE OF SONG 147 



in the earth's surface ; and so the distribution of 

 the bird population in any given area, though it 

 may be suitably adjusted for one year or even 

 for a period of years, is bound in the course of 

 time to require readjustment. Now there 

 cannot be readjustment without competition, 

 nor competition without combat. But the 

 appeal to physical force is only a means to an 

 end, and, since no male can endure incessant 

 warfare and the perpetual strain of always being 

 on the alert, without experiencing such physical 

 exhaustion as might affect his power of repro- 

 duction, its direct effect upon the combatants 

 cannot be otherwise than harmful — in fact it is 

 a necessary evil which for the good of the 

 species must be kept strictly within bounds. 

 Bearing in mind, then, these two facts, namely 

 that the distribution of the males is never stable 

 and that overmuch fighting may defeat the end 

 in view, we can appreciate the importance of 

 any factor which will lead to a more uniform 

 distribution and at the same time insure security 

 by peaceable means. 



The proximate end of the male's behaviour is 

 isolation — how is it to be obtained ? If, after 

 having occupied a territory, the bird were to 

 remain silent, it would run the risk of being 

 approached by rivals ; if, on the other hand, it 

 were merely to utter the recognition call of the 

 species, it would but attract them. In neither 

 case would the end in view be furthered, and 

 isolation would solely depend upon alertness and 

 the capacity to eject intruders. Supposing, 



