OF NATURAL SEL^CTIOtf. 253 



it to neglect other materials and to make its nest exclu- 

 sively 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. 



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 origi- 

 nated ; cases, in which no intermediate gradations are known 

 to exist ; cases of instincts of such trifling importance, that 

 they could hardly have been acted upon 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 progenitor, 

 and consequently must believe that they were independ- 

 ently 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 insuper- 

 able and actually fatal to the whole theory. I allude to the 

 neuters or sterile females in insect communities ; for these 

 neuters often differ widely in instinct and in structure from 

 both the males and fertile females, and yet, from being 

 sterile, they cannot propagate their kind. 



The subject well deserves to be discussed at great length, 

 but I will here take only a single case, that of working or 

 sterile ants. How the workers have been rendered sterile 

 is a difficulty ; but not much greater than that of any 

 other striking modification of structure; for it can be 

 shown that some insects and other articulate animals 

 in a state of nature occasionally become sterile ; and if 

 such insects had been social, and it had been profitable 

 to the community that a number should have been annually 

 born capable of work, but incapable of procreation, I can 

 see no especial difficulty in this having been effected through 

 natural selection. But I must pass over this preliminary diffi- 

 culty. The great difficulty lies in the working ants differ- 

 ing widely from both the males and the fertile females in 

 structure, as in the shape of the thorax, and in being desti- 

 tute of wings and sometimes of eyes, and in instinct. As 

 far as instinct alone is concerned, the wonderful difference 

 in this respect between the workers and the perfect females 

 would have been better exemplified by the hive-bee. If a 

 working ant or other neuter insect had been an ordinary 

 animal, I should have unhesitatingly assumed that all its 



