+-------------------------------------+-------------------+------+------+
| |Standard deviation | | |
| | of races crossed, | S. D.| S. D.|
| | same generation. | F₁ | F₂ |
+-------------------------------------+---------+---------+------+------+
|Plus-minus cross, series 1 (Table 50)| 0.49 | 0.50 | 0.71 | 1.01 |
|Plus-minus cross, series 2 (Table 50)| .36 | .24 | .60 | .87 |
|Mutant-minus cross, lower group | .25 | .24 | .77 | 1.17 |
|Mutant-minus cross, upper group | .19 | .24 | .31 | .44 |
|Mutant-plus cross, lower group | .25 | .36 | .24 | .35 |
|Mutant-plus cross, upper group | .19 | .36 | .23 | .15 |
+-------------------------------------+---------+---------+------+------+
The mutant-plus cross, it will be observed, shows no increase of
variability either in F₁ or in F₂, but crosses involving the minus race
show increase of variability both in F₁ and in F₂. Interpreted on a
Mendelian basis, this means that the mutant and plus races on the one
hand and the minus race on the other hand differ by more than a single
factor. If they differed by only a single factor, then crosses between
them should bring no increase of variability, either in F₁ or in F₂. This
appears to be true as regards the mutant and plus races when crossed with
each other. But if the races crossed differ by more than one factor, and
if, further, neither parent is homozygous as regards the factors in which
they differ, then we may expect an increase in variability both in F₁ and
in F₂. This is exactly what we observe when the minus race is crossed
with either the plus race or its derivative, the mutant race.
If we suppose that the plus race and the minus race differ from each
other by certain “modifiers,” we can not suppose that the plus and the
mutant races differ by these same modifiers. They differ in some other
_single_ respect; perhaps that in which they differ is the _main_ hooded
factor. Are we, then, to suppose that the plus and the minus races _do
not_ differ as regards this same main factor? This can not be stated,
but we see no reason for considering them identical as regards that
factor. It appears that the mutant race arose from the plus race by a
single large plus variation, which seems to have its determiner in some
single component of the germ-cell. But the fact that this change came as
a large quantitative variation does not show that small variations are
impossible in that same cell component. It seems to us quite improbable
that the plus mutation could have arisen in the minus selection
series. We believe that the repeated selection which was practised had
something to do with inducing this change in the plus direction. If one
can increase at will the “modifiers” which make the pigmentation more
extensive, it does not seem strange that after a time a readjustment
should occur within the cell which should incorporate modifiers in that
part of the cell which is responsible for the unit-character behavior of
the hooded pattern. This would amount to a quantitative change in the
unit-character for hooded pigmentation.