746 THE AMERICAN NATURALIST. [Vol. XXXVII. 



a possible species and the only selection which might be said to 

 act is that which determines the type able to live : this selec- 

 tion has nothing to do with the origin of the surviving form 

 however. 



Thus of the sixteen mutants discovered by DeVries one had 

 already established itself when found, although seen to arise anew 

 from the parent type subsequently. Perhaps one or two of the 

 others might have succeeded in gaining a foothold, but the major- 

 ity of the new forms must have inevitably perished if subjected 

 to the ordinary competition of the prevailing meadow species. 



As to the cause of mutation, and the mechanism of the proc- 

 ess but little except of a speculative nature may be offered. 

 Korschinsky assumes that heredity and variability are opposing 

 forces or tendencies which are ordinarily balanced. External 

 agencies such as successive seasons of good nutrition might 

 allow the tendency to variation to overcome the hereditary 

 stabiUty and allow the origination of a new form as a result 

 of the unloosed, superfluous unbalanced energy. He supposes 

 that whatever the agencies may be that cooperate to bring 

 about the mutative condition, these forces act upon the develop- 

 ing embryo in the seed, although he hazards no guess at the 

 manner in which this might be accomplished {Flora, 89 : 240, 

 1 901). The above it may be noted is in direct contrast with 

 the proposal of Darwin that the development of new types is 

 more rapid when species are competing under adverse condi- 

 tions, or when the struggle for existence is fiercest. 



So far as DeVries's theory of mutation is concerned it may be 

 said to be the logical outcome of, and to rest upon his hypothesis 

 of intracellular pangenesis. By this, protoplasm is taken to con- 

 sist of ideally minute pangens, which make up the living sub- 

 stance. The pangens and aggregations of pangens are the 

 bearers of the elementary characters of the species. Altera- 

 tions in the numerical relations of pangens are made to account 

 for fluctuating variability. The inactivity of pangens and groups 

 of these units would cause degressive or retrogressive mutation. 

 The formation of new characters in progi-essive mutations would 

 depend upon the development of new pangens, this process con- 

 stituting premutation. The formation of identical pangens in 

 separate species would account for parallel mutations. 



