物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 7. INSTINCT. Page 4
It will indeed be thought that I have an overweening confidence in theprinciple of natural selection, when I do not admit that suchwonderful and well-established facts at once annihilate my theory. Inthe simpler case of neuter insects all of one caste or of the samekind, which have been rendered by natural selection, as I believe tobe quite possible, different from the fertile males and females,--inthis case, we may safely conclude from the analogy of ordinaryvariations, that each successive, slight, profitable modification didnot probably at first appear in all the individual neuters in the samenest, but in a few alone; and that by the long-continued selection ofthe fertile parents which produced most neuters with the profitablemodification, all the neuters ultimately came to have the desiredcharacter. On this view we ought occasionally to find neuter-insectsof the same species, in the same nest, presenting gradations ofstructure; and this we do find, even often, considering how fewneuter-insects out of Europe have been carefully examined. Mr. F.Smith has shown how surprisingly the neuters of several British antsdiffer from each other in size and sometimes in colour; and that theextreme forms can sometimes be perfectly linked together byindividuals taken out of the same nest: I have myself compared perfectgradations of this kind. It often happens that the larger or thesmaller sized workers are the most numerous; or that both large andsmall are numerous, with those of an intermediate size scanty innumbers. Formica flava has larger and smaller workers, with some ofintermediate size; and, in this species, as Mr. F. Smith has observed,the larger workers have simple eyes (ocelli), which though small canbe plainly distinguished, whereas the smaller workers have theirocelli rudimentary. Having carefully dissected several specimens ofthese workers, I can affirm that the eyes are far more rudimentary inthe smaller workers than can be accounted for merely by theirproportionally lesser size; and I fully believe, though I dare notassert so positively, that the workers of intermediate size have theirocelli in an exactly intermediate condition. So that we here have twobodies of sterile workers in the same nest, differing not only insize, but in their organs of vision, yet connected by some few membersin an intermediate condition. I may digress by adding, that if thesmaller workers had been the most useful to the community, and thosemales and females had been continually selected, which produced moreand more of the smaller workers, until all the workers had come to bein this condition; we should then have had a species of ant withneuters very nearly in the same condition with those of Myrmica. Forthe workers of Myrmica have not even rudiments of ocelli, though themale and female ants of this genus have well-developed ocelli.
I may give one other case: so confidently did I expect to findgradations in important points of structure between the differentcastes of neuters in the same species, that I gladly availed myself ofMr. F. Smith's offer of numerous specimens from the same nest of thedriver ant (Anomma) of West Africa. The reader will perhaps bestappreciate the amount of difference in these workers, by my giving notthe actual measurements, but a strictly accurate illustration: thedifference was the same as if we were to see a set of workmen buildinga house of whom many were five feet four inches high, and many sixteenfeet high; but we must suppose that the larger workmen had heads fourinstead of three times as big as those of the smaller men, and jawsnearly five times as big. The jaws, moreover, of the working ants ofthe several sizes differed wonderfully in shape, and in the form andnumber of the teeth. But the important fact for us is, that though theworkers can be grouped into castes of different sizes, yet theygraduate insensibly into each other, as does the widely-differentstructure of their jaws. I speak confidently on this latter point, asMr. Lubbock made drawings for me with the camera lucida of the jawswhich I had dissected from the workers of the several sizes.
With these facts before me, I believe that natural selection, byacting on the fertile parents, could form a species which shouldregularly produce neuters, either all of large size with one form ofjaw, or all of small size with jaws having a widely differentstructure; or lastly, and this is our climax of difficulty, one set ofworkers of one size and structure, and simultaneously another set ofworkers of a different size and structure;--a graduated series havingbeen first formed, as in the case of the driver ant, and then theextreme forms, from being the most useful to the community, havingbeen produced in greater and greater numbers through the naturalselection of the parents which generated them; until none with anintermediate structure were produced.
Thus, as I believe, the wonderful fact of two distinctly definedcastes of sterile workers existing in the same nest, both widelydifferent from each other and from their parents, has originated. Wecan see how useful their production may have been to a socialcommunity of insects, on the same principle that the division oflabour is useful to civilised man. As ants work by inherited instinctsand by inherited tools or weapons, and not by acquired knowledge andmanufactured instruments, a perfect division of labour could beeffected with them only by the workers being sterile; for had theybeen fertile, they would have intercrossed, and their instincts andstructure 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 anticipatedthat natural selection could have been efficient in so high a degree,had not the case of these neuter insects convinced me of the fact. Ihave, therefore, discussed this case, at some little but whollyinsufficient length, in order to show the power of natural selection,and likewise because this is by far the most serious specialdifficulty, which my theory has encountered. The case, also, is veryinteresting, as it proves that with animals, as with plants, anyamount of modification in structure can be effected by theaccumulation of numerous, slight, and as we must call them accidental,variations, which are in any manner profitable, without exercise orhabit having come into play. For no amount of exercise, or habit, orvolition, in the utterly sterile members of a community could possiblyhave affected the structure or instincts of the fertile members, whichalone leave descendants. I am surprised that no one has advanced thisdemonstrative case of neuter insects, against the well-known doctrineof Lamarck.
SUMMARY.
I have endeavoured briefly in this chapter to show that the mentalqualities of our domestic animals vary, and that the variations areinherited. Still more briefly I have attempted to show that instinctsvary slightly in a state of nature. No one will dispute that instinctsare of the highest importance to each animal. Therefore I can see nodifficulty, under changing conditions of life, in natural selectionaccumulating slight modifications of instinct to any extent, in anyuseful direction. In some cases habit or use and disuse have probablycome into play. I do not pretend that the facts given in this chapterstrengthen in any great degree my theory; but none of the cases ofdifficulty, to the best of my judgment, annihilate it. On the otherhand, the fact that instincts are not always absolutely perfect andare liable to mistakes;--that no instinct has been produced for theexclusive good of other animals, but that each animal takes advantageof the instincts of others;--that the canon in natural history, of"natura non facit saltum" is applicable to instincts as well as tocorporeal structure, and is plainly explicable on the foregoing views,but is otherwise inexplicable,--all tend to corroborate the theory ofnatural selection.
This theory is, also, strengthened by some few other facts in regardto instincts; as by that common case of closely allied, but certainlydistinct, species, when inhabiting distant parts of the world andliving under considerably different conditions of life, yet oftenretaining nearly the same instincts. For instance, we can understandon the principle of inheritance, how it is that the thrush of SouthAmerica lines its nest with mud, in the same peculiar manner as doesour British thrush: how it is that the male wrens (Troglodytes) ofNorth America, build "cock-nests," to roost in, like the males of ourdistinct Kitty-wrens,--a habit wholly unlike that of any other knownbird. Finally, it may not be a logical deduction, but to myimagination it is far more satisfactory to look at such instincts asthe young cuckoo ejecting its foster-brothers,--ants makingslaves,--the larvae of ichneumonidae feeding within the live bodies ofcaterpillars,--not as specially endowed or created instincts, but assmall consequences of one general law, leading to the advancement ofall organic beings, namely, multiply, vary, let the strongest live andthe weakest die.