darwin and modern science-第20章
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It should here be pointed out that in order to be selected； a change must first have been produced。 This proposition； which now seems self…evident； has； however； been a source of much difference of opinion among Darwin's followers。 The opinion that natural selection produces changes in useful directions has prevailed for a long time。 In other words； it was assumed that natural selection； by the simple means of singling out； could induce small and useful changes to increase and to reach any desired degree of deviation from the original type。 In my opinion this view was never actually held by Darwin。 It is in contradiction with the acknowledged aim of all his work；the explanation of the origin of species by means of natural forces and phenomena only。 Natural selection acts as a sieve； it does not single out the best variations； but it simply destroys the larger number of those which are； from some cause or another； unfit for their present environment。 In this way it keeps the strains up to the required standard； and； in special circumstances； may even improve them。
Returning to the variations which afford the material for the sieving… action of natural selection； we may distinguish two main kinds。 It is true that the distinction between these was not clear at the time of Darwin； and that he was unable to draw a sharp line between them。 Nevertheless； in many cases； he was able to separate them； and he often discussed the question which of the two would be the real source of the differentiation of species。 Certain variations constantly occur； especially such as are connected with size； weight； colour； etc。 They are usually too small for natural selection to act upon； having hardly any influence in the struggle for life： others are more rare； occurring only from time to time； perhaps once or twice in a century； perhaps even only once in a thousand years。 Moreover； these are of another type； not simply affecting size； number or weight； but bringing about something new； which may be useful or not。 Whenever the variation is useful natural selection will take hold of it and preserve it； in other cases the variation may either persist or disappear。
In his criticism of miscellaneous objections brought forward against the theory of natural selection after the publication of the first edition of 〃The Origin of Species〃； Darwin stated his view on this point very clearly：〃The doctrine of natural selection or the survival of the fittest； which implies that when variations or individual differences of a beneficial nature happen to arise； these will be preserved。〃 （〃Origin of Species〃 （6th edition）； page 169； 1882。） In this sentence the words 〃HAPPEN TO ARISE〃 appear to me of prominent significance。 They are evidently due to the same general conception which prevailed in Darwin's Pangenesis hypothesis。 （Cf。 de Vries； 〃Intracellulare Pangenesis〃； page 73； Jena； 1889； and 〃Die Mutationstheorie〃； I。 page 63。 Leipzig； 1901。）
A distinction is indicated between ordinary fluctuations which are always present； and such variations as 〃happen to arise〃 from time to time。 （（I think it right to point out that the interpretation of this passage from the 〃Origin〃 by Professor de Vries is not accepted as correct either by Mr Francis Darwin or by myself。 We do not believe that Darwin intended to draw any distinction between TWO TYPES of variation； the words 〃when variations or individual differences of a beneficial nature happen to arise〃 are not in our opinion meant to imply a distinction between ordinary fluctuations and variations which 〃happen to arise；〃 but we believe that 〃or〃 is here used in the sense of ALIAS。 With the permission of Professor de Vries； the following extract is quoted from a letter in which he replied to the objection raised to his reading of the passage in question：
〃As to your remarks on the passage on page 6； I agree that it is now impossible to see clearly how far Darwin went in his distinction of the different kinds of variability。 Distinctions were only dimly guessed at by him。 But in our endeavour to arrive at a true conception of his view I think that the chapter on Pangenesis should be our leading guide； and that we should try to interpret the more difficult passages by that chapter。 A careful and often repeated study of the Pangenesis hypothesis has convinced me that Darwin； when he wrote that chapter； was well aware that ordinary variability has nothing to do with evolution； but that other kinds of variation were necessary。 In some chapters he comes nearer to a clear distinction than in others。 To my mind the expression 'happen to arise' is the sharpest indication of his inclining in this direction。 I am quite convinced that numerous expressions in his book become much clearer when looked at in this way。〃
The statement in this passage that 〃Darwin was well aware that ordinary variability has nothing to do with evolution； but that other kinds of variation were necessary〃 is contradicted by many passages in the 〃Origin〃。 A。C。S。）） The latter afford the material for natural selection to act upon on the broad lines of organic development； but the first do not。 Fortuitous variations are the species…producing kind； which the theory requires； continuous fluctuations constitute； in this respect； a useless type。
Of late； the study of variability has returned to the recognition of this distinction。 Darwin's variations； which from time to time happen to arise； are MUTATIONS； the opposite type being commonly designed fluctuations。 A large mass of facts； collected during the last few decades； has confirmed this view； which in Darwin's time could only be expressed with much reserve； and everyone knows that Darwin was always very careful in statements of this kind。
