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Show 214 INSTINCT. [Crr.AP. vn. lection, by acting on the fertile parei.lts, could form n. species which should regularly produce neuters, either all of large size with one forn1 of jaw, or all of small size with jaws having a widely different structure; or lastly, and this is our climax of difficulty, one set of workers of oue size and structure, and shnultaneously another set of workers of a different size and structure ;-a graduated series having been first formed, as in the case of the driver ant, and then the extrenw forn1s, from being the most useful to the community, having been produced in greater and greater nu1nbers through t11e natural selection of the parents which generated them; until none with an intermediate structure were produced. Thus, as I beHeve, the wonderful fact of two distinctly defined castes of sterile 'workers existing in the same nest, both_ widely different fro1n each other and from their parents, has originated. W o can see how useful their production may have been to a social community of Insects, on the sa1ne principle that the division of la'bour is useful to civilised man. As ants work by inherited instincts and by inherited 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 fertil e, they would have intercrossed, and their instincts and structure would have becoine 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 1nost 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 accuInulation of numerous, slight, and as ·we 1nust call them CH.A.P. VII.] SUMMARY. 215 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 attmnpted 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 aceumulating slight modifications of instinct to any extent, in any useful direction. In smne 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 n1istakes ;-that no instinct has been produced for the exclusive good of other animals, but that each animal tal~es advanta~e of the instincts of others ;-that the canon 1n natural history, of" natura non facit saltum" is ap:plicab~e to inst~ncts as well as to corporeal structure, and 1s plainly explicable on the foregoing views but is otherwise inexplicable,-all tend to corroborate th~ theory of natural selection. This theory is, also, strengthened by some few other facts in regard to instincts ; as by th3tt common case of ?l.osely. allied, but certainly distinct, species, when inhabIting d.1stant parts of the world and living under considerably different conditions of life, yet often retaining nearly the same instincts. For instance, we can understand on the principle of inheritance, how it is that the thrush of South America lines its nest with mud, in the same pecu- |