CHAP. VIII.] OF NATURAL SELECTION. 213 



change in the other would have been fatal." The force of this 

 objection rests 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 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 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 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 exclu- 

 sively of inspissated saliva ? And so in other cases. It must, how- 

 ever, 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 originated ; cases, in which 

 no intermediate gradations are known to exist ; cases of instincts 

 of such trifling importance, that they could hardly have been acted 

 on by natural selection ; cases of instincts almost identically the 

 same in animals so remote in the scale of nature, that we cannot 

 account for their similarity by inheritance from a common pro- 

 genitor, and consequently must believe that they were indepen- 

 dently acquired through natural selection. I will not here enter 

 on these several cases, but will confine myself to one special 

 difficulty, which at first appeared to me insuperable, and actually 

 fatal to the whole theory. I allude to the neuters or sterile 