From the same chapter I may here cite the following paragraph： 〃Thus as I am inclined to believe； morphological differences；。。。such as the arrangement of the leaves； the divisions of the flower or of the ovarium； the position of the ovules； etc。first appeared in many cases as fluctuating variations； which sooner or later became constant through the nature of the organism and of the surrounding conditions。。。but NOT THROUGH NATURAL SELECTION （The italics are mine （H。 de V。）。）； for as these morphological characters do not affect the welfare of the species； any slight deviation in them could not have been governed or accumulated through this latter agency。〃 （〃Origin of Species〃 （6th edition）； page 176。） We thus see that in Darwin's opinion； all small variations had not the same importance。 In favourable circumstances some could become constant； but others could not。
Since the appearance of the first edition of 〃The Origin of Species〃 fluctuating variability has been thoroughly studied by Quetelet。 He discovered the law； which governs all phenomena of organic life falling under this head。 It is a very simple law； and states that individual variations follow the laws of probability。 He proved it； in the first place； for the size of the human body； using the measurements published for Belgian recruits； he then extended it to various other measurements of parts of the body； and finally concluded that it must be of universal validity for all organic beings。 It must hold true for all characters in man； physical as well as intellectual and moral qualities； it must hold true for the plant kingdom as well as for the animal kingdom； in short； it must include the whole living world。
Quetelet's law may be most easily studied in those cases where the variability relates to measure； number and weight； and a vast number of facts have since confirmed its exactness and its validity for all kinds of organisms； organs and qualities。 But if we examine it more closely； we find that it includes just those minute variations； which； as Darwin repeatedly pointed out； have often no significance for the origin of species。 In the phenomena； described by Quetelet's law nothing 〃happens to arise〃； all is governed by the common law； which states that small deviations from the mean type are frequent； but that larger aberrations are rare； the rarer as they are larger。 Any degree of variation will be found to occur； if only the number of individuals studied is large enough： it is even possible to calculate before hand； how many specimens must be compared in order to find a previously fixed degree of deviation。
The variations； which from time to time happen to appear； are evidently not governed by this law。 They cannot； as yet； be produced at will： no sowings of thousands or even of millions of plants will induce them； although by such means the chance of their occurring will obviously be increased。 But they are known to occur； and to occur suddenly and abruptly。 They have been observed especially in horticulture； where they are ranged in the large and ill…defined group called sports。 Korschinsky has collected all the evidence which horticultural literature affords on this point。 （S。 Korschinsky； 〃Heterogenesis und Evolution〃； 〃Flora〃； Vol。 LXXXIX。 pages 240…363； 1901。） Several cases of the first appearance of a horticultural novelty have been recorded： this has always happened in the same way； it appeared suddenly and unexpectedly without any definite relation to previously existing variability。 Dwarf types are one of the commonest and most favourite varieties of flowering plants； they are not originated by a repeated selection of the smallest specimens； but appear at once； without intermediates and without any previous indication。 In many instances they are only about half the height of the original type； thus constituting obvious novelties。 So it is in other cases described by Korschinsky： these sports or mutations are now recognised to be the main source of varieties of horticultural plants。
As already stated； I do not pretend that the production of horticultural novelties is the prototype of the origin of new species in nature。 I assume that they are； as a rule； derived from the parent species by the loss of some organ or quality； whereas the main lines of the evolution of the animal and vegetable kingdom are of course determined by progressive changes。 Darwin himself has often pointed out this difference。 But the saltatory origin of horticultural novelties is as yet the simplest parallel for natural mutations； since it relates to forms and phenomena； best known to the general student of evolution。
The point which I wish to insist upon is this。 The difference between small and ever present fluctuations and rare and more sudden variations was clear to Darwin； although the facts known at his time were too meagre to enable a sharp line to be drawn between these two great classes of variability。 Since Darwin's time evidence； which proves the correctness of his view； has accumulated with increasing rapidity。 Fluctuations constitute one type； they are never absent and follow the law of chance； but they do not afford the material from which to build new species。 Mutations； on the other hand； only happen to occur from time to time。 They do not necessarily produce greater changes than fluctuations； but such as may become； or rather are from their very nature； constant。 It is this constancy which is the mark of specific characters； and on thi
