物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 6. DIFFICULTIES ON THEORY. Page 4
SUMMARY OF CHAPTER.
We have in this chapter discussed some of the difficulties andobjections which may be urged against my theory. Many of them are verygrave; but I think that in the discussion light has been thrown onseveral facts, which on the theory of independent acts of creation areutterly obscure. We have seen that species at any one period are notindefinitely variable, and are not linked together by a multitude ofintermediate gradations, partly because the process of naturalselection will always be very slow, and will act, at any one time,only on a very few forms; and partly because the very process ofnatural selection almost implies the continual supplanting andextinction of preceding and intermediate gradations. Closely alliedspecies, now living on a continuous area, must often have been formedwhen the area was not continuous, and when the conditions of life didnot insensibly graduate away from one part to another. When twovarieties are formed in two districts of a continuous area, anintermediate variety will often be formed, fitted for an intermediatezone; but from reasons assigned, the intermediate variety will usuallyexist in lesser numbers than the two forms which it connects;consequently the two latter, during the course of furthermodification, from existing in greater numbers, will have a greatadvantage over the less numerous intermediate variety, and will thusgenerally succeed in supplanting and exterminating it.
We have seen in this chapter how cautious we should be in concludingthat the most different habits of life could not graduate into eachother; that a bat, for instance, could not have been formed by naturalselection from an animal which at first could only glide through theair.
We have seen that a species may under new conditions of life changeits habits, or have diversified habits, with some habits very unlikethose of its nearest congeners. Hence we can understand, bearing inmind that each organic being is trying to live wherever it can live,how it has arisen that there are upland geese with webbed feet, groundwoodpeckers, diving thrushes, and petrels with the habits of auks.
Although the belief that an organ so perfect as the eye could havebeen formed by natural selection, is more than enough to stagger anyone; yet in the case of any organ, if we know of a long series ofgradations in complexity, each good for its possessor, then, underchanging conditions of life, there is no logical impossibility in theacquirement of any conceivable degree of perfection through naturalselection. In the cases in which we know of no intermediate ortransitional states, we should be very cautious in concluding thatnone could have existed, for the homologies of many organs and theirintermediate states show that wonderful metamorphoses in function areat least possible. For instance, a swim-bladder has apparently beenconverted into an air-breathing lung. The same organ having performedsimultaneously very different functions, and then having beenspecialised for one function; and two very distinct organs havingperformed at the same time the same function, the one having beenperfected whilst aided by the other, must often have largelyfacilitated transitions.
We are far too ignorant, in almost every case, to be enabled to assertthat any part or organ is so unimportant for the welfare of a species,that modifications in its structure could not have been slowlyaccumulated by means of natural selection. But we may confidentlybelieve that many modifications, wholly due to the laws of growth, andat first in no way advantageous to a species, have been subsequentlytaken advantage of by the still further modified descendants of thisspecies. We may, also, believe that a part formerly of high importancehas often been retained (as the tail of an aquatic animal by itsterrestrial descendants), though it has become of such smallimportance that it could not, in its present state, have been acquiredby natural selection,--a power which acts solely by the preservationof profitable variations in the struggle for life.
Natural selection will produce nothing in one species for theexclusive good or injury of another; though it may well produce parts,organs, and excretions highly useful or even indispensable, or highlyinjurious to another species, but in all cases at the same time usefulto the owner. Natural selection in each well-stocked country, must actchiefly through the competition of the inhabitants one with another,and consequently will produce perfection, or strength in the battlefor life, only according to the standard of that country. Hence theinhabitants of one country, generally the smaller one, will oftenyield, as we see they do yield, to the inhabitants of another andgenerally larger country. For in the larger country there will haveexisted more individuals, and more diversified forms, and thecompetition will have been severer, and thus the standard ofperfection will have been rendered higher. Natural selection will notnecessarily produce absolute perfection; nor, as far as we can judgeby our limited faculties, can absolute perfection be everywhere found.
On the theory of natural selection we can clearly understand the fullmeaning of that old canon in natural history, "Natura non facitsaltum." This canon, if we look only to the present inhabitants of theworld, is not strictly correct, but if we include all those of pasttimes, it must by my theory be strictly true.
It is generally acknowledged that all organic beings have been formedon two great laws--Unity of Type, and the Conditions of Existence. Byunity of type is meant that fundamental agreement in structure, whichwe see in organic beings of the same class, and which is quiteindependent of their habits of life. On my theory, unity of type isexplained by unity of descent. The expression of conditions ofexistence, so often insisted on by the illustrious Cuvier, is fullyembraced by the principle of natural selection. For natural selectionacts by either now adapting the varying parts of each being to itsorganic and inorganic conditions of life; or by having adapted themduring long-past periods of time: the adaptations being aided in somecases by use and disuse, being slightly affected by the direct actionof the external conditions of life, and being in all cases subjectedto the several laws of growth. Hence, in fact, the law of theConditions of Existence is the higher law; as it includes, through theinheritance of former adaptations, that of Unity of Type.