THE MUTATION THEORY 



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which species or genera are set apart from their allies by 

 the possession of a different number of chromosomes. 

 Such variations in chromosome number have been found 

 in many cases among the Kosaceas, a family noteworthy 

 for the complications which it presents to the systema- 

 tise Certain Japanese species of Viola exhibit variations 

 in chromosome number which give a clue to the way in 

 which the numerous forms of this complex genus have 

 evolved. Similar variations occur among the Orchid- 

 aceae, one of the largest families of flowering plants. 



There remain to be considered a large number of muta- 

 tions in which the chromosome complement has not been 

 shown to differ from that of the parent form. Such mu- 

 tations are frequent in Oenothera. De Vries has ob- 

 served them in the case of (E. Lamarckiana and more 

 recently he and other workers have observed them in 

 other species, belonging to the small-flowered, self-polli- 

 nating portion of the genus. In connection with these 

 mutations it will be necessary to consider more in detail 

 the criticisms brought against the mutation theory by 

 Bateson, Davis, Heribert-Nilsson and others. 



In the past, most objections to the mutation theory 

 have been based upon the supposition that Oenothera 

 Lcunarckiana is a hybrid of garden origin. I am forced 

 to admit that I am not satisfied with any evidence thus far 

 offered that this species, in the form familiar in cultiva- 

 tion, is or ever was a wild constituent of our flora. Never- 

 theless I venture to predict that it will eventually come 

 to light in some obscure locality and that its character as 

 a natural species will be established. Whether it is a 

 natural species or a product of floriculture is of relatively 

 little importance, however, in view of the fact that none 

 of the mutation phenomena are peculiar to it. Several 

 other species are known which are equally mutable and 

 which are now elements of our flora. Moreover, they are 



