218 SUMMARY. [CHAP VIIL 



may have been to a social community of ants, on the same 

 principle that the division of labour is useful to civilised man. 

 Ants, however, work by inherited instincts and by inherited 

 organs or tools, whilst 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 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 demonstra- 

 tive 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. 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 always absolutely perfect and are liable 

 to mistakes : that no instinct can be shown to have been produced 

 for the good of other animals, though animals take advantage of 

 the instincts of others ; that the canon in natural history, of 

 "Natura non facit saltum," is applicable to instincts as well as 

 to corporeal structure, and is plainly explicable on the foregoing 

 views, but is otherwise inexplicable, all tend to corroborate the 

 theory of natural selection. 



This theory is also strengthened by some few other facts in 

 regard to instincts ; as by that common case of closely allied, but 



