SUMMARY 299 



a third must secure a position for its nest within 

 the community. Hence it follows that the 

 degree of separation varies with the conditions 

 of existence. Since, however, the conditions in 

 the external world are constantly changing 

 according to the relative abundance or scarcity 

 of enemies, the rise or fall of rivals, the physical 

 changes in the earth's surface, and the alterations 

 of climate, it is clear that isolation can only be 

 obtained with difficulty, and that the competi- 

 tion for it must be severe. Some individuals 

 therefore fail to breed, whilst others, perhaps 

 because their impulse is stronger, persevere 

 and seek stations elsewhere. What are their 

 prospects of finding them ? By extending the 

 field of their activities, they will wander into 

 districts remote from the scene of competition, 

 districts where not only food is plentiful but 

 where enemies and rivals are scarce ; and to 

 these pioneers, if to any, success in reproduction 

 will most certainly be assured. But not only is 

 it they who will benefit ; their offspring also, 

 when the time comes for them to take their part 

 in the maintenance of the race, will share in the 

 success of their parents, for even though they 

 may not escape competition from individuals of 

 closely related forms, they will meet with but 

 little from those of their own kind. Now 

 species which live throughout the year in the 

 vicinity of their territory are comparatively few, 

 the majority are obliged to wander in search 

 of food so soon as reproduction is ended, and 

 their behaviour is determined not only by its 



