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 Ust 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 0. biennis leptomeres out of 0. biennis L. and 

 O. airovirens Bartlett (0. cruciate of my Gruppenweise Aribildung) , 

 by means of the expulsion of the undesirable characters in double 

 reciprocal crosses.' 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 allpgamous 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 sHghtest 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 hj^othesis of hybridism as a cause of the 

 mutable condition of the evening primroses, different authors have 



' For the hybrids of Cirsium see C. Nageli, Dispositio specierum generis Cirsii tarn 

 genuinarum quam hybridarum, in G. D. J. Koch, Synopsis Florae Germanicae et Helveti- 

 cae, pp. 743-760. 1857; and for the willows see Max Wichura, Die Bastardbefruchtung 

 im Pflanzenreich, eriautert an den Bastarden der Weiden. Breslau. pp. 95, mit zwei 

 Tafehi. 1865. 



' FoCKE, W., Die Pflanzenmischlinge. 465-468. 1881. 



' Gruppenweise Artbildung. Berlin. 311-312. 1913. 



'"Gates, R. R., Mutation in Oenothera. Amer. Nat. 45:577-606. igii; see 

 PP- 578-579- 



