EFFECT OF THE GREGARIOUS INSTINCT 289 



difficult to estimate their importance or to 

 indicate their precise effect ; the former, how- 

 ever, accelerate the rate of expansion, whilst the 

 latter retard it. Those individuals that wander 

 outwards and seek territory on the outskirts of 

 the range we have called " pioneers." They will 

 have advantages over others that, wandering 

 inwards, seek isolation in congested districts, 

 and will succeed where the latter fail ; and since 

 there is in the young an innate ability to 

 return to the district wherein they were reared, 

 the advantages so gained may be said to be 

 handed on from generation to generation. 



Let us now turn to the contra-phase, and 

 endeavour to ascertain whether the gregarious 

 instinct bears any relation to the seasonal 

 desertion of the breeding ground. The conclu- 

 sion at which we have already arrived regarding 

 this instinct is that it forms part of the inherited 

 nature of most species ; that its functioning is 

 suppressed when a bird is actually in occupation 

 of a territory ; and that it is serviceable in pro- 

 moting the welfare of the individual. We 

 cannot of course observe the instinct. What 

 we observe, when reproduction is ended, is a 

 change in the relations of different individuals ; 

 instead of arousing mutual hostility, they attract 

 one another, from which we infer the existence 

 of something which determines their conduct, 

 and this "something" we speak of as an 

 instinct. 



To what does this change lead ? Let us 

 suppose that there is an area inhabited by one 



