ISOLATION LEADS TO EXPANSION 287 



tion within a comparatively short distance of 

 their base, or they may come into competition 

 with rivals and fail, not necessarily on account 

 of any congenital weakness of ability, but 

 because, being warned by an alien song, they 

 may be precluded from coming into contact 

 with just the environing conditions which can 

 supply the stimulus and allow behaviour to run 

 its further course — and so be obliged to extend 

 their search into remoter districts. But it must 

 not be overlooked that they will be placed in a 

 most advantageous position so far as the attain- 

 ment of reproduction is concerned. In their 

 search for territory they will meet with little 

 opposition and will be free to select whatsoever 

 ground they will ; and be free also from 

 intrusion by neighbouring males, which is so 

 frequent in occurrence and continues for so 

 long in congested areas. Moreover, in thinly 

 populated districts, the pressure upon the 

 available means of support will not be so 

 great, neither will natural enemies be so plenti- 

 ful ; and since the offspring, guided by prior 

 experience, return to the neighbourhood of 

 their birthplace, the advantages thus gained 

 will be shared by the succeeding generation. 

 It follows, then, that the range of a species 

 will not always be continuous, will not, that 

 is to say, proceed by a series of successive 

 steps, but that sometimes in this direction and 

 at other times in that, the chain of territories 

 will be interrupted and different individuals 

 separated by distances of greater or lesser 



