378 
HOWARD B. FROST 
find this group characterized by a peculiar prevalence of hybridity. 
Apparently this hybridity is not due to unusual opportunities for 
interpollination of species in their past history; it is rather to be as- 
cribed to an exceptional degree of fertility between decidedly unlike 
forms, perhaps combined with an unusual tendency to produce widely 
divergent forms by mutation. Frequent partial sterility of the 
resulting hybrids, sometimes producing permanently heterozygous 
forms, seems also to be a factor (de Vries, 1913). 
It has been urged that all the Oenothera mutations should be 
charged to hybridization. There is obviously a remarkable preva- 
lence of hybridity in the genus, but we may well consider whether this 
hybridity may not be, in large part, a result rather than a cause of 
mutation. 
However this may be, Belling (191 4) seems to have shown that 
the artificial crossing of two "good species" may lead to the opposition 
(pairing), at the reduction division in the hybrid, of non-homologous 
genes, with the result that some germ-cells are not viable. That is, 
certain germ-cells may receive incompatible materials, or lack essential 
ones. If, then, a genus combines high mutability with wide limits of 
interfertility of forms, the resulting germ-plasms may be very unsym- 
metrical in the structure of their pairs of chromosomes, often producing 
non-viable gametes, and may therefore be apparently "non-Mende- 
lian" in transmission of characters. It seems very significant, in this 
connection, that the various ''species" of Oenothera and of Citrus are 
so generally interfertile. 
Von Tschermak (191 2) has made a specially extensive experimental 
test of the factor-hypothesis of heredity, and concludes that his evi- 
dence is very favorable to this hypothesis. One of the three plants 
which he used is the ten-weeks stock, Matthiola annua or M. incana 
var. annua, which Miss Saunders (191 1) also has found to give typical 
Mendelian results for flower-color and pubescence. Mutations similar 
to those of Oenothera, occurring in such material, acquire a special 
significance. 
In Annual Report 8 of the American Breeders' Association, I 
(Frost, 1 91 2) reported the discovery of a series of such mutations in 
stocks, discussing especiall}^ a few-noded type of apparently regular 
heredity. Six or seven of these types have reproduced themselves in 
progeny- tests, some through several generations, but only the early 
(few-noded) type has been found to be evidently homozygous in any 
