294 RELATION OF TERRITORY TO MIGRATION 



reason to believe that its origin dates back to 

 an early period in the evolution of the higher 

 forms of life ; and if in" the subsequent course 

 of evolution it could have been so organised 

 as to serve a double purpose, so much the more 

 reason would there have been for its survival. 

 In what does the instinct consist ? Is it merely 

 that the sight of this individual or the call of 

 that proves at some particular moment an 

 irresistible attraction, or does the appropriate 

 organic condition give rise, as is generally 

 supposed, to some preceding state of uneasiness ? 

 In the former case, the temporarily isolated 

 individual or colony would have but little 

 chance of sharing in the benefits which mutual 

 association confers upon the associates ; in the 

 latter, the feeling of discomfort would lead to 

 restlessness, and would thus bring the bird into 

 touch with the environing circumstances under 

 which instinctive behaviour could run its 

 further course. So that it is probable that the 

 movements of each individual, prior to its 

 becoming a unit in the flock, are not accidental 

 but are determined in some measure by racial 

 preparation. 



Now if the fundamental assumption of the 

 doctrine of the struggle for existence be true, 

 the gregarious instinct will not be quite alike in 

 all the members of different broods, nor even in 

 each member of the same brood ; that is, 

 variation will occur in all possible directions. 

 And we shall not, I think, exceed the limits of 

 probability if we assume that different individuals 



