MIGRATION 105 



the diminution of food at the end of the breeding season — 

 whether caused by climate or otherwise — while behind them 

 the supply was increasing. Thus they were drawn back 

 towards their starting-point. Again threatened by famine, they 

 once more turned outwards, finding the earlier depleted area 

 restocked. These movements, in short, were doubtless then, 

 as now, periodic, and determined largely, if not entirely, by 

 seasonal changes. Such species increasing numerically with 

 their increase in range were naturally automatically compelled 

 to still further extend this to obtain the means of sustenance. 

 That each individual would return by the route he came by is 

 but a natural inference, and the same is true of the offspring of 

 each pair- — -hence the "homing instinct" and the formation of 

 British — or other races — of migrating species. 



The range of a migratory species, in fact, would seem to 

 increase of necessity — so long as no severe check is placed on 

 the annual increase in numbers of that species, and this because 

 the younger generations have to stake out new claims for 

 territory, so to speak : since the older birds will return to the 

 breeding spots annexed by them previously, and will drive 

 away any younger would-be settlers, which must therefore 

 settle outside the range of the earlier occupiers of these sites. 



Thus is explained the fact that the Common Swallow 

 {Hirundo rustica\ for example, is found breeding throughout 

 the whole of its range from Southern Europe northwards, those 

 breeding farthest north having been forced thither by the settle- 

 ment of the earlier migrants : and thus is explained the general 

 rule that species which go farthest north in summer go farthest 

 south in winter. 



