^°lw^^] Taverner, The Origin of Migration. 329 



adjoining area whenever that area assumes conditions favorable 

 for the support of an increased population. The return of spring 

 causes the favorable conditions in the north, and the spring migra- 

 tion is the evidence of the overflow. The approach of winter influ- 

 ences life in the same manner, but the overflow, or migration is in 

 the opposite direction. 



Mr. Allen has very aptly applied the saying that " Nature abhors 

 a vacuum," and suggests that migration is the only manner in 

 which a zoological vacuum, in a country whose life-supporting 

 capacity is a regularly fluctuating quantity, can be filled by non- 

 hibernating animals. 



That this view is correct, I do not think can be doubted, but 

 there is another factor in the case that does not seem to have 

 been generally perceived, — a fact that strengthens the foregoing 

 reasoning manifold. True, Mr. Newton, in his ' Dictionary of 

 Birds ' has suggested it, but without apparently perceiving what 

 a powerful factor it must prove in the case. I refer to the effect 

 of the large increase of life in the breeding season, in an already 

 thickly populated country, such as the southern stations must be 

 just previous to the spring migration, coincident with the opening 

 up for settlement of a vast adjoining and practically unoccupied 

 territory, by the seasonal recession of the winter ice cap. Under 

 the "Law of Malthus" we find a country to the south of us, popu- 

 lated to its fullest extent during the winter. Spring comes, and 

 nearly every pair of birds has a nest full of young, requiring 

 great quantities of food. The food demand must be increased to 

 many times what it was before. There \vould, of course, be an 

 increase in this food supply, due to the influence of spring, but it 

 would not be in proportion to the demand. This inadequacy of 

 the food supply is brought home to us very clearly if we reflect 

 upon the fact that it takes the whole northern hemisphere to sup- 

 port the species in the summer that all through the winter were 

 confined to a very limited territory ; and that even then, during 

 the time of greatest dispersal and food supply, the competition is 

 always keen. Considering, then, that this great increase in popu- 

 lation happens contemporaneously with an equal growth of the 

 food producing territory due to the return of spring, it does not 

 seem at all wonderful that the birds should migrate to utilize a 



