Origin of Species Variorum

of the nuthatch, at the same time that hereditary habit, or compulsion from the want of other food, or the preservation of chance variations of taste, made the bird more and more of a seed-eater? In this case the beak is supposed to be slowly modified by natural selection, subsequently to, but in accordance with, slowly changing habit; but let the feet of the titmouse vary and grow larger from correlation with the beak, or from any other unknown cause, and is it very improbable that such larger feet might lead the bird to climb more and more until it acquired even the remarkable climbing instinct and capacity of the nuthatch? In this case a gradual change of structure is supposed to lead to changed instinctive habits of life. To take one more case: few instincts are more remarkable than that which leads the swift of the Eastern Islands to make its nest wholly of inspissated saliva. Some birds build their nests of mud, believed to be moistened with saliva; and one of the swifts of North America makes its nest (as I have seen) of sticks agglutinated with saliva, and even with flakes of this substance. Is it then very improbable that the natural selection of individual swifts, which secreted more and more saliva, should at last produce a species with instincts leading it to neglect other materials, and to make its nest exclusively of inspissated saliva? And so in other cases. It must be admitted that in many instances we cannot conjecture whether instinct or structure has first slightly changed; nor can we conjecture by what gradations many instincts have been developed when they relate to organs (such as the mammary glands) on the first origin of which we know nothing.

No doubt many instincts of very difficult explanation could be opposed to the theory of natural selection,— cases, in which we cannot see how an instinct could