SUMMARY. 275 



of two distinctly 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 ha"e 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 

 instruments. 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 con- 

 clusion. 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 amouut 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 of sterile females, however long they might be fol- 

 lowed, 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 in- 

 herited habit, as advanced 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 at- 

 tempted to show that instincts vary slightly in a state of 

 nature. No one will dispute that instincts are of the high- 

 est 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 



