darwin and modern science-第79章
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ust be attached to their opinion that the evidence from continuous deposits favours a somewhat sudden change from one specific form to another。 At the same time it will be well to bear in mind that the subject of the 〃absence of numerous intermediate varieties in any single formation〃 was fully discussed by Darwin。 （〃Origin of Species〃； pages 275…282； and page 312。）； the explanation which he gave may go a long way to account for the facts which recent writers have regarded as favouring the theory of saltatory mutation。
The rapid sketch given in the present essay can do no more than call attention to a few salient points； in which the palaeontological records of plants has an evident bearing on the Darwinian theory。 At the present day the whole subject of palaeobotany is a study in evolution； and derives its chief inspiration from the ideas of Darwin and Wallace。 In return it contributes something to the verification of their teaching； the recent progress of the subject； in spite of the immense difficulties which still remain； has added fresh force to Darwin's statement that 〃the great leading facts in palaeontology agree admirably with the theory of descent with modification through variation and natural selection。〃 （Ibid。 page 313。）
XIII。 THE INFLUENCE OF ENVIRONMENT ON THE FORMS OF PLANTS。
By GEORG KLEBS； PH。D。 Professor of Botany in the University of Heidelberg。
The dependence of plants on their environment became the object of scientific research when the phenomena of life were first investigated and physiology took its place as a special branch of science。 This occurred in the course of the eighteenth century as the result of the pioneer work of Hales； Duhamel； Ingenhousz； Senebier and others。 In the nineteenth century； particularly in the second half； physiology experienced an unprecedented development in that it began to concern itself with the experimental study of nutrition and growth； and with the phenomena associated with stimulus and movement； on the other hand； physiology neglected phenomena connected with the production of form； a department of knowledge which was the province of morphology； a purely descriptive science。 It was in the middle of the last century that the growth of comparative morphology and the study of phases of development reached their highest point。
The forms of plants appeared to be the expression of their inscrutable inner nature； the stages passed through in the development of the individual were regarded as the outcome of purely internal and hidden laws。 The feasibility of experimental inquiry seemed therefore remote。 Meanwhile； the recognition of the great importance of such a causal morphology emerged from the researches of the physiologists of that time； more especially from those of Hofmeister （Hofmeister； 〃Allgemeine Morphologie〃； Leipzig； 1868； page 579。）； and afterwards from the work of Sachs。 （Sachs； 〃Stoff und Form der Pflanzenorgane〃； Vol。 I。 1880； Vol。 II。 1882。 〃Gesammelte Abhandlungen uber Pflanzen…Physiologie〃； II。 Leipzig； 1893。） Hofmeister； in speaking of this line of inquiry； described it as 〃the most pressing and immediate aim of the investigator to discover to what extent external forces acting on the organism are of importance in determining its form。〃 This advance was the outcome of the influence of that potent force in biology which was created by Darwin's 〃Origin of Species〃 （1859）。
The significance of the splendid conception of the transformation of species was first recognised and discussed by Lamarck （1809）； as an explanation of transformation he at once seized upon the ideaan intelligible viewthat the external world is the determining factor。 Lamarck （Lamarck； 〃Philosophie zoologique〃； pages 223…227。 Paris； 1809。） endeavoured； more especially； to demonstrate from the behaviour of plants that changes in environment induce change in form which eventually leads to the production of new species。 In the case of animals； Lamarck adopted the teleological view that alterations in the environment first lead to alterations in the needs of the organisms； which； as the result of a kind of conscious effort of will； induce useful modifications and even the development of new organs。 His work has not exercised any influence on the progress of science： Darwin himself confessed in regard to Lamarck's work 〃I got not a fact or idea from it。〃 （〃Life and Letters〃； Vol。 II。 page 215。）
On a mass of incomparably richer and more essential data Darwin based his view of the descent of organisms and gained for it general acceptance； as an explanation of modification he elaborated the ingeniously conceived selection theory。 The question of special interest in this connection； namely what is the importance of the influence of the environment； Darwin always answered with some hesitation and caution； indeed with a certain amount of indecision。
The fundamental principle underlying his theory is that of general variability as a whole； the nature and extent of which； especially in cultivated organisms； are fully dealt with in his well…known book。 （Darwin； 〃The variation of Animals and Plants under domestication〃； 2 vols。； edition 1； 1868； edition 2； 1875； popular edition 1905。） In regard to the question as to the cause of variability Darwin adopts a consistently mechanical view。 He says： 〃These several considerations alone render it probable that variability of every kind is directly or indirectly caused by changed conditions of life。 Or； to put the case under another point of view； if it were possible to expose all the individuals of a species during many generations to absolutely uniform conditions of life； there would be no variability。〃 （〃The variation of Animals and Plants〃 （2nd edition）； Vol。 II。 page 242。） Darwin did not draw further conclusions from this general principle。
Variations produced in organisms by the environment are distinguished by Darwin as 〃the definite〃 and 〃the indefinite。〃 （Ibid。 II。 page 260。 See also 〃Origin of Species〃 （6th edition）； page 6。） The first occur 〃when all or nearly all the offspring of an individual exposed to certain conditions during several generations are modified in the same manner。〃 Indefinite variation is much more general and a more important factor in the production of new species； as a result of this； single individuals are distinguished from one another by 〃slight〃 differences； first in one then in another character。 There may also occur； though this is very rare； more marked modifications； 〃variations which seem to us in our ignorance to arise spontaneously。〃 （〃Origin of Species〃 （6th edition）； page 421。） The selection theory demands the further postulate that such changes； 〃whether extremely slight or strongly marked；〃 are inherited。 Darwin was no nearer to an experimental proof of this assumption than to the discovery of the actual cause of variability。 It was not until the later years of his life that Darwin was occupied with the 〃perplexing problem。。。what causes almost every cultivated plant to vary〃 （〃Life and Letters〃； Vol。 III。 page 342。）： he began to make experiments on the influence of the soil； but these were soon given up。
In the course of the violent controversy which was the outcome of Darwin's work the fundamental principles of his teaching were not advanced by any decisive observations。 Among the supporters and opponents； Nageli （Nageli； 〃Theorie der Abstammungslehre〃； Munich； 1884； cf。 Chapter III。） was one of the few who sought to obtain proofs by experimental methods。 His extensive cultural experiments with alpine Hieracia led him to form the opinion that the changes which are induced by an alteration in the food…supply； in climate or in habitat； are not inherited and are therefore of no importance from the point of view of the production of species。 And yet Nageli did attribute an important influence to the external world； he believed that adaptations of plants arise as reactions to continuous stimuli； which supply a need and are therefore useful。 These opinions； which recall the teleological aspect of Lamarckism； are entirely unsupported by proof。 While other far…reaching attempts at an explanation of the theory of descent were formulated both in Nageli's ti
