1915] DE VRIES—OENOTHERA BIENNIS 175 
acters has played in the production of species in nature is a question 
which it is impossible to answer at the present time. There is no 
doubt that numerous hybrids are continually produced in nature, 
but almost all of them disappear after a relatively short period 
of existence. Even in such genera as Cirsium and Salix, which 
are known to be rich in hybrids, our knowledge concerning the 
propagation of hybrid strains is very small.?' It is quite possible 
that some as yet undiscovered principle of purification (Selbst- 
reinigung der Arten) prevails on a large scale, and if this should 
be so, we must expect hybrid races to be rather rare in the field. 
Focke has published a list of forms which have been dupli- 
cated by means of artificial crosses,* and quite a number of later 
instances have been added to this list, the latest of them being 
the reconstruction of O. biennis leptomeres out of O. biennis L. and 
O. atrovirens Bartlett (O. cruciata of my Gruppenweise Artbildung), 
by means of the expulsion of the undesirable characters in double 
reciprocal crosses.2 But all such facts point rather to a relative 
rarity of hybrid races in nature, outside of the small number of 
well known polymorphic genera. . 
GATES assumes that crosses between species or between ele- 
mentary species often occur in nature among allogamous or open- 
flowered forms." But, according to my own experience, even in 
such cases hybrids are rare in the wild state, and hybrid races 
must be much rarer still. The slightest degree of weakening of 
the individual vigor will doom such hybrids to extermination, even 
as most of the occasional white flower mutations in nature dis- 
appear sooner or later, without starting a permanent variety. 
In order to save the hypothesis of hybridism as a cause of the 
mutable condition of the evening primroses, different authors have 
7 For the hybrids of Cirsium see C. NAGELI, Dispositio ppecierans generis i we 
genuinarum quam hybridarum, in G. D. J. Kocn, Synopsis 
cae, pp. 743-760. 1857; and for the willows see MAx WicuurRA, Die Bastardbefruchtung 
im Pflanzenreich, erliutert an den Bastarden der Weiden. Breslau. pp. 95, mit zwei 
Tafeln. 1865. 
§ Focke, W., Die Pflanzenmischlinge. 465-468. 1881. 
9» Gruppenweise Artbildung. Berlin. 311-312. 1913- 
toGates, R. R., Mutation in Oenothera. Amer. Nat. 45:577-606. IgII; see 
PP. 578-579. , 
