Modification of Unit Characters iii 



inhibiting factor is simple, the resulting ratio may be a 

 monohybrid ratio; that is, in the F^ generation from 

 such a cross the ratio of pattern to non-pattern will be 

 1:3, and this is exactly what Castle got and what would 

 be expected. At the same time the amount of pigmenta- 

 tion, determined by numerous cumulative factors, goes 

 on in the same intermediate condition, unaffected by the 

 cross. The relation of pattern to non-pattern is merely 

 a simple monohybrid system temporarily superimposed 

 upon the other more complex system without perma- 

 nently affecting it, any more than any inhibitory factor 

 permanently affects the factors it inhibits, or a dominant 

 permanently affects a recessive. 



It is in this way that MendeUans can explain away 

 Castle's results. Castle does not admit the justice 

 of the explanation, but continues to maintain that he has 

 modified a unit character by selection, and some geneti- 

 cists agree with him. Whether Castle is deaUng with 

 cumulative factors or not can never be settled by exact 

 demonstration, since the progeny of rats is too small. 

 It is rather significant, however, that in plants, where 

 larger progenies are involved, the cumulative factor 

 hypothesis is well established, and no one claims to have 

 modified a Mendelian unit in plants. The best that can I 

 be said concerning Castle's claim is that it cannot bel 

 proved false, but that it is possible to explain his results' 

 by the cumulative factor hypothesis. 



The question might be raised, however, why cling 

 so strongly to the cumulative factor hypothesis and 

 force Castle's results into this conception? Is there 

 anything sacred about a unit character that it should 

 not be modified just as complex chemical molecules may 



