Vol. XXI 

 1904 



J Taverner, The Origin of Migt-ation. 331 



aid in driving forward those that had not yet found suitable con- 

 ditions for nesting. 



In the incipiency of the migration habit, the individual move- 

 ments would be small, perhaps originating in a pair of birds 

 discovering an unexpected store of food on the side of a hill 

 opposite their usual haunts. The birds that were bred here 

 would find their way back the next year with greater ease than 

 their parents did originally, and would be in a position to make 

 further advances to the hill beyond. So each year, as the glacial 

 ice receded, the territory suited for summer occupancy would be 

 slightly enlarged, and the birds would each succeeding year, 

 during the period of greatest stress, find sustenance a little to the 

 northward of the preceding season's uttermost range. 



The migratory movements and the differentiations of the 

 breeding season are so closely connected that it is difficult to 

 determine which originated first. Migration would delay breeding 

 in the species that showed the slightest inclination towards the 

 habit ; and conversely, a delayed breeding season would actively 

 assist the evolution of migration. The origination of both may 

 have been simultaneous, though it is hard to imagine a time when 

 some slight traces of migration would not have been beneficial to 

 the races. At any rate, their effects would have been cumulative, 

 each increasing and fixing the others. Once started, then, either 

 or both would be rendered more and more pronounced, through 

 natural selection, until the extreme limit profitable for each 

 species was reached. 



The gradual extension of the extreme summer range, as the 

 glacial ice cap retreated, would most probably have been by 

 means of the younger individuals, or birds in their first breeding 

 season, of each species, as these would be weaker, and more easily 

 driven than the older ones that would have become more attached 

 to their local habitats. It seems universally true that young 

 birds do not often return to breed in the immediate vicinity of the 

 place where they are raised. There is a dispersing influence of 

 some sort at work here. It is said that the older ones drive their 

 offspring away from their hunting grounds when those offspring 

 are able to take care of themselves. I cannot say from actual 

 experience that they do this, but it seems so advantageous a 



