Chap. VIII.] OF NATURAL SELECTION. 355 



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 compul- 

 sion, 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 selec- 

 tion, 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 in- 

 stinct and power of the nuthatch. In this case a grad- 

 ual change of structure is supposed to lead to changed 

 instinctive habits. To take one more case: few in- 

 stincts 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? Ani 

 so in other eases. 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 explana- 

 tion could be opposed to the theory of natural selec- 

 tion — cases, in which we cannot see how an instinct 

 could have originated; cases, in which no intermediate 



