Table C.
| Standard deviation of races crossed, same generation. | S. D. F₁ | S. D. 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.