20 The Journal 
a few who are. Eugenists, for example, 
almost to a man have failed to adopt 
this hypothesis. 
FACTORS ARE NUMEROUS 
They tend to cling to the old Weis- 
mannian idea of one determiner for each 
visible ‘“‘unit character.’’ Morgan and 
most of the advanced students of the 
subject declare flatly that such an idea 
is, in the light of recent research, abso- 
lutely untenable. In place of it they 
have adopted the hypothesis that each 
visible character is due to the coopera- 
tion of an indefinitely large number of 
factors in the germ-plasm; and con- 
versely, that each single one of these 
factors produces an influence on an 
indefinitely large number of traits. 
It is of the utmost importance that 
this hypothesis should be understood, 
for it is the basis of a large part of the 
work in genetics today. Those who 
accept it must give up talking about, 
e. g., Roman nose being due to a deter- 
miner for Roman nose in the germ- 
plasm. The modern view would say 
that the ‘‘Romanness”’ of the nose 1s 
due to the interaction of a very large 
number of factors, each of which, in 
turn, is also probably influencing an 
indefinitely large number of other char- 
acters in the body. 
This is not a mere speculative hypoth- 
esis—the authors look on it as almost a 
demonstrated fact. For example, the 
little pomace fly, Drosophila, which they 
have been breeding in immense numbers, 
has normally a red eye. At an earlier 
day it would have been assumed that it 
the color of the eyes to be red. But in 
the course of the breeding, as many as 
twenty-five distinct mutations in this 
eye color have come to light. It is, 
therefore, their assumption that at 
least twenty-five different factors are 
concerned in the production of this red 
eye coler, and that when a single one of 
these factors changes, a different end 
result is produced, such as pink eyes, or 
vermilion eyes, or white eyes, or eosin 
eyes. 
‘Each such color may be the product 
of twenty-five factors (probably of many 
more) and each set of twenty-five or 
of Heredity 
more differs from the normal in a differ- 
ent factor. It is this one different factor 
that we regard as the ‘unit factor’ for 
this particular effect, but obviously it 
is only one of the twenty-five unit 
factors that are producing the effect. 
However, since it is only this one factor 
and not all twenty-five which causes 
the difference between this particular 
eye color and the normal, we get simple 
Mendelian segregation in respect to this 
difference. In this sense we may say 
that a particular factor (p) is the cause 
of pink, for we use cause here in the 
sense in which science always uses this 
expression, namely, to mean that a par- 
ticular system differs from another 
system only in one special factor. 
EFFECT OF ONE FACTOR 
“The converse relation is also true, 
namely, that a single factor may affect 
more than one character. For example, 
the factor for rudimentary wings in 
Drosophila affects not only the wings, 
but the legs, the number of eggs laid, 
the viability, etc. Indeed, in his defi- 
nition of mutation, De Vries supposed 
that a change in a unit factor involves 
all parts of the body. The germ-cells 
may be thought of as a mixture of many 
chemical substances, some of them more 
closely related to the production of a 
special character, color, for example, 
than are others. If any one of the sub- 
stances undergoes a change, however 
slight, the end product of the activity 
of the germ-cell may be different. All 
sorts of characters might be affected 
by the change, but certain parts might 
be more conspicuously changed than 
are others. It is these more obvious 
effects that we seize upon and call ‘unit 
characters.’ It is the custom of most 
writers to speak of the most affected 
part as a ‘unit character,’ and to disre- 
gard minor or less obvious changes in 
other parts. They frequently speak of a 
unit character as the result of a unit 
factor, forgetting that the unit character 
may be only one effect of the factor.”’ 
It is hardly necessary to insist on the 
far-reaching practical importance of 
this changed viewpoint. Nor is it neces- 
sary to insist on the slight degree to 
which it has so far been apprehended 
