7 20 Taverner, T/ie Origin of Migration. frulv 



plentiful food supply and escape death by the causes attendant 

 upon the evils of insufficient nourishment. 



Migration, if the outcome of these phenomena, probably would 

 have originated in the following manner. In the beginning of the 

 breeding season, the competition would originate in the areas 

 containing the earliest breeders, and would be severest in the 

 most productive districts. Here the strongest species would soon 

 drive out the weaker ones and the later breeders, which, having 

 no parental ties to bind them to any one locality, would be more 

 easily forced to leave than those already possessing nests — all 

 other things, of course, being equal. These species, driven away, 

 would encroach on others, forcing them out, in their turn, to tres- 

 pass upon a wider circle of species. Thus the pressure arising 

 from the congestion originating probably in the center of the win- 

 ter residential area, would be felt to the farthest points of the 

 populated territory. Any stringency of food supply invariably 

 causes greater exertions on the part of the inhabitants, and hence 

 wider ranging ; and the slightest increase in sustaining power of 

 adjoining lands would be immediately found and taken advantage 

 of. As these species moved into the new country, their places 

 would be quickly taken by those behind, and as the congestion 

 was relieved, the impelling force would be constantly reinforced 

 by the nesting of the later breeders as the season progressed. 



The increase of population and life-supporting area would pro- 

 ceed regularly and evenly, so that the pressure would never exceed 

 the relief. This nice balance would, of course, have been secured 

 according to the laws of survival of the fittest — undesirable forms 

 that would disturb the equilibrium, being either modified or elimi- 

 nated. 



Thus each species, crowded on by those behind, and enticed by 

 the advance of those in front, would proceed onward until their 

 own particular station had been reached. This point would be 

 determined by one or more of several factors. The most obvious 

 of these would be the failure of their particular food, the arrival 

 of their nesting season, and the absence of superior competitors. 

 When a species had reached this stage in its own particular migra- 

 tion, it would settle down and nest, and from then, to the end of 

 its nidification period, would be fixed, and by its own increase 



