EFFECT OF ISOLATION 271 



successfully accomplished providing that success 

 is attained at every stage, the probability is 

 that, of the impulses concerned, one is neither 

 more powerful nor less powerful than another. 



So that we have two impulses operating at 

 different seasons and guiding the behaviour 

 into widely divergent channels. But though 

 the proximate end to which the behaviour is 

 directed is apparently different, there are not 

 two biological ends in view, but one — the 

 attainment of reproduction ; and the changes 

 that we witness are not contrary but comple- 

 mentary, and their prospective value lies in the 

 circumstance that they contribute towards the 

 preservation of the race. 



If, then, every male is driven by inherited 

 impulse to seek the appropriate breeding ground 

 each recurring season ; if, having arrived there, 

 it is driven to seek a position of its own ; if, 

 in order to secure isolation it is obliged to 

 attack other males or to ward off the attacks 

 of intruders ; if, in short, success can only be 

 attained providing that the inherited nature is 

 so adjusted that the bird can accomplish all 

 that is here demanded — what will be the general 

 result ? That the individual will rear its off- 

 spring in safety and that they will inherit the 

 peculiarities of their parents, enabling them, 

 in their turn, to procreate their kind ; all this 

 will certainly follow. We are not concerned, 

 however, at the moment, with the direct effect 

 upon the individual, but with the consequences 

 that will accrue to the species as a whole. 



