darwin and modern science-第62章
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ctivity all the potentialities。
But the control of plant…form is only a preliminary stepthe foundation stones on which to erect a coherent scientific structure。 We must discover what are the internal processes in the cell produced by external factors； which as a necessary consequence result in the appearance of a definite form。 We are here brought into contact with the most obscure problem of life。 Progress can only be made pari passu with progress in physics and chemistry； and with the growth of our knowledge of nutrition； growth； etc。
Let us take one of the simplest casesan alteration in form。 A cylindrical cell of the alga Stigeoclonium assumes； as Livingstone （Livingstone； 〃On the nature of the stimulus which causes the change of form； etc。〃 〃Botanical Gazette〃； XXX。 1900； also XXXII。 1901。） has shown； a spherical form when the osmotic pressure of the culture fluid is increased； or a spore of Mucor； which； in a sugar solution grows into a branched filament； in the presence of a small quantity of acid （hydrogen ions） becomes a comparatively large sphere。 （Ritter； 〃Ueber Kugelhefe； etc。〃 〃Ber。 bot。 Gesell。〃 Berlin； XXV。 page 255； 1907。） In both cases there has undoubtedly been an alteration in the osmotic pressure of the cell…sap； but this does not suffice to explain the alteration in form； since the unknown alterations； which are induced in the protoplasm； must in their turn influence the cell…membrane。 In the case of the very much more complex alterations in form； such as we encounter in the course of development of plants； there do not appear to be any clues which lead us to a deeper insight into the phenomena。 Nevertheless we continue the attempt； seeking with the help of any available hypothesis for points of attack； which may enable us to acquire a more complete mastery of physiological methods。 To quote a single example； I may put the question； what internal changes produce a transition from vegetative growth to sexual reproduction？
The facts； which are as clearly established from the lower as for the higher plants； teach us that quantitative alteration in the environment produces such a transition。 This suggests the conclusion that quantitative internal changes in the cells； and with them disturbances in the degree of concentration； are induced； through which the chemical reactions are led in the direction of sexual reproduction。 An increase in the production of organic substances in the presence of light； chiefly of the carbohydrates； with a simultaneous decrease in the amount of inorganic salts and water； are the cause of the disturbance and at the same time of the alteration in the direction of development。 Possibly indeed mineral salts as such are not in question； but only in the form of other organic combinations； particularly proteid material； so that we are concerned with an alteration in the relation of the carbohydrates and proteids。 The difficulties of such researches are very great because the methods are not yet sufficiently exact to demonstrate the frequently small quantitative differences in chemical composition。 Questions relating to the enzymes； which are of the greatest importance in all these life…processes； are especially complicated。 In any case it is the necessary result of such an hypothesis that we must employ chemical methods of investigation in dealing with problems connected with the physiology of form。
II。 INFLUENCE OF ENVIRONMENT ON THE TRANSFORMATION OF SPECIES。
The study of the physiology of form…development in a pure species has already yielded results and makes slow but sure progress。 The physiology of the possibility of the transformation of one species into another is based； as yet； rather on pious hope than on accomplished fact。 From the first it appeared to be hopeless to investigate physiologically the origin of Linnean species and at the same time that of the natural system； an aim which Darwin had before him in his enduring work。 The historical sequence of events； of which an organism is the expression； can only be treated hypothetically with the help of facts supplied by comparative morphology； the history of development； geographical distribution； and palaeontology。 （See Lotsy； 〃Vorlesungen〃 （Jena； I。 1906； II。 1908）； for summary of the facts。） A glance at the controversy which is going on today in regard to different hypotheses shows that the same material may lead different investigators to form entirely different opinions。 Our ultimate aim is to find a solution of the problem as to the cause of the origin of species。 Indeed such attempts are now being made： they are justified by the fact that under cultivation new and permanent strains are produced； the fundamental importance of this was first grasped by Darwin。 New points of view in regard to these lines of inquiry have been adopted by H。 de Vries who has succeeded in obtaining from Oenothera Lamarckiana a number of constant 〃elementary〃 species。 Even if it is demonstrated that he was simply dealing with the complex splitting up of a hybrid （Bateson； 〃Reports to the Evolution Committee of the Royal Society〃； London； 1902； cf。 also Lotsy； 〃Vorlesungen〃； Vol。 I。 page 234。）； the facts adduced in no sense lose their very great value。
