GREGARIOUS IMPULSE 265 



chorus, there is reason to think that the impulse 

 which determines its movements is similar to 

 that which is temporarily aroused in the flock. 



Apart, however, from the evidence derived 

 from the affective aspect of the operation of the 

 instinct, the general course of behaviour lends 

 support to the view that the assemblies are 

 determined by internal factors, and are not 

 merely the outcome of circumstances in the 

 external environment. Observe, for example, 

 the manner in which the flock is built up. 

 Single individuals are content to rest alone so 

 long as no assembly is in sight, but they are 

 drawn towards their companions directly the 

 opportunity arises, just as surely as the smaller 

 aggregation is drawn towards the flock ; and so, 

 as the flock increases, it gradually absorbs all 

 the lesser flocks and smaller parties, for the 

 greater the flock the greater the attraction 

 seems to be ; and different individuals appear 

 to gain some satisfaction from being in close 

 bodily contact with one another. 



When the Curlew flies to that part of the 

 mud-flat which is first exposed by the receding 

 tide, and there associates with others, it does 

 not then do so because it has any interest in its 

 fellows, nor because they serve as an attraction, 

 but because it is constrained by hunger — in 

 other words, the association is determined by 

 accident. But when, during periods of repose, 

 it sees a flock, flies to it, and takes up a position 

 in the midst of it, it does so not because suitable 

 accommodation is lacking — not therefore because 



