Inheritance of Quantitative Characters 93 



factors, might go on in the same intermediate condition, unaffected 

 by the cross. The rehition of pattern to non-pattern is merely a 

 simple monohybrid system temporarily superimposed upon the 

 other more complex system without permanently affecting it, any 

 more than any inhibitory factor permanently affects the factors 

 it inhibits, or a dominant permanently affects a recessive. 



It was in this way that the mutationists attempted to explain 

 away Castle's results. Castle did not at first admit the ade- 

 quacy of this explanation, but continued to maintain that he had 

 modified a unit character by selection, and some geneticists agreed 

 with him. 



This question might be raised. 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 

 be modified in certain reactions'? Why not admit that Mendelian 

 factors may be modified, and explain Castle's results in this way ? 

 The reason is that when we begin to admit that unit characters 

 and single MendeUan factors may be modified, the whole con- 

 ception of inheritance becomes chaos. The great advantage of 

 the factor hypothesis is that it furnishes the clearest method of 

 describing breeding results. East (4) makes an eloquent state- 

 ment on this point. 



*' Taking into consideration all the facts, no one can well deny 

 that they are well described by terminology which requires hypo- 

 thetical segregating units, as represented by the term 'factor.' 

 What then is the object of having the units vary at will ? There 

 is then no value to the unit, the unit itself being only an assumption. 

 It is the expressed character that is seen to vary; and if one can 

 describe these facts by the use of hypothetical units, theoretically 

 fixed, but influenced by the environment and by other units, sim- 

 plicity of description is gained. If, however, one creates a hypo- 

 thetical unit by which to describe phenomena, and this unit varies, 

 he really has no basis for description." 



The question was finally settled in a very neat way by some 

 critical breeding experiments that Castle himself performed on 

 these same hooded rats (2). The degree of pigmentation on the 

 high pigmentation strain was designatetl as +3.73 (in terms of 



