Chap. viii. Summary. 233 



appearing under two or even three distinct female forms ; and by 

 Fritz Miiller, of certain Brazilian crustaceans likewise appearing 

 under two widely distinct male forms. But this subject need not 

 here be discussed. 



I have now explained how, as I believe, the wonderful fact of 

 two distinctly denned 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 ants, on the same principle 

 that the division of labour is useful to civilised man. Ants, how- 

 ever, work by inherited instincts and by inherited organs or tools, 

 whilst man works by acquired knowledge and manufactured instru- 

 ments. But I must confess, that, with all my faith in natural 

 selection, I should never have anticipated that this principle could 

 have been efficient in so high a degree, had not the case of these neuter 

 insects led me to this conclusion. 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 may be effected by the 

 accumulation of numerous, slight, spontaneous variations, which 

 are in any way profitable, without exercise or habit having been 

 brought into play. For peculiar habits confined to the workers or 

 sterile females, however long they might be followed, could not 

 possibly affect the males and fertile females, which alone leave 

 descendants. I am surprised that no one has hitherto advanced 

 this demonstrative case of neuter insects, against the well-known 

 doctrine of inherited habit, as advanced by Lamarck. 



Summary. 



I have endeavoured in this chapter briefly to show that the 

 mental qualities of our domestic animals vary, and that the varia- 

 tions are inherited. Still more briefly I have attempted to show 

 that instincts vary slightly in a state of nature. No one will dis- 

 pute that instincts are of the highest importance to each animal. 

 Therefore there is no real difficulty, under changing conditions of life, 

 in natural selection accumulating to any extent slight modifications 

 of instinct which are in any way useful. In many cases habit or 

 use and disuse have probably come into play. I do not pretend 

 that the facts given in this chapter strengthen in any great degree 

 my theory ; but none of the cases of difficulty, to the best of my 

 judgment, annihilate it. On the other hand, the fact that instincts 



