Seite - 233 - in The Origin of Species

THEORY OF NATURAL SELECTION 233 more favourable than others for the development of so large a quadruped as the giraffe. In order that an animal should acquire some structure specially and largely developed, it is almost indispensable that several other parts should be modified and co-adapted. Although every part of the body varies slightly, it does not follow that the necessary parts should always vary in the right direction and to the right degree. With the different species of our domesticated animals we know that the parts vary in a different manner and degree; and that some species are much more variable than others. Even if the fitting vari- ations did arise, it does not follow that natural selection would be able to act on them, and produce a structure which apparently would be beneficial to the species. For instance, if the number of individuals existing in a country is deter- mined chiefly through destruction by beasts of prey,—by ex- ternal or internal parasites, etc., —as seems often to be the case, then natural selection will be able to do little, or will be greatly retarded, in modifying any particular structure for ob- taining food. Lastly, natural selection is a slow process, and the same favourable conditions must long endure in order that any marked effect should thus be produced. Except by assigning such general and vague reasons, we cannot explain why, in many quarters of the world, hoofed quadrupeds have not acquired much elongated necks or other means for brows- ing on the higher branches of trees. Objections of the same nature as the foregoing have been advanced by many writers. In each case various causes, be- sides the general ones just indicated, have probably inter- fered with the acquisition through natural selection of struc- tures, which it is thought would be beneficial to certain species. One writer asks, why has not the ostrich acquired the power of flight? But a moment's reflection will show what an enormous supply of food would be necessary to give to this bird of the desert force to move its huge body through the air. Oceanic islands are inhabited by bats and seals, but by no terrestrial mammals ; yet as some of these bats are peculiar species, they must have long inhabited their present homes. Therefore Sir C. Lyell asks, and assigns certain rea- sons in answer, why have not seals and bats given birth on