222 EXPERIMENTAL EVOLUTION LECT. 



brought forward by Detmer, is that of the shoots of 

 Thiija occidentalis. These shoots contain in their 

 upper side green palisade cells, while the under sides 

 possess green spheroidal cells. And when the branches 

 are turned upside down, and are fixed in this position, 

 the anatomical structure of the shoots which put in 

 their appearance later is reversed ; the side which was 

 destined to become the upper side, and which a change 

 in the branch's position has made the lower side, 

 assumes the structure of the lower side, and vice versa. 

 A similar case occurs with the climbing shoots of the 

 ivy, and in Tropaeolum leaves important structural 

 changes have been noticed by Detmer in response to 

 differences in external influences, or change of envi- 

 ronment. Now Weismann says, " Such differences [in 

 structure] do not by any means afford proof of the 

 direct production of structural changes by means of 

 external influences. How would such an explanation 

 be consistent with the fact that the leaves are, in all 

 these cases, changed in a highly purposeful manner ? 

 Or is it assumed that these organs were so constituted 

 from the beginning that they are compelled to respond 

 to external conditions by the production of useful 

 changes ? Any one who made such an assertion now- 

 adays, or who even thought of such a thing as a 

 possibility, would prove that he is entirely ignorant of 

 the facts of organic nature, and that he has no claim 



