290 ORIGIN OF SPECIES 



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 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 in- 

 stincts almost identically the same in animals so remote in 

 the scale of nature, that we cannot account for their simi- 

 larity by inheritance from a common progenitor, and conse- 

 quently must believe that they were independently 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 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 difficulty. The great difficulty 

 lies in the working ants differing widely from both the males 

 and the fertile females in structure, as in the shape of the 

 thorax, and in being destitute 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 



