CAUSE AND ORIGIN OF MIGRATION 31 



be occasioned in another way, and the evolution 

 of migration assisted apart from any glacial in- 

 fluences. Each successive increase of the length 

 of the journey taken by the stronger and more 

 go-ahead individuals, leading them in advance of 

 the bulk of southward moving and competing 

 birds, would be a distinct advantage to the individual 

 and consequently to the species. The pioneer would 

 arrive, like the slower movers, in a land already 

 peopled with an avian population, but it would 

 not have its own fellows to add to the stress of 

 competition ; it would be ahead of the greatest 

 struggle. So the fittest would mould for the species 

 the most suitable journey both in distance and 

 route, and the laggards would gradually fall out 

 of the competition. 



Dr Wallace, without destroying these arguments, 

 has shown that the survival of the fittest has a 

 powerful influence. Those birds which do not 

 leave the breeding area at the proper season will 

 suffer and ultimately become extinct, and the 

 same will happen to those which fail to leave the 

 winter quarters when it would be a distinct ad- 

 vantage to the species to move into lands better 

 suited for reproduction. 



It has been put forward as a serious objection 

 to many arguments that migration, instead of 

 being advantageous to birds, is a danger to the 



