Nest-building, Incubation, and Migration. 259 



is surely," says Mr. Headley,* " a matter for wonder. Still 

 more astounding is it that the young birds, with defective 

 strength and no experience, should start on the great pil- 

 grimage alone, instead of waiting for the old birds to guide 

 them. And in spring, too, when you see the first swallow, 

 it is a startling thought that the small bird whom you see 

 practising his short swallow-flights, started perhaps only 

 some ten days before on his northward voyage from Natal." 

 As to suggestions with regard to the origin of the 

 instinct — the congenital tendency of a definite and deter- 

 minate nature — that of Mr. A. E. Wallace, quoted by 

 Professor Newton,t is as follows : " It appears to me 

 probable that here, as in so many other cases, ■ survival of 

 the fittest ' will be found to have had a powerful influence. 

 Let us suppose that in any species of migratory bird, breed- 

 ing can, as a rule, be only safely accomplished in a given 

 area; and further, that during a great part of the rest of the 

 year sufficient food cannot be obtained in that area. It will 

 follow that those birds which do not leave the breeding area 

 at the proper season will suffer, and ultimately become 

 extinct, which will also be the fate of those who do not 

 leave the feeding area at the proper time. Now if we 

 suppose that the two areas were (for some remote ancestor 

 of the existing species) coincident, but by geological and 

 climatic changes gradually diverged from each other, we 

 can easily understand how the habit of incipient and 

 partial migration at the proper seasons would at last 

 become hereditary, and so fixed as to be what we term an 

 instinct. It will probably be found that every gradation 

 still exists in many parts of the world, from a complete 

 coincidence to a complete separation of the breeding and 

 the subsistence areas ; and when the natural history of a 



* " The Structure and Life of Birds," p. 351. 



t " Dictionary of Birds," p. 556, quoted from Nature, vol. x. p. 459. 



