100 A HISTORY OF BIRDS 



On this theme much has ah'eady been written, but h'ttle to 

 any purpose. An American writer of good repute would have 

 us believe that when Professor Newton wrote that on this theme 

 "our ignorance is immense" he was really making mystery 

 where no mystery was. The whole matter, he assures us, is 

 simply one of securing suitable breeding sites ; and adduced in 

 evidence the migration of the shad and the salmon, which for 

 this purpose, and none other, leave the sea and ascend the 

 rivers, there to lay their eggs, just as the fresh-water eel for 

 a like purpose leaves the rivers to perform the duties of re- 

 production in the sea. Even so, what impels the one to seek 

 the fresh and the other the salt water ? When they start on 

 their perilous journeys have they any fixed ideas as to the 

 relative merits of fresh or salt water as repositories for their 

 eggs? The fresh-water eel and the species of salmon of which 

 he wrote perform these journeys but once. Reproduction over, 

 they die — every one, like Pharaoh's host, perishes. He further 

 seems to commit the very grave mistake of supposing that 

 what may be true of fishes is true of all other migrants. 



The veteran Darwinian, Alfred Russel Wallace, took a more 

 serious view of the matter. He says — in reference to birds — 

 {Nature, x., p. 459) : " 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 breeding can as a rule be safely 

 accomplished only 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 which 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 prob- 

 ably be found that every gradation still exists in various parts 

 of the world, from a complete coincidence to a complete 

 separation of the breeding and the subsistence areas ; and that 



