258 SUMMARY. 



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 commu- 

 nity of ants, on the same principle that the division of 

 labor is useful to civilized man. Ants, however, work by 

 inherited instincts, and by inherited organs or tools, while 

 man works by acquired knowledge and manufactured in- 

 struments. 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 instincts 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 numer- 

 ous, 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 of 

 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 ad- 

 vanced by Lamarck. 



SUMMARY. 



I have endeavored in this chapter briefly 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 instincts vary slightly in a state of 

 nature. No one will dispute 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 are not 



