232 THE ORIGIN OF SPECIES 



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 

 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 pro- 

 genitor, 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 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 ; 

 fmuch greater than that of any other striking modification of struc- i 

 ture; 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 com- 

 munity 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 re- 

 spect 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 unhesi- 

 tatingly assumed that all its characters had been slowly acquired 

 through natural selection; namely, by individuals having been 

 born with slight profitable modifications, which were inherited by 

 the offspring, and that these again varied and again were selected, 

 and so onward. But with the working ant we have an insect differ- 

 ing greatly from its parents, yet absolutely sterile; so that it could 

 never have transmitted successively acquired modifications of 



