TRANSMISSION OF ACQUIRED CHARACTERS. 403 



The assumption scarcely deserves to be called a scientific hypothesis, 

 and yet it must be made by all who accept the transmission of 

 acquired characters, that is unless they adopt the hypothesis of 

 pangenesis, which is quite as improbable, and which even Darwin 

 did not look upon as a real, but only as a formal explanation. 



Detmer is also greatly mistaken when he says that I refuse to 

 admit the transmission of acquired characters, because I am pre- 

 judiced in favour of my doctrine of the continuity of the germ- 

 plasm. This doctrine is either right or wrong-, and there is no 

 middle course : to this extent I quite admit that I am prejudiced. 

 But the question as to whether acquired characters can be im- 

 pressed upon the germ, and thus transmitted would not be by any 

 means settled in this way; for even if we admit that the germ- 

 plasm is not continuous from one generation to another, but that 

 it must be produced afresh in each individual, this would by no 

 means necessarily imply that it would potentially receive and retain 

 every change produced in every part of the individual, and at any 

 time in its life. It seems to me that the problem of the transmis-' 

 sion or non-transmission of acquired characters remains, whether the 

 theory of the continuity of the germ- plasm be accepted or rejected. 



I will now proceed to examine the last group of phenomena 

 which Detmer brings forward in favour of the transmission of 

 acquired characters. He charges me with not having taken into 

 account, in discussing the problem of heredity, the very important 

 facts which are known about the strange phenomena of 'after- 

 effect ' in plants. Among these ' after-effects ' are the following. 



If vigorous plants of the sun-flower, grown in the open air, be 

 cut off close to the ground and transferred to complete darkness, 

 the examination of a tube fixed to the cut surface of the stem will 

 show that the escape of sap does not take place uniformly, but 

 undergoes periodical fluctuation, being strongest in the afternoon 

 and weakest in the early morning. Now the cause of this daily 

 periodicity in the flow of sap depends upon the periodical changes 

 due to the light to which the plant was exposed when it was 

 growing under normal conditions. When plants which have been 

 grown in darkness from the first are similarly treated, the flow of 

 sap does not exhibit any such periodicity. 



Another instance is as follows : it is well known that darkness 

 accelerates, while light retards the growth of plants, and therefore 



D d 2 



