4 8 



MUTANTS AND HYBRIDS OF THE OENOTHERAS. 



that the selection of an extreme condition may result in increased 

 variability even after several generations ? 



The supposed effects of self and cross fertilization can hardly be 

 assumed as of any consequence in this connection, for both species 

 have been self-fertilized during a number of generations. Just what 

 effect this has had upon their variability is not known. De Vries 

 does not think that cross-fertilization, even hybridization, has any 

 appreciable influence on the frequency of origin of a given mutation 

 in a mutating species. (De Vries, 1901, p. 211-212; 1903, p. 425-426.) 



85-26 27-28 29-30 31-32 33-34 35-36 37-38 39-40 41-42 43-44 



Fig. 11. Variation in the mean ratio between width and length of leaves in 

 Onagra rubrinervis and O. lamarckiana. Curve for O. rubrinervis shaded 

 with lines rising to the right. Range: O. rubrinervis, 25.80 to 32.54 per cent.; O. 

 lamarckiana, 34.62 to 44.41 per cent. 



Weismann (1892) maintained in his earlier works that all heredi- 

 tary variation is due to cross-fertilization, but more lately he has 

 withdrawn from this extreme position, and now considers amphimixis 

 " nicht als die eigeutliche Wurzel der Variation selbst, deun diese kann 

 unmoglich auf einen blossen Austausch der Ide, sie muss vielmehr 

 auf einer Veranderung der Ide beruhen." He even looks upon cross- 

 fertilization as a process by which the range of variation is lessened, 

 and the variable forms which he thinks may arise at each ' ' Neuanpas- 

 sung," are condensed into a species and rendered constant. (Weis- 

 mann, 1902, 2:235.) 



