INSTINCT 217 



— a graduated series having been first formed, as in the 

 case of the driver ant, and then the extreme forms, 

 from being the most useful to the community, having 

 been produced in greater and greater numbers through 

 the natural selection of the parents which generated 

 them ; until none with an intermediate structure were 

 produced. 



Thus, as I believe, the wonderful fact of two dis- 

 tinctly defined castes of sterile workers existing in the 

 same nest, both widely different from each other and 

 from their parents, has originated. We can see how 

 useful their production may have been to a social 

 community of insects, on the same principle that the 

 division of labour is useful to civilised man. As ants 

 work by inherited instincts and by inherited organs or 

 tools, and not by acquired knowledge and manufac- 

 tured instruments, a perfect division of labour could 

 be effected with them only by the workers being 

 sterile ; for had they been fertile, they would have 

 mtercrossed, and their instincts and structure would 

 have become blended. And nature has, as I believe, 

 effected this admirable division of labour in the com- 

 munities of ants, by the means of natural selection. 

 But I am bound to confess, that, with all my faith in 

 this principle, I should never have anticipated that 

 natural selection could have been efficient in so high 

 a degree, had not the case of these neuter insects 

 convinced me of the fact. I have, therefore, discussed 

 this case, at some little but wholly insufficient length, 

 in order to show the power of natural selection, and 

 likewise because this is by far the most serious special 

 difficulty, which my theory has encountered. The 

 case, also, is very interesting, as it proves that with 

 animals, as with plants, any amount of modification 

 in structure can be effected by the accumulation of 

 numerous, slight, and as we must call them accidental, 

 variations, which are in any manner profitable, with- 

 out exercise or habit having come into play. For no 

 amount of exercise, or habit, or volition, in the utterly 

 sterile members of a community could possibly affect 



