404 ON THE SUPPOSED BOTANICAL PROOFS OF THE 



plants usually grow more strongly by night than by day. If now 

 plants arc transferred from the open air into constant darkness, the 

 periodicity in their growth does not immediately disappear, and 

 often persists for a long time as a phenomenon of after-effect. 



The opening and closing of the leaves of Mimosa pudica also takes 

 place periodically under natural conditions, the leaves closing at 

 dusk as a result of changes in the stimulus provided by the light. 

 In this case also, when the plants are transferred to constant dark- 

 ness, 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 imme- 

 diately arrested when the stimulus is withdrawn, and only become 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 transmission of acquired characters. All these peculiar- 

 ities produced by external influences remain restricted to the indi- 

 vidual 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 pudica by keeping them in dark- 

 ness 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. 



Detiner himself admits this when he says that the after-effects 

 are only witnessed during the life of the individual, but he never- 

 theless 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 similarity between the natures 

 of these two classes of phenomena cannot escape the intelligent 

 observer. 



It seems to me that this question does not demand the attention 



