750 PLANT RESPONSE 



that were best fitted to their external conditions. Going 

 back a step, however, to the question of the origin of these 

 variations themselves, we find that by some writers they 

 are held to be due to spontaneous unknown causes, in- 

 herent in the organism. Now it would obviously be more 

 satisfactory, since no effect can occur without a cause, if we 

 could assign at least some of these variations to something 

 more definite than this. And Darwin himself was of opinion 

 that variability of every kind was due, directly or indirectly, 

 to changes in the conditions of life. It was difficult, however, 

 to distinguish clearly how much of any given variation was 

 due ' to the accumulative action of natural selection, and 

 how much to the definite action of the conditions of life.' ^ 



It was this difficulty which, for example, compelled him 

 to ascribe the movements of plants to specific heliotropic 

 or geotropic sensibilities, acquired as the result of natural 

 selection. The particular reaction or reply to stimulus, 

 manifested by the plant in any special case, was thus to be 

 regarded, not as the direct and necessary result of changes 

 in the environment, but as an adaptive act forced on the 

 species by the struggle for life. 



Variation as induced by external forces. - With the 

 delicate modes of investigation, however, which are now at 

 the disposal of observers, it has become possible to demon- 

 strate a direct connection between some of the differentia- 

 tions induced in plant-organs and the conditions of the 

 environment. The factors which are conceivable as bringing 

 about variation may be classed under two heads, first, 

 internal or spontaneous, and second, those which arise from 

 external stimulus ; but with regard to the first of these we 

 have seen that, as far as experiment has carried us, there is 

 no such thing as spontaneity, in the sense of an effect which 

 occurs without antecedent cause, for the internal energy to 

 which such seeming actions have hitherto been vaguely 

 ascribed has been shown to be itself traceable to external 

 stimulus. And as regards the effect of external stimulus, on 



' Darwin, Origin of Species, p. 107. 



