Origin of Species Variorum

Comparison with 1861

than at present; for then, as we have seen, the spherical surfaces would wholly disappear, and would all be replaced by plane surfaces; and the Melipona would make a comb as perfect as that of the hive-bee. Beyond this stage of perfection in architecture, natural selection could not lead; for the comb of the hive-bee, as far as we can see, is absolutely perfect in economising wax.

Thus, as I believe, the most wonderful of all known instincts, that of the hive-bee, can be explained by natural selection having taken advantage of numerous, successive, slight modifications of simpler instincts; natural selection having by slow degrees, more and more perfectly, led the bees to sweep equal spheres at a given distance from each other in a double layer, and to build up and excavate the wax along the planes of intersection. The bees, of course, no more knowing that they swept their spheres at one particular distance from each other, than they know what are the several angles of the hexagonal prisms and of the basal rhombic plates. The motive power of the process of natural selection having been economy of wax, together with cells of due strength, and of the proper size and shape for the larvæ; that individual swarm which made the best cells, and wasted least honey in the secretion of wax, having succeeded best, and having transmitted by inheritance their newly acquired economical instincts to new swarms, which in their turn will have had the best chance of succeeding in the struggle for existence. ↑ ↑

It has been objected to the foregoing view on the origin of instinct that "the variations of structure and of instinct must have been simultaneous and accurately adjusted to each other, as a modification in the one without an immediate corresponding change in the other would have been fatal." The force of this objection seems entirely to rest on the assumption that the changes in both instinct and structure are abrupt. To take as an illustration the case of the larger titmouse (Parus major) alluded to in the last chapter: this bird often holds the seeds of the yew between its feet on a branch, and hammers away till it gets into the kernel. Now what special difficulty would there be in natural selection preserving each slight variation of beak, better and better adapted to break open seeds, until a beak was formed, as well constructed for this purpose as that 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 possibly have originated; cases, in which no intermediate gradations are known to exist; cases of instinct of apparently such trifling importance, that they could

Thus, as I believe, the most wonderful of all known instincts, that of the hive-bee, can be explained by natural selection having taken advantage of numerous, successive, slight modifications of simpler instincts; natural selection having by slow degrees, more and more perfectly, led the bees to sweep equal spheres at a given distance from each other in a double layer, and to build up and excavate the wax along the planes of intersection. The bees, of course, no more knowing that they swept their spheres at one particular distance from each other, than they know what are the several angles of the hexagonal prisms and of the basal rhombic plates. The motive power of the process of natural selection having been economy of wax; that individual swarm which wasted least honey in the secretion of wax, having succeeded best, and having transmitted by inheritance its newly acquired economical instinct to new swarms, which in their turn will have had the best chance of succeeding in the struggle for existence. ↑ ↑