We must look at the problem in its simplest form； we find it in every case where a new race differs essentially from the original type in a single character only； for example； in the colour of the flowers or in the petalody of the stamens （doubling of flowers）。 In this connection we must keep in view the fact that every visible character in a plant is the resultant of the cooperation of specific structure； with its various potentialities； and the influence of the environment。 We know； that in a pure species all characters vary； that a blue…flowering Campanula or a red Sempervivum can be converted by experiment into white…flowering forms； that a transformation of stamens into petals may be caused by fungi or by the influence of changed conditions of nutrition； or that plants in dry and poor soil become dwarfed。 But so far as the experiments justify a conclusion； it would appear that such alterations are not inherited by the offspring。 Like all other variations they appear only so long as special conditions prevail in the surroundings。
It has been shown that the case is quite different as regards the white… flowering； double or dwarf races； because these retain their characters when cultivated under practically identical conditions； and side by side with the blue； single…flowering or tall races。 The problem may therefore be stated thus： how can a character； which appears in the one case only under the strictly limited conditions of the experiment； in other cases become apparent under the very much wider conditions of ordinary cultivation？ If a character appears； in these circumstances； in the case of all individuals； we then speak of constant races。 In such simple cases the essential point is not the creation of a new character but rather an ALTERATION OF THIS CHARACTER IN ACCORDANCE WITH THE ENVIRONMENT。 In the examples mentioned the modified character in the simple varieties （or a number of characters in elementary species） appears more or less suddenly and is constant in the above sense。 The result is what de Vries has termed a Mutation。 In this connection we must bear in mind the fact that no difference； recognisable externally； need exist between individual variation and mutation。 Even the most minute quantitative difference between two plants may be of specific value if it is preserved under similar external conditions during many successive generations。 We do not know how this happens。 We may state the problem in other terms； by saying that the specific structure must be altered。 It is possible； to some extent； to explain this sudden alteration； if we regard it as a chemical alteration of structure either in the specific qualities of the proteids or of the unknown carriers of life。 In the case of many organic compounds their morphological characters （the physical condition； crystalline form； etc。） are at once changed by alteration of atomic relations or by incorporation of new radicals。 （For instance ethylchloride （C2H5Cl） is a gas at 21 deg C。； ethylenechloride （C2H4Cl2） a fluid boiling at 84 deg C。； beta trichlorethane （C2H3Cl3） a fluid boiling at 113 deg C。； perchlorethane （C2Cl6） a crystalline substance。 Klebs； （〃Willkurliche Entwickelungsanderungen〃 page 158。） Much more important； however； would be an answer to the question； whether an individual variation can be converted experimentally into an inherited charactera mutation in de Vries's sense。
In all circumstances we may recognise as a guiding principle the assumption adopted by Lamarck； Darwin； and many others； that the inheritance of any one character； or in more general terms； the transformation of one species into another； is； in the last instance； to be referred to a change in the environment。 From a causal…mechanical point of view it is not a priori conceivable that one species can ever become changed into another so long as external conditions remain constant。 The inner structure of a species must be essentially altered by external influences。 Two methods of experimental research may be adopted； the effect of crossing distinct species and； secondly； the effect of definite factors of the environment。
The subject of hybridisation is dealt with in another part of this essay。 It is enough to refer here to the most important fact； that as the result of combinations of characters of different species new and constant forms are produced。 Further； Tschermack； Bateson and others have demonstrated the possibility that hitherto unknown inheritable characters may be produced by hybridisation。
The other method of producing constant races by the influence of special external conditions has often been employed。 The sporeless races of Bacteria and Yeasts （Cf。 Detto； 〃Die Theorie der direkten Anpassung。。。〃； pages 98 et seq。； Jena； 1904； see also Lotsy； 〃Vorlesungen〃； II。 pages 636 et seq。； where other similar cases are described。） are well known； in which an internal alteration of the cells is induced by the influence of poison or higher temperature； so that the power of producing spores even under normal conditions appears to be lost。 A similar state of things is found in some races which under certain definite conditions lose their colour or their virulence。 Among the phanerogams the investigations of Schubler on cereals afford parallel cases； in which the influence of a northern climate produces individuals which ripen their seeds early； these seeds produce plants which seed early in southern countries。 Analogous results were obtained by Cieslar in his experiments； seeds of conifers from the Alps when planted in the plains produced plants of slow growth and small diameter。
All these observations are of considerable interest theoretically； they show that the action of environment certainly induces such internal changes； and that these are transmitted to the next generation。 But as regards the main question； whether constant races may be obtained by this means； the experiments cannot as yet supply a definite answer。 In phanerogams； the influence very soon dies out in succeed
