tools or weapons, and not by acquired knowledge and manufactured instruments, a perfect division of labour could be effected with them only by the workers being sterile; for had they been fertile, they would have intercrossed, and their instincts and structure would have become blended.  And nature has, as I believe, effected this admirable division of labour in the communities of ants, by the means of natural selection.  But I am bound to confess, that, with all my faith in this principle, I should never have anticipated that natural selection could have been efficient in so high a degree, had not the case of these neuter insects convinced me of the fact.  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 in structure can be effected by the accumulation of numerous, slight, and as we must call them accidental, variations, which are in any manner profitable, without exercise or habit having come into play.  For no amount of exercise, or habit, or volition, in the utterly sterile members of a community could possibly have affected the structure or instincts of the fertile members, which alone leave descendants.  I am surprised that no one has advanced this demonstrative case of neuter insects, against the well-known doctrine of Lamarck.

Summary.

I have endeavoured briefly in this chapter 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 I can see no difficulty, under changing conditions of life, in natural selection accumulating slight modifications of instinct to any extent, in any useful direction.  In some 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 has been produced for the exclusive good of other animals, but that each animal takes 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.