290 RELATION OF TERRITORY TO MIGRATION 



species ; that the number of inhabitants has 

 reached the maximum that the means of 

 sustenance will allow ; and that the season 

 of reproduction is drawing to a close. The 

 position will then be as follows. All the 

 available breeding ground is divided into terri- 

 tories ; each territory is occupied by one unit, 

 the family, and each individual is able to fend 

 for itself; changes both internal and external 

 begin to take place, the gregarious instinct 

 comes into functional operation, and the supply 

 of food diminishes that roughly is the position. 

 The internal factor operates so that the sight 

 of this individual or the call of that, instead of 

 evoking hostility as heretofore and keeping 

 different units apart, proves now an irresistible 

 attraction ; so that in place of a number of 

 individuals evenly dispersed over the whole of 

 this area, a small number of flocks of various 

 dimensions are stationed at certain points, which 

 points are determined partly by experience, 

 partly by the supply of food, and partly by 

 accident. This implies for each individual some 

 movement in some direction. But since the 

 population of this imaginary area has reached 

 the maximum, and the supply of food, though 

 limited in distribution, is nevertheless plentiful, 

 such movements will be irregular and will pro- 

 ceed in no definite direction. 



Now let us suppose that the breeding range 

 extends and that fresh ground is occupied by 

 pioneers. When reproduction and the rearing 

 of broods are ended and the gregarious instinct 



