OBJECTIONS TO THE THEORY 289 



OBJECTIONS TO THE THEORY OF NATURAL SELECTION AS APPLIED 

 TO INSTINCTS: NEUTER 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 correspond- 

 ing 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 ham- 

 mers with its beak till it gets at the kernel. Now what spe- 

 cial diihculty 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 nuthatch, at the same time that habit, or com- 

 pulsion, 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 aggluti- 

 nated 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 



J— HC XI 



