242 INSTINCT. Chap. VII. 



labour is useful to civilised man. As ants work by 

 inherited instincts and by inherited tools or weapons, 

 and not by acquired knowledge and manufactured instru- 

 ments, a perfect division of labour could be effected 

 with them only by the workers being sterile ; for had 

 they been fertile, they would have intercrossed, and 

 their instincts and structure would have become blended. 

 And nature has, as I believe, effected this admirable 

 division of labour in the communities 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, there- 

 fore, discussed this case, at some little but wholly insuf- 

 ficient 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 encoun- 

 tered. The case, also, is very interesting, as it proves 

 that with animals, as with plants, any amount of modi- 

 fication in structure can be effected by the accumula- 

 tion of numerous, slight, and as we must call them acci- 

 dental, variations, which are in any manner profitable, 

 without 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 have af- 

 fected the structure or instincts of the fertile members, 

 which alone leave descendants. I am surprised that no 

 one has advanced this demonstrative case of neuter in- 

 sects, against the well-known doctrine of Lamarck. 



Summary. — I have endeavoured briefly in this chapter 

 to show that the mental qualities of our domestic 

 animals vary, and that the variations are inherited. 

 Still more briefly I have attempted to show that in- 



