THE THEORY OF MUTATION 155 



continuous variability which is often so considerable, 

 and of the inheritance of this kind of differences which 

 the biometricians have been at so much pains to prove ? 

 De Vries points out that for no two plants are the con- 

 ditions of life exactly the same ; a considerable degree 

 of diversity among the plants themselves is therefore 

 advantageous, even when these belong to the same 

 specific type. Upon continuous variability depend 

 local races, forms adapted to wetter and drier situa- 

 tions, highland and lowland races, and the like, but 

 none of these are permanent. As regards the cause 

 of this variability, apart from the effect of sexual 

 reproduction, which combines the tendency to vary 

 of two separate parents, de Vries believes that indi- 

 vidual variability depends entirely upon nutrition ; 

 but under this head he includes practically the whole 

 environment of plants light, space, soil, moisture, 

 and the like. Characters acquired in a similar way by 

 previous generations are inherited, and the effect of 

 conditions upon the developing seed whilst still borne 

 upon the parent plant may be considerable. Thus 

 easily does de Vries dispose of the puzzling question 

 of the inheritance or non-inheritance of acquired 

 characters. Acquired characters are inherited; they 

 are not of any importance in the origin of species. 



With regard to the causes of mutations, little is 

 known. Still, it is no longer incumbent upon us, as it 

 was a few years ago, to admit that we know nothing at 

 all about the means by which this form of variation 

 can be produced. W. L. Tower, in his ' Evolution in 

 Chrysomelid Beetles,' has shown that variations due 



