Chap. VIII.] SUMMARY. 363 



organs or tools, whilst man works by acquired knowl- 

 edge and manufactured instruments. But I must con- 

 fess, 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 encoun- 

 tered. The case, also, is very interesting, as it proves 

 that with animals, as with plants, any amount of modi- 

 fication 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 

 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 in- 

 herited 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 condi- 

 tions of life, in natural selection accumulating to any ex- 



