276 THE FACTORIAL HYPOTHESIS 



can be due to different kinds of factors, all of them 

 "presences." 



A word here may not be out of place concerning 

 inhibitors. As pointed out, the adherents of presence 

 and absence generally interpret the absence of a char- 

 acter to mean the absence of a factor; they also inter- 

 pret recessiveness to mean the absence of a factor. 

 When cases come up in which a character is absent, 

 as horns in cattle, but the absence of the character 

 is* dominant, an attempt is made to reconcile fact and 

 theory by assuming that the factor for the absent 

 character is not really absent, but that an inhibitor 

 is present whose activity prevents the appearance of 

 the character. 



Those who do not accept the presence and ab- 

 sence hypothesis need make no such assumption here 

 of course. To them there is no reason why a factor 

 for hornless should not dominate a factor for horns. 

 Moreover, the facts do not even require one to assume 

 that the hornless race differs from the horned because 

 of the lack or inhibition of certain reactions, for it is 

 possible in such cases that the reaction merely takes 

 a different course, or may even proceed beyond the 

 usual point. 



These statements are not, however, intended to 

 mean that factors may not at times act as inhibitors, 

 but rather that we do not know, and in most cases 

 can not know, in a single case enough about the 

 nature of the reaction to demonstrate the existence 

 of a factorial inhibitor. 



