414 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
by the facts and was the explanation of the mechanism generally 
accepted. 
According to the presence and absence hypothesis, however, the 
situation is looked at from an entirely different point of view. Tall- 
ness is the result of a determiner, but dwarfness is merely the result 
of the absence of the determiner for tallness. ‘The dominant character 
is produced by an inheritable determiner, but the recessive character 
appears only when the dominant determiner is lacking. This con- 
ception has some evident advantages and may modify the previous 
Mendelian diagram, as shown in Fig. 76. This appears to be a simpler 
mechanism to account for the phenomenon called dominance. In the 
case of the dwarf form there is a normal course of development; in the 
case of the tall parent or hybrid, however, an additional determiner 
tole OO rs 
=> ue O 
Dwarf Parent Gametes 
Fic. 76.—Diagram showing how the original scheme must be modified to 
satisfy the presence and absence hypothesis. (From Coulter and Coulter.) 
stimulates cell growth, or cell division, or both. It is a simpler and 
more useful conception, so long as it fits the facts. Some investigators, 
however, claim that it cannot be applied to all the situations that have 
been discovered. 
This hypothesis introduces some additional terminology suggested 
by Bateson. In our illustration the tall parent has two determiners 
for tallness and therefore Bateson calls it duplex, having a double dose. 
For the same reason the F, individuals, having only one determiner for 
tallness, he calls simplex. According to the same terminology the 
dwarf parent is nulliplex with respect to its character of tallness. _ 
Additiona] advantages of the presence and absence hypothesis will 
appear in connection with a consideration of blending inheritance and 
of cumulative factors in inheritance. Attention, however, should be 
called to the fact that those who accept the presence and absence 
