the origin of species(物种起源)-第48节
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e that of any other known bird。 Finally； it may not be a logical deduction； but to my imagination it is far more satisfactory to look at such instincts as the young cuckoo ejecting its foster…brothers； ants making slaves； the larvae of ichneumonidae feeding within the live bodies of caterpillars； not as specially endowed or created instincts； but as small consequences of one general law； leading to the advancement of all organic beings； namely； multiply； vary； let the strongest live and the weakest die。
Chapter 8 … Hybridism
THE view generally entertained by naturalists is that species； when intercrossed； have been specially endowed with the quality of sterility； in order to prevent the confusion of all organic forms。 This view certainly seems at first probable； for species within the same country could hardly have kept distinct had they been capable of crossing freely。 The importance of the fact that hybrids are very generally sterile； has； I think； been much underrated by some late writers。 On the theory of natural selection the case is especially important； inasmuch as the sterility of hybrids could not possibly be of any advantage to them； and therefore could not have been acquired by the continued preservation of successive profitable degrees of sterility。 I hope； however； to be able to show that sterility is not a specially acquired or endowed quality； but is incidental on other acquired differences。
In treating this subject； two classes of facts； to a large extent fundamentally different； have generally been confounded together； namely； the sterility of two species when first crossed； and the sterility of the hybrids produced from them。
Pure species have of course their organs of reproduction in a perfect condition； yet when intercrossed they produce either few or no offspring。 Hybrids； on the other hand； have their reproductive organs functionally impotent； as may be clearly seen in the state of the male element in both plants and animals； though the organs themselves are perfect in structure； as far as the microscope reveals。 In the first case the two sexual elements which go to form the embryo are perfect； in the second case they are either not at all developed； or are imperfectly developed。 This distinction is important； when the cause of the sterility； which is common to the two cases； has to be considered。 The distinction has probably been slurred over； owing to the sterility in both cases being looked on as a special endowment； beyond the province of our reasoning powers。
The fertility of varieties； that is of the forms known or believed to have descended from common parents； when intercrossed； and likewise the fertility of their mongrel offspring； is； on my theory； of equal importance with the sterility of species； for it seems to make a broad and clear distinction between varieties and species。
First； for the sterility of species when crossed and of their hybrid offspring。 It is impossible to study the several memoirs and works of those two conscientious and admirable observers； K？lreuter and G？rtner； who almost devoted their lives to this subject； without being deeply impressed with the high generality of some degree of sterility。 K？lreuter makes the rule universal； but then he cuts the knot； for in ten cases in which he found two forms； considered by most authors as distinct species； quite fertile together； he unhesitatingly ranks them as varieties。 G？rtner； also； makes the rule equally universal； and he disputes the entire fertility of K？lreuter's ten cases。 But in these and in many other cases； G？rtner is obliged carefully to count the seeds； in order to show that there is any degree of sterility。 He always compares the maximum number of seeds produced by two species when crossed and by their hybrid offspring； with the average number produced by both pure parent…species in a state of nature。 But a serious cause of error seems to me to be here introduced： a plant to be hybridised must be castrated； and； what is often more important； must be secluded in order to prevent pollen being brought to it by insects from other plants。 Nearly all the plants experimentised on by G？rtner were potted； and apparently were kept in a chamber in his house。 That these processes are often injurious to the fertility of a plant cannot be doubted； for G？rtner gives in his table about a score of cases of plants which he castrated； and artificially fertilised with their own pollen； and （excluding all cases such as the Leguminosae； in which there is an acknowledged difficulty in the manipulation） half of these twenty plants had their fertility in some degree impaired。 Moreover； as G？rtner during several years repeatedly crossed the primrose and cowslip； which we have such good reason to believe to be varieties； and only once or twice succeeded in getting fertile seed； as he found the common red and blue pimpernels （Anagallis arvensis and coerulea）； which the best botanists rank as varieties； absolutely sterile together； and as he came to the same conclusion in several other analogous cases； it seems to me that we may well be permitted to doubt whether many other species are really so sterile； when intercrossed； as G？rtner believes。
It is certain； on the one hand； that the sterility of various species when crossed is so different in degree and graduates away so insensibly； and； on the other hand； that the fertility of pure species is so easily affected by various circumstances； that for all practical purposes it is most difficult to say where perfect fertility ends and sterility begins。 I think no better evidence of this can be required than that the two most experienced observers who have ever lived； namely； K？lreuter and G？rtner； should have arrived at diametrically opposite conclusions in regard to the very same species。 It is also most instructive to compare but I have not space here to enter on details the evidence advanced by our best botanists on the question whether certain doubtful forms should be ranked as species or varieties； with the evidence from fertility adduced by different hybridisers； or by the same author； from experiments made during different years。 It can thus be shown that neither sterility nor fertility affords any clear distinction between species and varieties； but that the evidence from this source graduates away； and is doubtful in the same degree as is the evidence derived from other constitutional and structural differences。
In regard to the sterility of hybrids in successive generations； though G？rtner was enabled to rear some hybrids； carefully guarding them from a cross with either pure parent； for six or seven； and in one case for ten generations； yet he asserts positively that their fertility never increased； but generally greatly decreased。 I do not doubt that this is usually the case； and that the fertility often suddenly decreases in the first few generations。 Nevertheless I believe that in all these experiments the fertility has been diminished by an independent cause； namely； from close interbreeding。 I have collected so large a body of facts； showing that close interbreeding lessens fertility； and； on the other hand； that an occasional cross with a distinct individual or variety increases fertility； that I cannot doubt the correctness of this almost universal belief amongst breeders。 Hybrids are seldom raised by experimentalists in great numbers； and as the parent…species； or other allied hybrids； generally grow in the same garden； the visits of insects must be carefully prevented during the flowering season： hence hybrids will generally be fertilised during each generation by their own individual pollen； and I am convinced that this would be injurious to their fertility； already lessened by their hybrid origin。 I am strengthened in this conviction by a remarkable statement repeatedly made by G？rtner； namely； that if even the less fertile hybrids be artificially fertilised with hybrid pollen of the same kind； their fertility； notwithstanding the frequent ill effects of manipulation； sometimes decidedly increases； and goes on increasing。 Now； in artificial fertilisation pollen is as often taken by chance （as I know from my own experience） from the anthers of another flower； as from the anthers of the flower itself which is to be fertilised； so that a cross between two flowers； though probably on the same plant； would be thus effected。 Moreover； whenever complicated experiments are in progress； so careful an observer as G？rtner would have castrated his hybrids； and this would have insured in each generation a cross with the pollen from a distinct flower； either from the same plant or from another plant of the same hybrid nature。 And thus； the strange fact of the increase of fertility in the successive generations of artificially fertilised hybrids may； I believe； be accounted for by close interbreeding having been avoided。
Now let us turn to the results arrived at by the third most experienced hybridiser； namely； the Hon。 and Rev。 W。 Herbert。 He is as emphatic in his conclusion that some hybrids are perfectly fertile as fertile as the pure parent…species as are K？lreuter and G？rtner that some degree of sterility between distinct species is a universal law of nature。 He experimentised
