512 Umversity of California Publications in Botany [Vol.5 



there is evidently in Tahacuni, to those who accept current interpre- 

 tations of heredity, a series of allelomorphic contrasts, the number of 

 which cannot even be guessed, but which need not perhaps be more nu- 

 merous or striking than those which have been discovered in Drosophila. 

 But whereas in Drosophila the factors have been kept in stocks in- 

 volving for the most part single factor differences from a common wild 

 type, in Tahacum, and in other cultivated crop plants such as barley, 

 maize, oats, rice, wheat, etc., these factor differences have been shuffled 

 about through long periods of cultivation until existing varieties are 

 no longer related clearly to a common form or to each other. In some 

 instances in such groups certain factor differences have a more strik- 

 ing visible effect than in others. In such instances we have an obvious 

 mode of classification based not upon number of factor differences so 

 much as upon the striking character differences which arise from 

 certain factor contrasts. Thus in barley we have the classification of 

 varieties advocated by Harlan (1918) based upon recognition of a 

 number of major morphological distinctions, some of which at least 

 have been clearly analyzed in Mendelian fashion ; and the same prin- 

 ciple has been recognized in the classification of varieties of maize, 

 where it has led to the absurdity of erection of a heterozygous form, 

 podded maize (vide Collins), as one of the primary group distinctions. 

 In some instances, doubtless, the sorting of factors may give rise to 

 certain recombinations which are more favorable to life processes than 

 others, as Muller has pointed out in another connection, and such 

 genotypes may act as centers around which groups of varieties may 

 be built up, thus giving rise to more or less obvious grouping of varie- 

 ties. The attempt to base a system of classification upon reference to 

 certain fundamental types does not, however, promise much simpli- 

 fication of the difficulty ; moreover, such an attempt rests upon the 

 rather naive assumption that it is unnecessary to account for the 

 fundamental types. 



From a genetic standpoint, therefore, it w^ould appear that in 

 attacking the problem of classification and interrelations of varieties 

 in a polymorphic species the major premise should be a recognition 

 of the fundamental equivalence of every homozj^gous genotype. Start- 

 ing from this premise a system of dichotomy beginning with those 

 factor contrasts which produce the most striking, visible effects and 

 proceeding to those of lesser effect might be set in operation. Such a 

 system obviously would in certain cases separate some similar varie- 

 ties into separate groups, and would lead to recognition of group 

 differences without obvious morphological distinctions, but the system 



