136 Shull. 



ization and cell-dmsion; (3) If tlie distribution of the chromosomes is 

 the determining cause of the distribution of the unit-characters, it is 

 iuiniaterial for most of the l^nown phenomena of heredity, whether these 

 genes are whole chromosomes parts of chromosomes, or physical or 

 energic differences in the chromosomes or their parts. With the under- 

 standing that the chromosomes simply represent a type of behav/ur 

 which Mendelian genes probably also exhibit, they may be substituted 

 with some degxee of reserve for the Mendelian determiners, in picturing 

 to ourselves the probable relations and movements of such determiners. 



Without abandoning any of these propositions, except for an im- 

 mediate didactic purpose, I shall frankly assume in the following dis- 

 cussion of the genotypic and evolutionary significance of duplicate 

 determiners, that the genes are definitely associated with the chromo- 

 somes. 



If any Mendelian character be inherited as a single unit, its 

 heredity may be explained by assuming that the gene Ä for that partic- 

 ular character occurs in only one pair of (homologous) chromosomes 

 (1,1, Fig. 6) in the one parent (the positive homozygote), and that it 

 is absent from all the chromosomes of the other parent (the negative 

 homozygote). In order that another character may be independently 

 inherited in the same manner, its determiner, B, must occur in a differ- 

 ent pair of chromosomes (2,2) either of the same parent that contained 

 A or in the other parent, but must be absent from all other chromo- 

 somes of the two chosen parents. In the same manner, as many in- 

 dependently inherited determiners, A, B, C, D, E, etc., can be accounted 

 for as there are haploid chromosomes. All the phenomena of duplicate 

 determiners will be fully accounted for, then, by assuming that B =^ A, 

 that C=5=.4, that D = C=B=A, etc., the total number of independently 

 inheiitalile duplicate determiners being likewise limited by the number 

 of haploid ciiromosomes. 



When we try to picture to ourselves how duplicate determiners 

 have originated, several possibilities at once present themselves. In 

 the first place such duplication of determiners may be either a i)rinii- 

 tive or a derivative condition. Emerson (1911) has suggested that 

 many fundanicntal rliaracteristics of any biotype may be "represented" ') 



') It will be understood, of course, that this convenient mode of expression is 

 purely figurative. The true relation between the genes and the characters toward whose 

 devebjpmcnt they make essential contributions, has been indicated with sufficient ac- 

 curacy elsewhere in this paper that no misconception will arise from the adoption here 

 of this more figurative phraseology. 



