INSTINCT 383 



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 

 BO 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 originated; cases, in which no intermediate grada- 

 tions are known to exist; cases of instincts of such triflmg 

 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 

 itt' 1 from a common progenitor, and consequently must he- 

 ni n lieve that they were independently acquired through nat- 

 a ^ ural selection. I will not here enter on these several 

 ciHw cases, but will confine myself to one special difficulty, 



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