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 practica]ly 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 
