242 Influence of Environment on Plants 
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 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 
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 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 ethylehloride (C,H,Cl) is a gas at 21°C., ethylenechloride (C,H,Cl,) a 
fluid boiling at 84°C., 8 trichlorethane (C,H,Cl,) a fluid boiling at 118°C., perchlorethane 
(C,Cl,) a crystalline substance. Klebs, Willkiirliche Entwickelungsinderungen, p. 158. 
2 Cf. Detto, Die Theorie der direkten Anpassung..., pp. 98 et seg., Jena, 1904; see also 
Lotsy, Vorlesungen, 11. pp. 636 et seg., where other similar cases are described. 
