472 BOTANICAL GAZETTE [DECEMBER 
favor of the Darwinian rather than the DeVriesian hypothesis 
of evolution. 
In contrast to this slow transformation in plants, there are 
larger and more inconstant variations which are brought about as 
a result of hybridization. These differences, thus formed, may 
follow a strict Mendelian ratio, or become segregates which breed 
true to type. This manner of fluctuation is so well known, how- 
ever, that any further discussion of it is manifestly unnecessary. 
Turning again to the consideration of the probable origin of 
species by mutation, as advanced by those who are in favor of this 
view, it is at least significant that by far the greater majority of the 
so-called species thus produced have descended from ancestry 
whose genetical purity is questionable. The genus Oenothera has 
been mentioned so frequently in this connection that it naturally 
deserves special consideration. This genus has long been known 
to hybridize freely, and also to have produced constant generations 
from crosses as a result of a segregation of characters. To place 
so much weight upon such precarious evidence as a distinction 
between segregates and so-called mutants, therefore, appears from 
the morphological standpoint, at least, to be unjustifiable. 
Since the question of mutation so intimately concerns the 
Onagraceae, it seems advisable to consider the probable occurrence 
of hybrids in a few genera other than Oenothera, as indicated by the 
morphological evidence of pollen sterility. In this connection the 
following species will be discussed in respect to the probable rela- 
tion of pollen sterility to hybridism and geographical distribution, 
namely: Epilobium angustifolium L., E. latifolium L., and E. 
Dodonaei Vill. of the subgenus Chamaenerion; Epilobium hirsutum 
L., E. luteum Pursh, and E. obcordatum Gray of the section Lysi- 
machion; Clarkia pulchella Pursh, C. rhomboidea Dougl., and C: 
Xanthina Gray; Oenothera serrulata Nutt.; Gongylocarpus fruti- 
culosus Benth. and G. rubicaulis Cham. and Schl.; Diplandra 
lepezioides Hook. and Arn.; and Zauschneria californica Presl. 
The material for investigation was selected from herbarium 
specimens, representing widely separated stations throughout the 
greater part of the ranges of these species. Mature buds were 
chosen and treated with alcohol and NaOH, which caused them to 
