Origin of Species Variorum

Objections to the Theory of Natural Selection as applied to Instincts: Neuter and Sterile Insects .

It has been objected to the foregoing view of the origin of instincts 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 rests entirely .. 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 all the slight individual variations in the shape of the beak, which were better and better adapted to break open the seeds, until a beak was formed, as well constructed for this purpose as that of the nuthatch, at the same time that .. habit, or compulsion, or spontaneous variations of taste, led the bird to become 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 habits or taste; but let the feet of the titmouse vary and grow larger from correlation with the beak, or from any other unknown cause, and .. it is not improbable that such larger feet would lead the bird to climb more and more until it acquired .. the remarkable climbing instinct and power of the nuthatch. In this case a gradual change of structure is supposed to lead to changed instinctive habits. .. .. 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.