of birds, since the number of individuals is greater than in any other genus.
We can not believe that all migratory birds inherit the habit from a common parent which was migratory, nor is it probable that in all cases it owes its origin to the same influences; but if the view which is here advanced be correct, we must believe that in most migratory birds it has been brought into existence by the needs which are involved in reproduction, and not by the supply of food, and that the winter home of birds in tropical and temperate regions, and not the birthplace of modern birds, must be regarded as the starting point for the migratory habit.
While Wallace was the first to recognize the importance of selection in the formation of this and other instincts, he seems to think selection alone, without the influence of geological change, is inadequate to explain all the facts of migration. He says: "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 birds breeding can, as a rule, be only safely accomplished in a given area; and, further, that during the great part of the rest of the year sufficient food can not 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 or 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 become what we term an instinct. It will probably 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 when the natural history of a sufficient number of species is thoroughly worked out, we may find every link between species which never have a restricted area where they breed and live the whole year round to others in which the two areas are absolutely separated."
Modern zoölogy owes its scientific basis to the work of Wallace and Darwin on the distribution of birds, which, in their hands, has led to a revolution in our conceptions of Nature, and has given so much weight to their opinions that no one would venture to differ from them inconsiderately, although when we try to interpret, in the light of his other writings, Wallace's assertion that "the habit of incipient and partial migration" may "at last become hereditary," we must doubt whether he has carefully weighed his words.
