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ic 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 not been produced。
Another means of producing new races has been attempted by Blaringhem。 （Blaringhem； 〃Mutation et Traumatisme〃； Paris； 1907。） On removing at an early stage the main shoots of different plants he observed various abnormalities in the newly formed basal shoots。 From the seeds of such plants he obtained races； a large percentage of which exhibited these abnormalities。 Starting from a male Maize plant with a fasciated inflorescence； on which a proportion of the flowers had become male； a new race was bred in which hermaphrodite flowers were frequently produced。 In the same way Blaringhem obtained； among other similar results； a race of barley with branched ears。 These races； however； behaved in essentials like those which have been demonstrated by de Vries to be inconstant； e。g。 Trifolium pratense quinquefolium and others。 The abnormality appears in a proportion of the individuals and only under very special conditions。 It must be remembered too that Blaringhem worked with old cultivated plants； which from the first had been disposed to split into a great variety of races。 It is possible； but difficult to prove； that injury contributed to this result。
A third method has been adopted by MacDougal （MacDougal； 〃Heredity and Origin of species〃； 〃Monist〃； 1906； 〃Report of department of botanical research〃； 〃Fifth Year…book of the Carnegie Institution of Washington〃； page 119； 1907。） who injected strong （10 percent） sugar solution or weak solutions of calcium nitrate and zinc sulphate into young carpels of different plants。 From the seeds of a plant of Raimannia odorata the carpels of which had been thus treated he obtained several plants distinguished from the parent…forms by the absence of hairs and by distinct forms of leaves。 Further examination showed that he had here to do with a new elementary species。 MacDougal also obtained a more or less distinct mutant of Oenothera biennis。 We cannot as yet form an opinion as to how far the effect is due to the wound or to the injection of f