3 
wind, and the presence of large bodies of water are factors that 
modify climate in relation to plant hardiness or over-wintering. 
De Candolle (1904), referring to the various barriers limiting 
the distribution of species, finds not the slightest indication that 
perennial species have become adapted to greater cold and thus 
extended their range northward to any great degree within the 
historical period in spite of the fact that their seeds are in one way 
or another continually being carried north of their present range. 
‘Periods of more than four or five thousand years, or changements of form and 
uration, are needed apparently to produce a modification in a plant which 
will allow it to support a greater degree of cold.’ 
In other words the temperature-requirement limits of species 
are now practically or for the most part fixed. In general this 
is the current belief among botanists, horticulturists, and others 
that have given attention to the subject. 
However, since the rediscovery of Mendel’s laws in 1900, 
and the subsequent development of the experimental study of 
heredity and variation, only two general methods by which 
inherited variations can arise are recognized. One method in- 
volves hybridization through which new combinations of hered- 
itary units or genes take place, resulting in new characters. 
The other method is mutation. New genes arise only through 
some form of mutation and, since these form the bases of all 
characters, they are the material which natural selection sorts. 
As the ability to stand cold is an inherited characteristic its basis 
is dependent on the presence or absence of certain genes, though 
very little is known about genes of this character or of the 
number of genes involved. The distribution of herbaceous 
perennials and woody plants, then, as far as temperature is con- 
cerned, is determined by their gene make-up. On this hypothe- 
sis, the only method (after selection has ended its sorting) by 
which such plants could extend their northern range limits— 
their ability to stand lower degrees of temperature—is through 
mutation, which means in most cases gene mutation. Do walnuts 
from Minnesota differ then from those in Texas and Alabama by 
a gene (or a series of genes) that determines a walnut’s ability 
to stand different winter temperatures and in no other way 
expresses itself, at least so far as the systematist is concerned? 
