254 
EDWIN BLAKE PAYS ON 
since no representatives have been reported from the Rocky Moun- 
tains. Of these fifteen, four are peculiar to Idaho and seven are found 
farther west in Nevada, CaHfornia and Oregon. Sixteen of the twenty- 
two are not found as far north as the southern boundary of Washington 
and perhaps none extend farther south than the southern boundary 
of Colorado and Utah. It can thus be seen that the great multipli- 
cation of species occurs between the thirty-seventh and forty-sixth 
parallels of north latitude and west of the main chain of the Rocky 
Mountains. This region is characterized by many more or less isolated 
mountain ranges, ranges often separated by many miles of low terri- 
tory. Since climatic and edaphic conditions are in general very similar 
in most alpine regions and the specific distinctions are not of such a 
nature as to be considered adaptive, we can scarcely believe that the 
differentiation of species has come about by selection. There is, 
however, a close relation between isolation and the multiplication of 
species and this factor must be considered seriously in any attempt 
to account for the evolution of this group. 
Characters that do not vary with ecological conditions and 
remain constant in a given species are often quite different in 
different families and even genera. Of paramount importance to 
the systematist in these Drabas is the character of the pu- 
bescence. Classification must always be based primarily upon this 
character. The uniformity of the pubescence within a species is 
most interesting and the use of a lens or even a compound micro- 
scope is imperative. Some species never bear truly stellate hairs, 
as for example, D. alpina and D. fladnizensis . One very inter- 
esting species bears no other hairs on the leaves except strong marginal 
cilia. In some species no cilia are present and the leaf surface is 
covered with sessile stellate hairs as in Lesquerella. D. cruciata 
and D. aster ophora are of especial interest because of the prevalence 
of cruciform hairs. Nearly every species might be determined by 
the character of the pubescence alone. Other characters of consider- 
able importance are the presence or absence of pubescence on the pods, 
the length of the style, the shape of the pod, the shape of the leaf and 
the form of the fruiting inflorescence. 
The material cited under the specific names may be found in the 
Rocky Mountain Herbarium at the University of Wyoming and in 
the herbarium of the Missouri Botanical Garden at St. Louis. To the 
curators of these institutions the author wishes to express his gratitude. 
