ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



451 



mutation produces all new and heritable variations, the persistence of 

 these depends on selection. 



Darwin assigned to the selection of fluctuations (quantitative, plus 

 or minus variations) a major part in evolution ; De Vries assigned to it 

 a minor part ; and Johannsen no part at all. As regards sports or dis- 

 continuous variations, Darwin assigned to their selection a minor part in 

 evolution (chiefly among cultivated plants and domestic animals) ; De 

 Vries ascribed to a particular kind of sports (his " mutations ") a major 

 part in evolution ; and Johannsen ascribes an exclusive part to a type 

 of variation wliich would include both Darwin's sports and De Vries' 

 mutations. Johannsen classifies variations into those which are inherited 

 (genotypic) and those which are not inherited (phenotypic), but they 

 cannot be securely distinguished except by experiment. 



[The recorder would put in a plea for the usage which distinguishes 

 exogenous somatic modijications from endogenous germinal variations, 

 the latter then admitting of further classification.] 



The discovery of Mendelian unit-characters introduces a new un- 

 certainty. Are these fluctuations or sports ? Do they arise solely by 

 mutation, or also by the cumulation of fluctuations ? De Vries showed 

 that certain mutations were associated with chromosome changes and 

 involve Mendelian unit-characters. Morgan has shown that in Drosophila 

 a unit-character change almost certainly involves a change in a definitely 

 localized part of a single chromosome. He regards unit-character 

 variations as the only kind of genotypic variations, and these as fluctuat- 

 ing (if at all) only through the interaction of other unit-characters, each 

 one by itself being incapable of fluctuation. 



According to Darwin. 



1. New types are for the most part 



created gradually. 



2. New types are for the most part 



plastic. 

 ?>. One evolutionary change fol- 

 lows upon and is made pos- 

 sible by another. 



4. Natural selection determines 



what classes of variations 

 shall survive, and, in con- 

 sequence, what shall be the 

 variable material subjected to 

 selection in the next genera- 

 tion. 



5. The further evolution of our 



domestic animals and culti- 

 vated plants (and of man 

 himself) is to some extent 

 controllable, because we can 

 by selection influence the va- 

 riability of later generations. 



According to De Vries. 



1. New types are created only 



abruptly. 



2. New types are fully stable. 



3. One evolutionary change has 



no necessary relation to an- 

 other. 



4. Natural selection determines 



only what classes of variations 

 shall survive, and exercises no 

 influence on the subsequent 

 variability of the race. 



5. Evolution is beyond our con- 

 trol, except as we discover 

 . and isolate variations. 



2 H 2 



