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THE AMERICAN NATURALIST [Vol. XLVIII 



Castle a. id Phillips/' The ffa-e.looms champion the multiple 

 factor hypothesis as an explanation of Castle's results, and also 

 cite certain rather inconclusive experiments of their own to sup- 

 port this point of view. They err, however, in supposing that 

 the factors concerned must he incompletely dominant; as Ave 

 have seen, this is not a necessary assumption, if we admit that 

 in the case of some modifiers the " minus" allelomorph dominates, 

 in others the "plus." They also err in denying the possibility, 



inluv. din- he strictly followed. In fart they offer this as a test 



Finally, papers haw recently appeared by MacDowell/ in 

 which he gives evidence that certain other cases of inheritance 

 (e. g., head size in rabbits), formerly considered by Castle to 



pn.bably hest interpreted on the view of multiple factors instead. 

 His evidence consists in the fact that the characters concerned 

 are somewhat more variable in the offspring of back-crosses than 

 in as we should expect on the basis of recombination of 



explained otherwise. 



Hermann J. Muller 



