CiiAi-. VII. OBJECTIONS TO NATDKAL SELECTION. 225 



Objectiojis to the Tlicory of Natural Selection as cq^plied to 

 Instincts y' Axuter 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 corre- 

 sponding change in the other Avould have been fatal." The 

 force of this olijection 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) al- 

 luded to in the last chapter : this l)ird often holds the seeds of 

 the yew between its feet on a branch, and hannners away till 

 it gets into the kernel. Now what special diiliculty would 

 there be in natural selection preserving all the slight individ- 

 ual variations in the shape of the beak, Avhich were better and 

 better adapted to break ojien the seeds, until a beak was 

 formed, as well constructed for this purpose as that of the nut- 

 hatch, at the same time that habit, or compulsion, or sponta- 

 neous 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 tlie beak, 

 or from any other imknown 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 

 jwwer of the nuthatch. In this case a gradual change of struct- 

 ure 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. iSoine birds build their nests 

 of nnid, 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 selec- 

 tion of individual swifts, wliich secreted more and more saliva, 

 should at last produce a species with instincts leading it to 

 neglect other matc^rials^ and to make its nest exclusively of 

 inspissated saliva? And so in other cases. It must, however, 

 be admitted tliat in many instances we cannot conjecture 

 whetlier it was instinct or structure which first varied. 



No doubt many instincts of very dillicult explanation could 



