EFFECT OF THE GREGARIOUS INSTINCT 291 



becomes dominant, these pioneers, or at least 

 some of them, will revisit the area wherein 

 formerly they associated with companions. 

 Their offspring, however, though they will 

 have the inherited impulse and the innate 

 tendency, will not have the experience ; how 

 then will they behave ? There can be no doubt 

 that some will accompany the older birds, and, 

 being led by them, will share the experience of 

 a foriper generation ; nor any question that 

 others will collect together in the neighbourhood 

 of their birthplace and, if their impulse is 

 satisfied, will remain there so long as food is to 

 be found. Thus the gregarious instinct, work- 

 ing in close relation with acquired experience, 

 will on the one hand lead to the formation of 

 organised movements in certain directions, 

 whilst on the other it will lead to the formation 

 of new areas of association which will follow in 

 the wake of the expansion. 



We have assumed, in the imaginary case 

 which we have just taken, that the conditions 

 in the external world are such as enable the 

 birds to endure throughout the year — in short, 

 that there are no complications regarding the 

 supply of food. But we must bear in mind 

 that so long as conditions are favourable during 

 the period of reproduction, which is of short 

 duration, the breeding range can continue to 

 expand,, and that therefore, in the course of 

 centuries, regions will come to be occupied 

 wherein, owing to alternations of climate or 

 physical changes in the surface of the earth, 



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