VII.] TRANSMISSION OF ACQUIRED CHARACTERS. 417 



to constant darkness, the periodicity in the movements of the 

 leaves continues for several days. 



All this is certainly very interesting, and it proves that 

 periodical stimuli produce periodical processes in the plant, 

 which are not immediately arrested when the stimulus is 

 withdrawn, and only become uniform gradually and after the 

 lapse of a considerable time. But I certainly claim the right to 

 ask what connexion there is between these facts and the trans- 

 mission of acquired characters ? All these peculiarities pro- 

 duced by external influences remain restricted to the individual 

 in which they arose ; most of them disappear comparatively 

 soon, and long before the death of the individual. No example 

 of the transmission of such a peculiarity is known. Although 

 successive generations of sunflowers have been exposed for 

 thousands of years to the daily alternation of light and darkness, 

 the periodicity in the flow of sap has not become hereditary, 

 and does not take place at all in plants which have always been 

 kept in darkness. Detmer specially tells us that we can even 

 reverse the periods of opening and closing the leaves in Mimosa 

 piidica hy keeping them in darkness during the day, but exposed 

 to light at night ; an experiment which was performed by 

 Pfeffer. Here again we see the proof that influences which 

 have acted upon countless generations have left no impression 

 whatever upon the germ-plasm. 



Detmer himself admits this when he says that the after- 

 effects are only witnessed during the life of the individual, but 

 he nevertheless adds that he has been for many years convinced 

 that the phenomena of heredity and after-effect differ in degree 

 and not in kind. He even goes so far as to assert that, in spite 

 of the obvious non-transmission of after-effect, the similarit}^ 

 between the natures of these two classes of phenom na cannot 

 escape the intelligent observer. 



It seems to me that this question does not demand the 

 attention of the observer (for the observations have already been 

 made) so much as that of the thinker. It is not a correct train 

 of reasoning to conclude that after-effect and heredity are 

 identical in nature, from the fact that certain periodical in- 

 fluences, acting upon a single individual, set up periodical 

 physiological processes which continue for a time after the 

 influences have ceased to act. We might almost as well argue 



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