SHORTER ARTICLES AND DISCUSSION 



MODIFYING FACTORS AND MULTIPLE ALLELO- 

 MORPHS IN RELATION TO THE RESULTS 

 OF SELECTION 



In the prevailing controversy as to the ctVcctivt iu'ss df selec- 

 tion, those who reject such effectiveness pnt forth inultiph modi- 

 fying factors as the explanation of the results observed. The 

 given character (for example, the coat color in Castle's hooded 

 rats) is held to depend on one main factor (determining tlif 

 presence or absence of the character) and upon numerous modi- 

 fying factors; the number of the latter present in a given ease 

 determines the degree of expression of the character. Selective 

 breeding is then held to act, as it does with relation to all other 

 Mendelian factors, merely by making diverse combinations of 

 such factors. Many are gathered together in certain individuals, 

 few in others, and the degree of expression of the inherited char- 

 acter variefi accordingly. 



Much evidence is presented that this is actually the mode of 

 ()]it'i';iti()ii ill many cases where selection is effective. Thus are 

 explained the visible results of selection in Castle's rats; thus 

 the iiiiexpeeted fact that many of the mutations of Drosophila 

 have shown themselves (in accordance with Castle's prediction ) 

 to be amenable to selection, although in other respects they be- 

 have like alterations of a single unit factor. This is indeed the 

 usual explanation for that effectiveness of selection which is 

 coming to liglit in so many eases; and in many of the eases it ap- 



h.'reditavy elia I'acteis is eeie-eriird. P.ut if they shniild theni- 



tutiou that are aeeuiuulat.'d by seleetioii. then the answer eeii- 

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