Chap. VHL] SUMMARY 363 



organs or tools, whilst man works by acquired know- 

 ledge 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 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 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. There- 

 fore there is no real difficulty, under changing conditions 

 of life, in natural selection accumulating to any extent 



