144 Shull. 



new genotypic elements by out-crossing. If, on the other hand, the 

 observed progress in the intensification or elimination of any unit- 

 character under selection, be due to segregation of plural Mendelian 

 determiners, it should be impossible by a reversal of selection, without 

 out-crossing, to completely reach the one extreme after having attained 

 a definite advance in the direction of the opposite extreme. The degree 

 to which such a reversal may prove effective in any case will certainly 

 be illuminating. 



d) To make it probable that a supposed recessive mutant has re- 

 sulted from the complete lack of homology of all duplicate Mendelian 

 determiners for the dominant parental character in the one parent, with 

 any of the duplicate genes of the same character in the other parent, it 

 is only necessary to self-fertilize a sufficiently large number of individuals 

 of the dominant type among the progeny in which the new form appeared, 

 and so to demonstrate that some of them yield monohybrid ratios, some 

 dihybrid ratios, and so on. If the immediate progeny in which the 

 recessive mutant (?) made its appearance is not available for such a 

 test, it may be necessary or advisable to cross with one another, a large 

 number of individuals belonging to the stock in which the new form 

 made its appearance. If the hypothesis has any considerable degree of 

 probability, some combination should be found by this process, in which 

 the mutant (?) form is repeated. Then this progeny in which the new 

 mutant occurs, should be analyzed by an adequate number of self-fertili- 

 zations among the dominant individuals, as already indicated. The 

 crossing of the new type extensively with individuals of the parent-type, 

 the latter being taken from as many independent sources as possible, 

 - might discover a duplication of Mendelian determiners, but failure to 

 find them by this process would not absolutely disprove the hypothesis, 

 because no two duplicate determiners might happen to occur together 

 in any single individual, in which case all crosses with the recessive 

 type would give monohybrid ratios. 



It is thus seen that the discovery of duplicate determiners, not 

 only because of their own direct implications, but also by calling at- 

 tention to the widespread existence of plural determiners, has tapped a 

 rich mine of new and important genetic problems. The discovery that 

 a given characteristic is probably determined by plural genes should be 

 the beginning and not the end of investigations dealing with this 

 characteristic. The chief value of the hypothesis at present must be 

 determined by the extent to which it is made a working hypothesis. 



