INSTINCT 391 



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

 insects led me to this conclusion. I have, therefore, dis- 

 cussed 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 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 



