332 , BOTANICAL GAZETTE [NOVEMBER 



" Presence and absence" came into use in the first place, simply as 

 a convenient method of expression to avoid the confusion which 

 arises when the same dominant character is described as an alterna- 

 tive of several different characters which are hypos ta tic to it, and 

 which may themselves be present or absent in any particular 

 instance. The very general applicability of this mode of expres- 

 sion naturally suggested to various writers (HURST 18, SHULL 27, 

 etc.) that it might have a more fundamental significance than merely 

 as a convenient form of description. These authors considered it 

 simpler and more practical to suppose that the heterozygous genes 

 are unpaired, and that the "absence" of a character 2 is unrepresented 

 by any internal unit corresponding with the gene which determines the 

 "presence" of that character. The "presence and absence " hypothe- 

 sis need not be associated, however, with the conception of unpaired 

 determiners in the heterozygote, for in any pair of organs there may 

 be present a function or feature in one member of the pair which is 

 absent in the other member, or both members may be alike in kind 

 but different in quantity or activity, the differential between the 

 two being in this case the determiner of the alternative characters 

 involved. This excess in one member of the pair would be present, 

 of course, in that member only, and must be absent in its mate. 

 Whether the hypothesis of unpaired genes or that of paired 

 genes represents the true condition in any particular instance, 

 and whether the absence of a character is absolute or only rela- 

 tive, will not interfere in the least with the use of " presence 

 and absence" as the most convenient method of stating a great 

 majority of the alternative characters with which the student of 

 heredity has to deal. For the application of these different phases 

 of the " presence and absence" hypothesis to the sex problem in 

 Lychnis, attention is directed to the following table: 



2 It is to be regretted that some writers have misconstrued the meaning attached 

 by most geneticists to the expression "absence of a character." The absence of the 

 Angora character in cats, rabbits, etc., does not result in a hairless animal, but one 

 with short hair. In Oenothera the absence of rubricalyx pigmentation does not prevent 

 the production of anthocyan in the amount and localization characteristic of O. 

 rubrinervis (see GATES, R. R., Studies on the variability and heritability of pigmenta- 

 tion in Oenothera. Zeitsch. Ind. Abst. Vererb. 4:337-372. 1911). 



