292 THE BIOLOGY OF THE SEASONS 



pointed out that the more prolific birds tend to have the 

 wider migratory range a fact which fits in well with the 

 theory outlined. We venture to quote a statement of the 

 case from Mr. W. P. Pycraft's excellent History oj Birds 

 (1910) : 



" We may assume that the migratory species owe their 

 origin to the matter of food-supply. Composed of indi- 

 viduals subsisting on a food of universal range, but limited 

 in supply, they were enabled to roam farther afield as their 

 numbers strained this supply in their immediate neigh- 

 bourhood. Annually, however, a check was placed on 

 further extensions of range by the cares of breeding, and 

 by 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 farther extend 

 this to obtain the means of sustenance. That each indi- 

 vidual 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/' 



Dr. Alfred Russel Wallace has laid emphasis on the 

 abundance of food in the Far North as a factor in establish- 

 ing migration. He quotes Mr. Seebohm : " Birds go to the 

 Arctic regions to breed, not by thousands, but by millions. 

 The cause of this migration is to be found in the lavish 

 prodigality with which Nature has provided food. Seed- 

 or fruit-eating birds find an immediate and abundant 



