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THE AMERICAN NATURALIST [Vol. LI 



fact has, perhaps, not been sufficiently emphasized in 

 work involving material which displays such partial ster- 

 ility. Obviously, however, it is impossible to regard 

 ratios obtained from such material as of any significance, 

 unless it be possible to demonstrate definitely the nature 

 of such gametes as fail to function. It is, therefore, not 

 strange that species hybrids as a class require a some- 

 what different sort of treatment from that applicable to 

 intervarietal crosses involving relatively few factor dif- 

 ferences. 



In the extensive work which has been done with the 

 various species and forms of (Enothera this influence of 

 partial sterility has undoubtedly played an important 

 part, but at the same time one which has not been clearly 

 defined. Jeffrey (1914) in particular has sought to es- 

 tablish the hybrid nature of 0. LamarcUana on the basis 

 of the high percentage of abortive pollen grains which are 

 found quite generally in the genus, and Heribert-Nilsson 

 (1912) has attempted to analyze the material from a Men- 

 delian standpoint. These attempts have not, however, 

 led to a consistent explanation of the results observed, 

 although they have established certain peculiar condi- 

 tions in (Enothera which practically preclude the apphca- 

 tion of a rigid Mendelian analysis to such a type of be- 

 havior. On the other hand, a critical examination of the 

 phenomena displayed by the various forms of (Enothera 

 clearly indicates that these belong to several distinct cate- 

 gories, and not to one as is very generally assumed m 

 discussions bearing on this subject. In this brief discus- 

 sion we propose to classify them roughly as follows : 



1. Strict factor mutations arising from unknown causes 

 but clearly referable to specific germinal changes in iso- 

 lated loci in the hereditary system. These display a 

 simple, consistent Mendelian behavior when tested with 

 the forms from which they arise. They usually show 

 relatively simple and definite character differences when 

 compared with the parent forms rather than complex dif- 

 ferences throughout. 



2. Segregation phenomena of a complex type resulting 



