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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 succeeding generations； in the case of bacteria； in which it is only a question of the loss of a character it is relatively easy for this to reappear。 It is not impossible； that in all such cases there is a material hanging…on of certain internal conditions； in consequence of which the modification of the character persists for a time in the descendants； although the original external conditions are no longer present。
Thus a slow dying…out of the effect of a stimulus was seen in my experiments on Veronica chamaedrys。 （Klebs； 〃Kunstliche Metamorphosen〃； Stuttgart； 1906； page 132。） During the cultivation of an artificially modified inflorescence I obtained a race showing modifications in different directions； among which twisting was especially conspicuous。 This plant； however； does not behave as the twisted race of Dipsacus isolated by de Vries （de Vries； 〃Mutationstheorie〃； Vol。 II。 Leipzig； 1903； page 573。）； which produced each year a definite percentage of twisted individuals。 In the vegetative reproduction of this Veronica the torsion appeared in the first； also in the second and third year； but with diminishing intensity。 In spite of good cultivation this character has apparently now disappeared； it disappeared still more quickly in seedlings。 In another character of the same Veronica chamaedrys the influence of the environment was stronger。 The transformation of the inflorescences to foliage…shoots formed the starting…point； it occurred only under narrowly defined conditions； namely on cultivation as a cutting in moist air and on removal of all other leaf… buds。 In the majority （7/10） of the plants obtained from the transformed shoots； the modification appeared in the following year without any interference。 Of the three plants which were under observation several years the first lost the character in a short time； while the two others still retain it； after vegetative propagation； in varying degrees。 The same character occurs also in some of the seedlings； but anything approaching a constant race has