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 variations 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 determined chiefly through destruction by beasts of prey--by external 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 obtaining 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 browsing 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, besides the general ones just indicated, have probably interfered with the acquisition through natural selection of structures, 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.