OF NATURAL SELEGTION. 269 



The force of this objection I'ests entirely on the assumption 

 that the changes in the instincts and structure are abrupt. 

 To take as an illustration the case of the larger titmouse, 

 (Parus major) alluded to in a previous chapter; this bird 

 often holds the seeds of the yew between its feet on a 

 branch, and hammers with its beak till it gets at the 

 kernel. Now what special difficulty would there be in natu- 

 ral 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 nut- 

 hatch, 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, subse- 

 quently 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. 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, however, be admitted that in many 

 instances we cannot conjecture whether it was instinct or 

 structure which first varied. 



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 have origin- 

 ated; cases, in which no intermediate gradations are known 

 to exist; cases of instincts of such trifling importance, that 



