280 RELATION OF TERRITORY TO MIGRATION 



matter of chance. A male in a congested 

 district, having no incentive to seek fresh 

 ground, would remain inactive until a female 

 happened to cross its path and stimulate its 

 sexual impulse, when its activity would take 

 another form. Hence some districts would be 

 over-populated, whilst others would remain 

 unexplored. But the system of reproduction 

 does not consist merely of a search for the 

 breeding ground, and of the discharge of the 

 sexual function ; it is a much more complex 

 business, yet withal more complete. Nothing 

 is left to chance ; the end is attained step by 

 step ; and each successive stage marks the 

 appearance of some specific factor which con- 

 tributes towards the success of the whole. We 

 start with the appropriate organic condition 

 under which, when adequate stimulation is 

 provided, the disposition to secure a territory 

 comes into functional activity. Within the 

 field of this disposition we can distinguish 

 certain specific impulses. In sequential order 

 we have the impulse to seek the breeding 

 ground ; the appropriate situation which gives 

 rise to an impulse to dwell in it ; and the act 

 of establishment which supplies the condition 

 under which the impulse to drive away intruders 

 is rendered susceptible to stimulation. Grouping 

 these impulses, for the convenience of treat- 

 ment, under one general heading, I speak of 

 an impulse to seek isolation. It implies some 

 kind of action with some kind of change as 

 its correlated effect ; and from it there flows 



