370 STRUGGLE BETWEEN PLANT-COMMUNITIES SECT, xvn 



Here we must pass over such factors in the origin of species as the 

 crossing of different species, as correlation among different parts of the 

 organism, as Vesque's 1 variability phyletique (an inherited variability 

 dependent on the ancestry of species and responsible for the progressive 

 evolution of plants and animals). 



(iii) In 1809 Lamarck published, in his famous Philosophic Zoologique, 

 speculations on the origin of species. Like Darwin and other believers 

 of the theory of evolution he assumed that forms showed variation ; and, 

 like Darwin, he devoted consideration to domesticated plants and animals. 

 According to Lamarck, evolution in Nature, or the natural modification 

 of forms, goes on ceaselessly. Time and nature of environment are the 

 two weightiest factors responsible for the natural production of all the 

 various forms. The environment acts on the organism, so that when it 

 changes, or when the organism migrates and becomes exposed to different 

 surroundings, the animal feels the necessity (besoin) to adapt itself to the 

 new conditions, naturally makes different use of its members or ceases to 

 use them, and thus causes them to undergo change. Geoff roy St. Hilaire 

 laid greater stress upon the direct action of the surrounding medium on 

 the organism, but Lamarck also assumed this to take place especially 

 in the case of plants. In neo-Lamarckism the former type of active 

 adaptation is scarcely discussed, but passive adaptation or direct self- 

 regulation (self-adaptation, epharmosis 2 ) receives greater consideration. 

 Lamarck assumed without discussion that the new forms arising in this 

 manner transmitted the acquired characters to their offspring, and were 

 thus preserved by generation ; but this is the weak point of the hypothesis, 

 and at present is a much-discussed question which is still open. 



Warming assumes that plants possess a peculiar inherent force or 

 faculty by the exercise of which they directly adapt themselves to new 

 conditions, that is to say, they change in such a manner as to become 

 fitted for existence in accordance with their new surroundings. He thus 

 assumes that between external influences and the utility of variation there 

 is a definite connexion, which is of obscure nature (self -regulation or direct 

 adaptation, epharmosis). 



It is certainly a fact that the plant is extremely plastic, and that 

 external factors can evoke numerous changes in it. This is proved by 

 numbers of facts and experiments recorded during the last few decades, 

 by Costantin, Volkens, Lothelier, Stahl, Vochting, Schenck, Lesage, 

 G. Karsten, Frank, Dufour, Vesque, Bonnier, Askenasy, Klebs, Massart, 

 Gobel, Lewakoffsky, Grabner, and others, who have investigated the 

 morphological and anatomical plasticity of individuals 3 . The result of 

 these investigations has been to show that by change in external con- 

 ditions there is set up a course of development tending to adapt the plant 

 to its environment in precisely the same manner as plants or plant-com- 

 munities growing under natural conditions normal to them are adapted. 



The preceding remarks are elucidated by examples discussed in the 

 succeeding paragraphs. 



The characteristics of sun-leaves and shade-leaves have already been 

 dealt with in Chapter VI. Change in the illumination can evoke tor- 



1 Vesque, 1882 a, 1889-92. * Vesque, loc. cit. ; seeDiels, 1906. 



3 Gobel, 1908. 



