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Show 242 INSTINCT. CHAP. VII. labour is useful to civilised man. As ants work by I.l l h eri' te d I.l lS tI'l lC ts and by inherited tools or we.a pons, and not by acquired knowledge and manufactured Instru-ment s, a per11.c ect division of labou.r could . be effected with them only by the workers bmng. sterile ; for had they been fertile, they would have Intercrossed, and th · · nstincts ·and structure would have become blended. mr I . d . bl And nature has, as I believe, effected this a mua e 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 hav~ 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.-! have endeavoured briefly in this chapt~r to show that the mental qualities of our domestic animals vary, and that the variations are I.n h en't~ d· Still more briefly I have attempted to show that m- CHAP. vrr. SUMMARY. 243 stincts vary slightly in a state of nature N '11 d. . . · o one WI Ispute that Instincts are of the highest impo t t h . 1 r ance ·O eac anima . Therefore I can see no difficult d h . d' . . . y, un er c ~ng1ng. con Itio~s of hfe, In I).atural selection accumu-lating shght modifications of instinct to any ext t I·n an~ use f uI di recti·o n. In some cases habit or eunse' and disuse have probably come into play. I do not rretend that the facts given in this chapter strengthen m 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 ~bs~lutely perfect and are liable to mistakes; -that no mstinct ?as been produced for the exclusive good of othe!-' a~Imals, 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 instinc. ts as well as to co~pore~l structure, and is plainly e~phcable on the foregoing VIews, but is otherwise inexplicab! e,-all tend to corroborate the theory of natural selectwn. Th~s theory is, also, strengthened by some few other facts In regard to instincts ; as by that common case of closely allied, but certainly distinct, species, when inhabi~ ing dista~t parts of the world and living under ~onsiderably 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 ~he thrush of South America lines its nest with mud, In the same peculiar manner as does our British thrush: ho": it is t~at the male wrens (Troglodytes) of North Amenca, build "cock-nests," to roost in like the males of our distinct Kitty-wrens,-a habit wholly unlike that ~f any other known bird. Finally, it may not be a logiCa~ deduction, but to my imagination it is far more satisfactory to look at such instincts as the young M2 |