2io POPULAR SCIENCE MONTHLY. 



tuating variability from going any further. The existence of such 

 limits compels us to acknowledge that there is no possibility that spe- 

 cies might arise in nature according to the same plan by which certain 

 breeds originate under artificial selection. 



On this point de Vries and Wallace differ essentially. The latter 

 is convinced that the fluctuating variability is the only source from 

 which new species have gradually originated; de Vries, however, is 

 quite justified in claiming that the examples of the increase and the 

 accumulation of certain variable characters do not prove that a new 

 species or subspecies has ever arisen in that way in its natural environ- 

 ment. 



But in addition to the fluctuating variability we have now to con- 

 sider another variability, regarding which both Darwin and Wallace 

 have collected numerous data, the so-called 'single variations,' which 

 do not follow the Galton curve. They are not connected with their 

 starting-point by very gradual transformations, but are separated from 

 it by a measurable distance which they have overcome, not by degrees, 

 but by starts. They have, therefore, been named 'sports' or 'saults,' 

 the leap being in different cases larger or smaller. 



We have seen that fluctuating variability leads to slow changes and 

 furnishes farmers with the material to improve the races of animals 

 and plants. The 'chance variations' in their turn are valued quite 

 especially by horticulturists and nursery-gardeners. 



The English name 'single variations' expresses very well, indeed, 

 the difference between the two kinds of variability. Fluctuating 

 variability shows us simultaneously all the different degrees between 

 extremes, as represented by the descendants of a single parental pair. 

 The single variations, on the contrary, stand isolated; they are discon- 

 tinuous ; between them and the original parent form we do not observe 

 any gradation. This difference has long been noticed, and on several 

 occasions the difference between these 'single variations' and fluctu- 

 ating' or 'oscillating variations' has been insisted upon. 



De Vries has accepted the name 'mutation' and has submitted the 

 phenomenon to a severe experimental test. The chief result of this 

 has been the conclusion which has at the same time become the basis 

 of his own mutation theory — that by means of fluctuating variability 

 certain local and improved races may indeed be bred, but that in 

 nature new species never arise through its agency. These latter owe 

 their origin exclusively to mutation, to 'discontinuous' variability. 

 He is here entirely opposed to Wallace, who looked upon fluctuating 

 variability as the real source from which species gradually originated. 

 With Darwin, de Vries is less at variance, and a quotation from the 

 'Origin of Species' leaves no doubt that Darwin fully appreciated the 

 value of the single variations for the formation of new species. We 

 read on page 66 of the edition of 1872 : 



