1916] '  FORSAITH—ONAGRACEAE 483 
a consideration of the probable origin of species. As has already 
been stated, there are 2 distinct views in reference to this question. 
On the one hand, there is the idea of a gradual and continuous 
development; and on the other hand, the sudden and intermittent 
acquisition or loss of certain characters. These variations are 
said to breed true to type, and to form new and distinct species. 
The first of these conceptions receives very strong support from 
the characteristically gradual development of the woody elements 
in the lignified stem. These organs, as revealed by anatomical 
investigations, show a slow and continuous transition from the 
organization normally present in the lowest of the vascular cryp- 
togams, to the form typical of the higher angiosperms. This 
support of the Darwinian hypothesis is still further strengthened 
by a characteristic lack of contradictory proof in the form of an 
intermittent course of development. Examples in favor of the 
Opposing view represent isolated cases in certain groups of plants, 
of which the genus Oenothera has furnished the largest percentage. 
As has already been stated, there is a fundamental objection to the 
doctrine of mutation, especially in connection with Oenothera. 
This genus is well known to hybridize freely, and consequently 
the lineage of its so-called mutants must invariably be open to 
question. 
This part of the discussion naturally leads into even narrower 
limits, that is, a recognition of hybrids in nature. In this con- 
nection there exists a general morphological proof of hybrid deri- 
vation. This condition is indicated by a distinct although 
variable percentage of abortive pollen grains in an anther. It 
should also be mentioned in this connection that most of the 
so-called mutants show this very evidence of hybrid descent. 
It can readily be seen that the geographical distribution of a 
plant might have a direct influence on hybridization, and such has 
proved to be the case. Epilobium, especially the section Chamae- 
nerion, furnishes an excellent example in this connection. £&. 
angustifolium from those regions where it is essentially monotypic 
(the southern part of its range in the United States) presented 
generally perfect pollen. In contrast to this state, in specimens 
from the northern zone (where it is in contact with the allied 
