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. 
