darwin and modern science-第61章
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e by no means isolated examples。 All plants exhibit a definite alteration in form as the result of prolonged cultivation in moist or dry air； in strong or feeble light； or in darkness； or in salt solutions of different composition and strength。
Every individual which is exposed to definite combinations of external factors exhibits eventually the same type of modification。 This is the type of variation which Darwin termed 〃definite。〃 It is easy to realise that indefinite or fluctuating variations belong essentially to the same class of phenomena； both are reactions to changes in environment。 In the production of individual variations two different influences undoubtedly cooperate。 One set of variations is caused by different external conditions； during the production； either of sexual cells or of vegetative primordia； another set is the result of varying external conditions during the development of the embryo into an adult plant。 The two sets of influences cannot as yet be sharply differentiated。 If； for purposes of vegetative reproduction； we select pieces of the same parent…plant of a pure species； the second type of variation predominates。 Individual fluctuations depend essentially in such cases on small variations in environment during development。
These relations must be borne in mind if we wish to understand the results of statistical methods。 Since the work of Quetelet； Galton； and others the statistical examination of individual differences in animals and plants has become a special science； which is primarily based on the consideration that the application of the theory of probability renders possible mathematical statement and control of the results。 The facts show that any character； size of leaf； length of stem； the number of members in a flower； etc。 do not vary haphazard but in a very regular manner。 In most cases it is found that there is a value which occurs most commonly； the average or medium value； from which the larger and smaller deviations； the so…called plus and minus variations fall away in a continuous series and end in a limiting value。 In the simpler cases a falling off occurs equally on both sides of the curve； the curve constructed from such data agrees very closely with the Gaussian curve of error。 In more complicated cases irregular curves of different kinds are obtained which may be calculated on certain suppositions。
The regular fluctuations about a mean according to the rule of probability is often attributed to some law underlying variability。 （de Vries； 〃Mutationstheorie〃； Vol。 I。 page 35； Leipzig； 1901。） But there is no such law which compels a plant to vary in a particular manner。 Every experimental investigation shows； as we have already remarked； that the fluctuation of characters depends on fluctuation in the external factors。 The applicability of the method of probability follows from the fact that the numerous individuals of a species are influenced by a limited number of variable conditions。 （Klebs； 〃Willkurl。 Ent。〃 Jena； 1903； page 141。） As each of these conditions includes within certain limits all possible values and exhibits all possible combinations； it follows that； according to the rules of probability； there must be a mean value； about which the larger and smaller deviations are distributed。 Any character will be found to have the mean value which corresponds with that combination of determining factors which occurs most frequently。 Deviations towards plus and minus values will be correspondingly produced by rarer conditions。
A conclusion of fundamental importance may be drawn from this conception； which is； to a certain extent； supported by experimental investigation。 （Klebs； 〃Studien uber Variation〃； 〃Arch。 fur Entw。〃 1907。） There is no normal curve for a particular CHARACTER； there is only a curve for the varying combinations of conditions occurring in nature or under cultivation。 Under other conditions entirely different curves may be obtained with other variants as a mean value。 If； for example； under ordinary conditions the number 10 is the most frequent variant for the stamens of Sedum spectabile； in special circumstances （red light） this is replaced by the number 5。 The more accurately we know the conditions for a particular form or number； and are able to reproduce it by experiment； the nearer we are to achieving our aim of rendering a particular variation impossible or of making it dominant。
In addition to the individual variations of a species； more pronounced fluctuations occur relatively rarely and sporadically which are spoken of as 〃single variations；〃 or if specially striking as abnormalities or monstrosities。 These forms have long attracted the attention of morphologists； a large number of observations of this kind are given in the handbooks of Masters （Masters； 〃Vegetable Teratology〃； London； 1869。） and Penzig （Penzig； 〃Pflanzen…Teratologie； Vols I。 and II。 Genua； 1890…94。） These variations； which used to be regarded as curiosities； have now assumed considerable importance in connection with the causes of form… development。 They also possess special interest in relation to the question of heredity； a subject which does not at present concern us； as such deviations from normal development undoubtedly arise as individual variations induced by the influence of environment。
