Adaptations may relate to habits and behavior as well as to structure. Plainly adaptive are such habits as the migration of birds and some other animals, most of the habits connected with food-getting, and especially striking and interesting those connected with the production and care of the young, including nest-making and home-building. =Species-forming.=--It is evident that through the cumulative action of natural selection, animals of a structural type considerably (even unlimitedly) different from any original type may in time be produced by the gradual modification of the original type under new conditions. If, for example, a few individuals of a mainland species should come to be thrown as waifs of wave and storm upon an island, and if these should be able to maintain themselves there and produce young, increasing so as to occupy the new territory, there would be produced in time a new type of individual conforming or adapted to the conditions obtaining in the island, these conditions being, of course, almost certainly different from those of the mainland. Thus as an offshoot or derivation from the original type still existing on the mainland we should have the new island-inhabiting type. Now when these island individuals come to differ so much, structurally and physiologically, from the mainland type that they cannot, even if opportunity offers, successfully mate or interbreed with mainland individuals the island type constitutes a new species. That is, our distinction between species rests not only on structural differences, but on the impossibility of interbreeding (at least for the production of fertile young). Such a combination of the action of natural selection and the condition of isolation (as illustrated by the case of island animals), is probably the most potent factor in the production of new species of animals (and plants). For accounts of the struggle for existence, variations, adaptations, natural selection and species-forming see Darwin's "Origin of Species," Wallace's "Island Life," and Romanes' "Darwin and After Darwin," I. =Artificial selection.=--When a selection among the individuals of a species, that is, the choosing and preserving of individuals which show a certain trait or traits and the destroying of those individuals not possessing this trait, is done by man, it is called artificial selection. To artificial selection we chiefly owe all the many races or varieties of our domesticated animals and plants. For example, from the ancestral jungle fowl have been developed by artificial selection (and by cross-breeding) all our kinds of domestic fowl, as Brahmas, black Spanish, bantams, game-cocks, etc.; from the wild rock-dove have been developed our various fancy pigeons, as carriers, pouters, fantails, etc. For an account of artificial selection see Darwin's "Plants and Animals under Domestication," and Romanes' "Darwin and After Darwin," I. FOOTNOTE: