MENDELIAN CHARACTERS IN PLANTS AND ANIMALS. 127 
of presence and absence of unit-characters, however, necessitates a 
somewhat different view of the process. 
On this view, a yellow pea is based on green, and produces gametes 
carrying two factors—one for yellowness (Y) and one for greenness (G). 
When self-fertilised, or fertilised with another yellow pea, the corre- 
sponding factors pair and the resulting zygote is yellow based on green 
(YY + GG). When the zygote produces gametes the corresponding 
factors segregate and each gamete has the constitution (Y + G).  ~ 
Similarly a green pea produces gametes carrying two factors—one 
for greenness (G) and one for absence of yellowness (y). When self- 
fertilised, or fertilised with another green pea, the corresponding factors 
pair, and the resulting zygote is green with the constitution (yy + GG). 
When the zygote produces gametes the corresponding factors segregate, 
and each gamete has the constitution of (y + G). 
So much for the process of pure breeding, 
In cross-breeding the process would be as follows:—A green pea, 
producing gametes (y + G), crossed with a yellow pea producing 
gametes (Y + G), would produce a hybrid yellow zygote of the 
constitution (Yy + GG). The hybrid zygote would produce two 
kinds of gametes (Y + G) and (y + G), one representing yellow based 
on green, and the other absence of yellow based on green. The practical 
result is, of course, the same on Mendel’s view and on the hypothesis 
of presence and absence; it is the interpretation of the process that is 
fundamentally different. ° 
THe NATURE OF THE ‘ABSENCE’ Factor. 
Presuming that the presence and absence hypothesis is the correct 
interpretation, the question arises :— 
What is the nature of the presumed gametic factor for absence? 
A factor for presence is concrete and tangible, but a factor for absence is 
not so easily comprehended. 
With regard to the possible nature of the ‘absence’ factor, three distinct 
views present themselves. 
(1) There may be a concrete factor literally representing ‘ absence.’ 
(2) The factor for ‘absence’ may represent simply ‘presence’ in a 
dormant or latent state. 
(3) There may be no factor at all, the presumed factor for ‘absence’ 
being simply nothing. 
The first view is perhaps the simplest in the abstract, yet it is difficult 
to comprehend, and also to understand how such a negative factor could 
have originated. 
The second view is, perhaps, the most plausible, but it is open to the 
Serious objection that it implies that ‘absence’ is not real but only 
apparent ; there is also the further objection that many cases are now 
known where the ‘ presence’ factor itself exists in a dormant or latent 
state. 
The third view is, perhaps, the most practical, inasmuch as ‘ absence ’ 
is not represented by anything, but this implies a non-pairing of factors 
in cross-breeding, and the question arises as to how segregation takes 
