DISPERSAL AND MIGRATIONS 163 



other factors. Young birds are not hatched 

 with this hereditary knowledge, only with the 

 inherited impulse to undertake the habit, as the 

 many blunders committed by migrant birds in 

 undertaking their annual journeys conclusively 

 prove, which we shall learn later on. The won- 

 derful amount of variation in the length of the 

 migration journey, between the breeding-places 

 on the one hand and the winter or home centres 

 on the other, seems to me conclusively to prove 

 that the habit of migration is purely and simply 

 an acquired habit one, in fact, that is subject to 

 almost individual variation and exigencies. The 

 migration journey when first initiated must have 

 been insignificantly short, and only reached its 

 present amazing length in many species gradually 

 with the expansion, or even entire change, of the 

 breeding area. As instances of birds with very 

 short migration flights, we may name the Grey 

 Phalarope and the various Eider Ducks ; greater 

 flights are presented by such birds as the Wood- 

 cock and the Rufous Warbler, with a length of 

 from 1000 to 2000 miles ; moderate flights by the 

 Crane and the Lapwing, say from 3000 to 5000 

 miles ; long flights by the Cuckoo and the Corn- 

 crake, say from 6000 to 7000 miles ; whilst the 

 most extended are presented by such species as 



