breeding season, the natural food supply would be taxed to its limits; the falling temperature drove some and finally all to seek food further south, and their short migration to lands already filled with old and young birds, caused pressure and overcrowding further south. Further outward and usually southward movement was necessary and the zone of stress was gradually extended, though probably in those early days no particular species took long passages. The winter passed and the vacuum was again provided, and the rebound to fill it would create a slackening force all along the line; birds would spread from congested districts so soon as food supplying areas opened to receive them.
Mr Taverner, arguing on these lines (51), shows that competition would be originated in areas containing the earliest breeders, and be severest in the most productive districts. Weaker and later breeders would be driven out or prevented from colonizing by the stronger and earlier species, and the evicted ones would encroach on others, forcing them in turn to trespass on a wider circle of species. He then argues how the gradual recession of the glacial ice would increase the possible northward breeding area, and cause longer migration, and that this migration would delay breeding and conversely delayed breeding would assist the evolution of migration.
But the lengthening of the journey might surely
