THE AMERICAN XATURALIST [Vol. XLIII 



so-called discontinuous variations, play an important role 

 in the origination of new species. 



A considerable proportion of what is described as 

 fluctuating variability is, in many cases, simply somatic 

 variation, having no relation to the germ plasm. It is 

 evident, however, that all fluctuating variability can not 

 be such, otherwise species could not be modified by ordi- 

 nary methods of continued selection. Our mathematical 

 curves represent two kinds of variability lumped together 

 and which it is in most cases practically impossible to 

 separate. The character of height, for instance, in 

 human beings is to a certain extent an inherited one, but 

 it is determined to a marked degree by influences opera- 

 ting after birth. The usual curves of variation represent 

 both and may even include also variations in the nature 

 of mutations which fail of discrimination from the rest 

 of the aggregate. 



De Vries distinguished three kinds of germinal varia- 

 tions, elementary species, retrograde varieties and fluctu- 

 ations. These three kinds he conceives to be sharply 

 distinguished and produced in different ways. All con- 

 genital variability is regarded by him as resting upon 

 qualitative or quantitative changes in the pangens or the 

 organic units of which he conceives living matter to be 

 built up. The pangens form the basis of the unit char- 

 acters, or independently variable elements of the organ- 

 ism, there being a special kind of pangen for each such 

 character. Variations in the number of pangens cause 

 variations of the fluctuating type which obey Quetelet's 

 law of chance frequency distribution. De Vries main- 

 tains and attempts to prove by the citation of several 

 examples that through the selection of such variations 

 modification may be carried to a certain point, but soon 

 a limit is reached beyond which selection is incompetent 

 to effect further improvement. Moreover, continued 

 selection must be practised in order to maintain the con- 

 dition which has been reached, else the stock will in the 

 course of a few generations revert more or less completely 



