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modifications and hereditary properties. A problem hes between these two 

 phenomena, but still there will not be a single investigator into heredity 

 who will not feel that an inconsequence exists in the separation of the 

 two phenomena. This separation is highly unsatisfactory and has the 

 character of artificiality. The whole of my investigation has brought to 

 light how that of a single "external" property: "teaf-lengt/i", further factors 

 are present, moreover that with one and the same tree these factors are 

 not activated in precisely the same manner but that in different parts 

 of the tree, i. e. in different phases of development different factors pre- 

 dominate. This is but to say that different circumstances of growth have 

 a different influence on the activity of the great, the vastly great, com- 

 plexity of factors of which the tree is built up. External circumstances 

 have therefore indeed a like influence upon the activity of the factors. 

 This is the principle whereon the foundations of "modifications" rest. Yet 

 it is beyond dispute that the seed again produces the complete resulting 

 or standard plant or at any rate is in the condition to produce it, so 

 that the generative part apparently has nothing to do with that influence 

 through external circumstances. Still it is illogical and inconsistent to 

 make a division between these two phenomena. They tnusf be bridged 

 over, and it seems to me that it is not difficult to bring these two phe- 

 nomena into a single point of view. 



I therefore submit the following hypothesis to consideration: 



The foundation is of course the doctrine of factors. In the plant are 

 found the factors which form the basis of the properties of the habitus. 

 Let us now regard it an open question how we have to demonstrate the 

 factors, be it as living substances, or be it as chemical materials; princi- 

 pally this is of little importance. 



The essential point is then that these factors are not of tiiemselves 

 sufficient to give the plant a habitus. 



We must picture to ourselves that these factors infiuence the plant. 

 They act as stimuli to the plant. A definite factor stimulates in a definite 

 manner, and the plant reacts on this stimulation; it reacts in a definite 

 manner, and that reaction displays itself to us by the assumption of a 

 definite habitus. The factors are thus real things. These factors form a 

 definite limited group, a definite quantity etc., all these factors are found 

 in the generative cells, and so the offspring of the plant has thus the 

 same factors, is thus precisely as the mother plant. Thus, the factors are 

 real things. Stimulation, however, is something energetic, something unreal. 

 Now throughout physiology of stimulation is to a great extent dependant 

 on external circumstances. We can miagine that through different external 

 circumstances some factors stimulate more powerfully, and thus tlwse factors 

 will determine the habitus of the plant. So tlic modifications arise. Yet 

 these modifications are thus quite temporary are only inherent to definite 



