242 Influence of Unvironment on Plants 



termed a Mutation. In this connection we must bear in mind the 

 fact that no dilFerence, recognisable externally, need exist between 

 individual variation and mutation. Even the most minute quanti- 

 tative 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 \ Much more important, however, would be an answer to the 

 question, whether an individual variation can be converted experi- 

 mentally into an inherited character— a 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 trans- 

 formation of one species into another, is, in the last instance, to be 

 i-eferred 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 essen- 

 tially 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- 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 



^ For instance ethylchloride (Cf^TIfil) is a gas at 21° C, ethylenechloride (CgHjCL,) a 

 fluid boiling at 84° C, j3 trichlorethane (CgHjClg) a fluid boiling at 113" C, perchlorethane 

 (CjClg) a crystalline substance. Klebs, Willkilrliche Entwickelungsanderungen, p. 158. 



^ Cf. Detto, Die Throrie der dirchten Anpassung..., pp. 98 et seq., Jena, 1904; see also 

 Lotsy, Vorlesungcn, ii. pp. G3G et scq., where other similar cases are described. 



