darwin and modern science-第65章
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the sound produced by a fiddle depends entirely upon the hand which plays upon it。 It would be improper to apply the term 〃mutation〃 to those genetic characters which are not new characters or new variants of old characters； but such genetic characters are of the same nature as those characters to which the term mutation has been applied。 It may be noticed in passing that it is very questionable if the modern biologist has acted in the real interests of science in applying the term mutation in the sense in which he has applied it。 The genetic characters of organisms come from one of two sources： either they are old characters and are due to the action of what we call inheritance or they are new and are due to what we call variation。 If the term mutation is applied to the actual alteration of the machinery of the protoplasm； no objection can be felt to its use； but if it be applied； as it is； to the product of the action of the altered machine； viz。 to the new genetic character； it leads to confusion。 Inheritance is the persistence of the structure of the machine； characters are the products of the working of the machine； variation in genetic characters is due to the alteration （mutation） in the arrangement of the machinery； while variation in acquired characters （Lamarckian） is due to differences in the mode of working the machinery。 The machinery when it starts （in the new zygote） has the power of grinding out certain results； which we call the characters of the organism。 These appear at successive intervals of time； and the orderly manifestation of them is what we call the life…history of the organism。 This brings us back to the question with which we started this discussion； viz。 what is the relation of these variations in structure； which successively appear in an organism and constitute its life…history； to the mutational variations which appear in different organisms of the same brood or species。 The question is brought home to us when we ask what is a bud…sport； such as a nectarine appearing on a peach…tree？ From one point of view； it is simply a mutation appearing in asexual reproduction； from another it is one of these successional characters （〃growth variations〃） which constitute the life…history of the zygote； for it appears in the same zygote which first produces a peach。 Here our analogy of a machine which only works in one way seems to fail us； for these bud…sports do not appear in all parts of the organism； only in certain buds or parts of it； so that one part of the zygotic machine would appear to work differently to another。 To discuss this question further would take us too far from our subject。 Suffice it to say that we cannot answer it； any more than we can this further question of burning interest at the present day； viz。 to what extent and in what manner is the machine itself altered by the particular way in which it is worked。 In connection with this question we can only submit one consideration： the zygotic machine can； by its nature； only work once； so that any alteration in it can only be ascertained by studying the replicas of it which are produced in the reproductive organs。
It is a peculiarity that the result which we call the ripening of the generative organs nearly always appears among the final products of the action of the zygotic machine。 It is remarkable that this should be the case。 What is the reason of it？ The late appearance of functional reproductive organs is almost a universal law； and the explanation of it is suggested by expressing the law in another way； viz。 that the machine is almost always so constituted that it ceases to work efficiently soon after the reproductive organs have sufficiently discharged their function。 Why this should occur we cannot explain： it is an ultimate fact of nature； and cannot be included in any wider category。 The period during which the reproductive organs can act may be short as in ephemerids or long as in man and trees； and there is no reason to suppose that their action damages the vital machinery； though sometimes； as in the case of annual plants （Metschnikoff）； it may incidentally do so； but； long or short； the cessation of their actions is always a prelude to the end。 When they and their action are impaired； the organism ceases to react with precision to the environment； and the organism as a whole undergoes retrogressive changes。
It has been pointed out above that there is reason to believe that at the dawn of life the life…cycle was； EITHER IN ESSE OR IN POSSE； at least as long as it is at the present time。 The qualification implied by the words in italics is necessary； for it is clearly possible that the external conditions then existing were not suitable for the production of all the stages of the potential life…history； and that what we call organic evolution has consisted in a gradual evolution of new environments to which the organism's innate capacity of change has enabled it to adapt itself。 We have warrant for this possibility in the case of the Axolotl and in other similar cases of neoteny。 And these cases further bring home to us the fact； to which I have already referred； that the full development of the functional reproductive organs is nearly always associated with the final stages of the life…history。
On this view of the succession of characters in the life…history of organisms； how shall we explain the undoubted fact that the development of buds hardly ever presents any phenomena corresponding to the embryonic and larval changes？ The reason is clearly this； that budding usually occurs after the embryonic stage is past； when the characters of embryonic life have been worked out by the machine。 When it takes place at an early stage in embryonic life； as it does in cases of so…called embryonic fission； the product shows； either partly or entirely； phenomena similar to those of embryonic development。 The only case known to me in which budding by the adult is accompanied by morphological features similar to those displayed by embryos is furnished by the budding of the medusiform spore…sacs of hydrozoon polyps。 But this case is exceptional； for here we have to do with an attempt； which fails； to form a free…swimming organism； the medusa； and the vestiges which appear in the buds are the umbrella…cavity； marginal tentacles； circular canal； etc。； of the medusa arrested in development。
But the question still remains； are there no cases in which； as implied by the recapitulation theory； variations in any organ are confined to the period in which the organ is functional and do not affect it in the embryonic stages？ The teeth of the whalebone whales may be cited as a case in which this is said to occur； but here the teeth are only imperfectly developed in the embryo and are soon absorbed。 They have been affected by the change which has produced their disappearance in the adult； but not to complete extinction。 Nor are they now likely to be extinguished； for having become exclusively embryonic they are largely protected from the action of natural selection。 This consideration brings up a most important aspect of the question； so far as disappearing organs are concerned。 Every organ is laid down at a certain period in the embryo and undergoes a certain course of growth until it obtains full functional development。 When for any cause reduction begins； it is affected at all stages of its growth； unless it has functional importance in the larva； and in some cases its life is shortened at one or both ends。 In cases； as in that of the whale's teeth； in which it entirely disappears in the adult； the latter part of its life is cut off； in others； the beginning of its life may be deferred。 This happens； for instance； with the spiracle of many Elasmobranchs； which makes its appearance after the hyobranchial cleft； not before it as it should do； being anterior to it in position； and as it does in the Amniota in which it shows no reduction in size as compared with the other pharyngeal clefts。 In those Elasmobranchs in which it is absent in the adult but present in the embryo （e。g。 Carcharias） its life is shortened at both ends。 Many more instances of organs； of which the beginning and end have been cut off； might
