PRESroENx's ADDRESS. — SECTION D. 265 



men will find that they cannot treat it lightly. It will have become a 

 question in Applied Science, and that is a field in which the layman 

 walks with the feeling of a proprietor, often unappreciatingly and 

 with little thought to bestow on the workers in pure science that have 

 placed it at his disposal. 



I propose to bring before you some of the most striking of the 

 recent experimental evidence as to direct modification of the germ 

 plasm by that portion of its environment that lies outside the body in 

 which it is lodged ; and then to refer to some of the evidence, less 

 fitriking perhaps, but incontrovertible, as to modification of the germ 

 plasm by the portion of its environment constituted by the soma, the 

 body in which it is lodged. 



That the germ plasm itself may be influenced by environment that 

 does not permanently afiect the soma in which it is lodged was shown 

 in the now well-known experiments of Tower (" Evolution of the 

 Chrysomelid Beetles of the Genus Leptinotarsa," Carnegie Institution, 

 1906). En viro mental stimuli applied to Leptinotarsa while the germ 

 cells were maturing led to pronounced saltatory action. The young 

 produced from the germinal material so acted upon comprised a very 

 large proportion of mutants, animals with distinctive somatic modifi- 

 cations that they were able to transmit in a normal environment. The 

 environment that modified the maturing germ plasm, however, did not 

 modify the parent beetles themselves, nor did it modify the non- 

 maturing germ plasm. Later broods from these parents, when the 

 parents were returned to a normal environment, presented no special 

 features. These series of experiments are probably the most 

 important that have been made in this field in the light they throw on 

 the heritability of variations acquired in a known fashion. They 

 are now, however, so well known that there is no need to describe them 

 fully in this place. I should like to emphasize the fact that the germ 

 plasm that was not maturing for the next brood was not affected. I 

 do not question the continuity of the germ plasm ; but those that do 

 question it should pay particular attention to this feature in Tower's 

 results. 



Tower's experiments should be considered with MacDougal's, 

 which seem to me not to have attracted the attention that their import- 

 ance would lead one to expect. 



MicDaugal and his collaborators (MacDougal, Vail and Shull, 

 " Illustrations, Variations, and Relationships of the Oenotheras," 

 Carnegie lastitution, 1907, and other papers) were investigating 

 saltatory action in the Evening Primroses, with a view to verifying and 

 extending the observations of De Vries. They experimented with 

 Oenotheras still confined to their native America, as well as with others. 

 It may be mentioned, in passing, as a matter that does not bear directly 

 on the subject of this address, that they were very successful in the 



