Chap. VHL] 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 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, 

 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 in- 

 stinctive 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 

 Xorth 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 in- 

 stincts 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. 



Xo 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 originated ; cases, in which no intermediate grada- 