Abnormal developments of all kinds in stems； leaves； and flowers； may be produced by parasites； insects； or fungi。 They may also be induced by injury； as Blaringhem （Blaringhem； 〃Mutation et traumatismes〃； Paris； 1907。） has more particularly demonstrated； which； by cutting away the leading shoots of branches in an early stage of development； caused fasciation； torsion； anomalous flowers； etc。 The experiments of Blaringhem point to the probability that disturbances in the conditions of food…supply consequent on injury are the cause of the production of monstrosities。 This is certainly the case in my experiments with species of Sempervivum （Klebs； 〃Kunstliche Metamorphosen〃； Stuttgart； 1906。）； individuals； which at first formed normal flowers； produced a great variety of abnormalities as the result of changes in nutrition； we may call to mind the fact that the formation of inflorescences occurs normally when a vigorous production of organic compounds； such as starch； sugar； etc。 follows a diminution in the supply of mineral salts。 On the other hand； the development of inflorescences is entirely suppressed if； at a suitable moment before the actual foundations have been laid； water and mineral salts are supplied to the roots。 If； during the week when the inflorescence has just been laid down and is growing very slowly； the supply of water and salts is increased； the internal conditions of the cells are essentially changed。 At a later stage； after the elongation of the inflorescence； rosettes of leaves are produced instead of flowers； and structures intermediate between the two kinds of organs； a number of peculiar plant…forms are thus obtained （Cf。 Lotsy； 〃Vorlesungen uber Deszendenztheorien〃； Vol。 II。 pl。 3； Jena； 1908。） Abnormalities in the greatest variety are produced in flowers by varying the time at which the stimulus is applied； and by the cooperation of other factors such as temperature； darkness； etc。 In number and arrangement the several floral members vary within wide limits； sepals； petals； stamens； and carpels are altered in form and colour； a transformation of stamens to carpels and from carpels to stamens occurs in varying degrees。 The majority of the deviations observed had not previously been seen either under natural conditions or in cultivation； they were first brought to light through the influence of external factors。
Such transformations of flowers become apparent at a time； which is separated by about two months from the period at which the particular cause began to act。 There is； therefore； no close connection between the appearance of the modifications and the external conditions which prevail at the moment。 When we are ignorant of the causes which are operative so long before the results are seen； we gain the impression that such variations as occur are spontaneous or autonomous expressions of the inner nature of the plant。 It is much more likely that； as in Sempervivum； they were originally produced by an external stimulus which had previously reached the sexual cells or the young embryo。 In any case abnormalities of this kind appear to be of a special type as compared with ordinary fluctuating variations。 Darwin pointed out this difference； Bateson （Bateson； 〃Materials for the study of Variation〃； London； 1894； page 5。） has attempted to make the distinction sharper； at the same time emphasising its importance in heredity。
Bateson applies the term CONTINUOUS to small variations connected with one another by transitional stages； while those which are more striking and characterised from the first by a certain completeness； he names DISCONTINUOUS。 He drew attention to a great difficulty which stands in the way of Lamarck's hypothesis； as also of Darwin's view。 〃According to both theories； specific diversity of form is consequent upon diversity of environment； and diversity of environment is thus the ultimate measure of diversity of specific form。 Here then we meet the difficulty that diverse environments often shade into each other insensibly and form a continuous series； whereas the Specific Forms of life which are subject to them on the whole form a Discontinuous Series。〃 This difficulty is； however； not of fundamental importance as well authenticated facts have been adduced showing that by alteration of the environment discontinuous variations； such as alterations in the number and form of members of a flower； may be produced。 We can as yet no more explain how this happens than we can explain the existence of continuous variations。 We can only assert that both kinds of variation arise in response to quantitative alterations in external conditions。 The question as to which kind of variation is produced depends on the greater or less degree of alteration； it is correlated with the state of the particular cells at the moment。
In this short sketch it is only possible to deal superficially with a small part of the subject。 It has been clearly shown that in view of the general dependence of development on the factors of the environment a number of problems are ready for experimental treatment。 One must； however； not forget that the science of the physiology of form has not progressed beyond its initial stages。 Just now our first duty is to demonstrate the dependence on external factors in as many forms of plants as possible； in order to obtain a more thorough control of all the different plant…forms。 The problem is not only to produce at will （and independently of their normal mode of life） forms which occur in nature； but also to stimulate into operation potentialities which necessarily lie dormant under the conditions which prevail in nature。 The constitution of a species is much richer in possibilities of development than would appear to be the case under normal conditions。 It remains for man to stimulate into activity all the potentialities。
But the control of plant…form is only a preliminary stepthe found
