6 THE COEFFICIENT OF MUTATION IN OENOTHERA BIENNIS L. 



bility as it has been observed, up to this time, in 0. biennis, which 

 is, next to 0. Lamarckiana, the most suitable species for this kind 

 of research. The mutations in the other forms seem to be far more 

 rare, and therefore require many more thousands of individuals 

 for a statistical study or for experiments upon their causes. 



Besides the assumption that 0. Lamarckiana might be a hybrid, 

 some authors have recently pointed out that hybridism may be one 

 of the chief ways in which species are produced in nature, especially 

 in the larger or so called polymorphous genera. Linnaeus was 

 the first to propose this hypothesis, at the time when the number of 

 discovered forms was growing so fast as to make it almost impossible 

 to assume a separate creation for every one of them. I have not 

 the least doubt that Linnaeus and his followers were right in this 

 point, and that many wild species have been produced by the 

 sexual combination of the characters of their allies. How great 

 a r61e this kind of hybridization or of the recombination of charac- 

 ters 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. 1 ) 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 duplicated 

 by means of artificial crosses, 2 ) and quite a number of later in- 

 stances have been added to this list, the latest of them being the 

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

 0. atrovirens Bartlett (0. cruciata of my Gruppenweise Artbildung), 

 by means of the expulsion of the undesirable characters in double 

 reciprocal crosses. 3 ) 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. 



i) 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 Helveticae, pp. 743-760, 1857; and for the willows 

 see Max Wichura, Die Bastardbefruchtung im Prlanzenreich, erlautert an 

 den Bastarden der VVeiden. Breslau, pp. 95, mit zwei Tafeln, 1865. 



2) Focke, W., Die Pfianzenmischlinge, 465—408, 1881. 



3) Gruppenweise Artbildung, Berlin, 311 — 312, 1913. 



