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In the nineteenth chapter of the 〃Generelle Morphologie〃a part of which has been republished； without any alteration； after a lapse of forty years I made a critical study of Lamarck's theory of descent and of Darwin's theory of selection； and endeavoured to bring the complex phenomena of heredity and adaptation under definite laws for the first time。 Heredity I divided into conservative and progressive： adaptation into indirect （or potential） and direct （or actual）。 I then found it possible to give some explanation of the correlation of the two physiological functions in the struggle for life （selection）； and to indicate the important laws of divergence （or differentiation） and complexity （or division of labour）； which are the direct and inevitable outcome of selection。 Finally； I marked off dysteleology as the science of the aimless （vestigial； abortive； atrophied； and useless） organs and parts of the body。 In all this I worked from a strictly monistic standpoint； and sought to explain all biological phenomena on the mechanical and naturalistic lines that had long been recognised in the study of inorganic nature。 Then （1866）； as now； being convinced of the unity of nature； the fundamental identity of the agencies at work in the inorganic and the organic worlds； I discarded vitalism； teleology； and all hypotheses of a mystic character。
It was clear from the first that it was essential； in the monistic conception of evolution； to distinguish between the laws of conservative and progressive heredity。 Conservative heredity maintains from generation to generation the enduring characters of the species。 Each organism transmits to its descendants a part of the morphological and physiological qualities that it has received from its parents and ancestors。 On the other hand； progressive heredity brings new characters to the species characters that were not found in preceding generations。 Each organism may transmit to its offspring a part of the morphological and physiological features that it has itself acquired； by adaptation； in the course of its individual career； through the use or disuse of particular organs； the influence of environment； climate； nutrition； etc。 At that time I gave the name of 〃progressive heredity〃 to this inheritance of acquired characters； as a short and convenient expression； but have since changed the term to 〃transformative heredity〃 （as distinguished from conservative）。 This term is preferable； as inherited regressive modifications （degeneration； retrograde metamorphisis； etc。） come under the same head。
Transformative heredityor the transmission of acquired charactersis one of the most important principles in evolutionary science。 Unless we admit it most of the facts of comparative anatomy and physiology are inexplicable。 That was the conviction of Darwin no less than of Lamarck； of Spencer as well as Virchow； of Huxley as well as Gegenbaur； indeed of the great majority of speculative biologists。 This fundamental principle was for the first time called in question and assailed in 1885 by August Weismann of Freiburg； the eminent zoologist to whom the theory of evolution owes a great deal of valuable support； and who has attained distinction by his extension of the theory of selection。 In explanation of the phenomena of heredity he introduced a new theory； the 〃theory of the continuity of the germ…plasm。〃 According to him the living substance in all organisms consists of two quite distinct kinds of plasm； somatic and germinal。 The permanent germ…plasm； or the active substance of the two germ…cells （egg… cell and sperm…cell）； passes unchanged through a series of generations； and is not affected by environmental influences。 The environment modifies only the soma…plasm； the organs and tissues of the body。 The modifications that these parts undergo through the influence of the environment or their own activity （use and habit）； do not affect the germ…plasm； and cannot therefore be transmitted。
This theory of the continuity of the germ…plasm has been expounded by Weismann during the last twenty…four years in a number of able volumes； and is regarded by many biologists； such as Mr Francis Galton； Sir E。 Ray Lankester； and Professor J。 Arthur Thomson （who has recently made a thoroughgoing defence of it in his important work 〃Heredity〃 （London； 1908。））； as the most striking advance in evolutionary science。 On the other hand； the theory has been rejected by Herbert Spencer； Sir W。 Turner； Gegenbaur； Kolliker； Hertwig； and many others。 For my part I have； with all respect for the distinguished Darwinian； contested the theory from the first； because its whole foundation seems to me erroneous； and its deductions do not seem to be in accord with the main facts of comparative morphology and physiology。 Weismann's theory in its entirety is a finely conceived molecular hypothesis； but it is devoid of empirical basis。 The notion of the absolute and permanent independence of the germ…plasm； as distinguished from the soma…plasm； is purely speculative； as is also the theory of germinal selection。 The determinants； ids； and idants； are purely hypothetical elements。 The experiments that have been devised to demonstrate their existence really prove nothing。
It seems to me quite improper to describe this hypothetical structure as 〃Neodarwinism。〃 Darwin was just as convinced as Lamarck of the transmission of acquired characters and its great importance in the scheme of evolution。 I had the good fortune to visit Darwin at Down three times and discuss with him the main principles of his system； and on each occasion we were fully agreed as to the incalculable importance of what I call transformative inheritance。 It is only proper to point out that Weismann's theory of the germ…plasm is in express contradiction to the fundamental principles of Darwin and Lamarck。 Nor is it more acceptable in what one may call its 〃ultradarwinism〃the idea that the theory of selection explains everything in the evolution of the organic world。 This belief in the 〃omnipotence of natural selection〃 was not shared by Darwin himself。 Assuredly； I regard it as of the utmost value； as the process of natural selection through the struggle for life affords an explanation of the mechanical origin of the adapted organisation。 It solves the great problem： how could the finely adapted structure of the animal or plant body be formed unless it was built on a preconceived plan？ It thus enables us to dispense with the teleology of the metaphysician and the dualist； and to set aside the old mythological and poetic legends of creation。 The idea had occurred in vague form to the great Empedocles 2000 years before the time of Darwin； but it was reserved for modern research to give it ample expression。 Nevertheless； natural selection does not of itself give the solution of all our evolutionary problems。 It has to be taken in conjunction with the transformism of Lamarck； with which it is in complete harmony。
The monumental greatness of Charles Darwin； who surpasses every other student of science in the nineteenth century by the loftiness of his monistic conception of nature and the progressive influence of his ideas； is perhaps best seen in the fact that not one of his many successors has succeeded in modifying his theory of descent in any essential point or in discovering an entirely new standpoint in the interpretation of the organic world。 Neither Nageli nor Weismann； neither De Vries nor Roux； has done this。 Nageli； in his 〃Mechanisch…Physiologische Theorie der Abstammungslehre〃 （Munich； 1884。）； which is to a great extent in agreement with Weismann； constructed a theory of the idioplasm； that represents it （like the germ…plasm） as developing continuously in a definite direction from internal causes。 But his internal 〃principle of progress〃 is at the bottom just as teleological as the vital force of the Vitalists； and the micellar structure of the idioplasm is just as hypothetical as the 〃dominant〃 structure of the germ…plasm。 In 1889 Moritz Wagner sought to explain the origin of species by migration and isolation； and on that basis constructed a special 〃migration…theory。〃 This； however； is not out of harmony with the theory of selection。 It merely elevates one single factor in the theory to a predominant position。 Isolation is only a special case of selection； as I had pointed out in the fifteenth chapter of my 〃Natural history of creation〃。 The 〃mutation…theory〃 of De Vries （〃Die Mutationstheorie〃； Leipzig； 1903。）； that would explain the origin of species by sudden and saltatory variations rather than by gradual modification； is regarded by many botanists as a great step in advance； but it is generally rejected by zoologists。 It affords no explanation of the facts of adaptation； and has no causal value。
Much more important than these theories is that of Wilhelm Roux （〃Der Kampf der Theile im Organismus〃； Leipzig； 1881。） of 〃the struggle of parts within the organism； a supplementation of the theory of mechanical adaptation。〃 He explains the functional autoformation of the purposive structure by a combination of Darwin's principle of selection with Lamarck's idea of transformative heredity； and applies the two in conjun