296 RELATION OF TERRITORY TO MIGRATION 



experience, they will be guided by them and 

 Avill find themselves in regions where food is 

 plentiful. Hence in each generation those will 

 survive that, owing to some congenital variation 

 of their instinct, seek satisfaction for their 

 impulse in a direction which brings them under 

 the influence of tradition. And though at first 

 but slight and not in themselves of survival 

 value, such variations, since they coincide with 

 modifications of behaviour due to acquired 

 experience, will be preserved and in the process 

 of time so accumulated as to be capable of 

 determining the direction and extent of the 

 movement. 



But the young Cuckoo deserts this country 

 many weeks after its parents, and there is no 

 reason to suppose that it lives in society when 

 eventually its destination is reached ; and the 

 young Falcon passes to the south, and is 

 certainly not gregarious — how then can we 

 explain their behaviour in terms of something 

 which they show no signs of possessing ? I do 

 not wish to make light of a difficulty which 

 admittedly, at first sight, is a grave objection to 

 the view that the gregarious instinct has been 

 operative in the manner here claimed for it. It 

 must, however, be borne in mind that this 

 instinct, though originally developed to serve 

 the purpose of mutual protection, supplies the 

 material upon which evolution works when the 

 extension of the breeding range creates a 

 situation requiring readjustment on the part of 

 the organism to new conditions of life ; and that 



