300 RELATION OF TERRITORY TO MIGRATION 



abundance or scarcity, but also by the powerful 

 gregarious impulse which waxes in proportion 

 as the instincts connected with reproduction 

 wane. If, then, when the sexual instinct again 

 becomes predominant, the experience of the 

 former season nowise affects their movements, 

 little or no progress will be made in the expan- 

 sion of the range. But just as a certain entrance 

 into the bush and pathway through it, when 

 once made use of in the process of building, 

 becomes so firmly established as to form the 

 sole highway to and from the nest, so likewise, 

 when the impulse to seek isolation repeats 

 itself, the bird is constrained to seek the 

 neighbourhood wherein it had experienced the 

 enjoyment of breeding or of birth. Thus the 

 little that is added one year becomes the basis 

 for further additions in the next, and new 

 centres of distribution are continually being 

 formed from which expansion proceeds anew. 



Now as the range gradually extends into 

 regions where the climate alternates and food at 

 certain seasons is consequently scarce, the 

 distance between the customary area of associa- 

 tion and that of reproduction must perforce 

 widen. The question then arises : How will the 

 young that have no experience find their way to 

 regions wherein they can endure ? The forces 

 which may have been organised to subserve the 

 end in view are three : (1) Acquired experience, 

 (2) tradition, (3) the gregarious instinct. The 

 pioneer that carries the range a little further 

 forward starts from a base where it has associated 



