That the variability of the mutants is unaffected by a cross with the plus race, but that it is increased by a cross with the minus race, and that, further, the mean of the mutants is affected little or none by a cross with the plus race, but that it is lowered by a cross with the minus race—these several facts are all conformable with the hypothesis that _the change in variability due either to crossing or to selection results from modifying factors_ which, as they are independent of the main factor concerned, are probably transmitted in a different part or component of the germ-cell than that factor. For if the mutant and the plus race are alike as regards the modifiers, but differ only in the main factor, then no change in variability should result from intercrossing them, but only alternative conditions as regards the main factor. This is the observed result. But if the mutant and the minus race differ not only in the main factor, but also in modifiers which are independent of it, then, when they are crossed, we may expect that through independent segregation of main factor and modifiers the extracted minus race will be raised in grade, while the extracted mutants are lowered, and both will become more variable. This also is the observed result. One objection may be offered to this interpretation, namely, that the increased variability is not delayed until F₂, but is already in evidence to some extent in F₁. The same thing was observable in the crosses of the plus and minus series (Table 50). From that table, Series 1, it will be observed that when the plus and minus races had standard deviations of 0.49 and 0.50, respectively, their F₁ offspring had a standard deviation of 0.71, an increase by nearly one-half; F₂ showed a further increase to 1.01. In series 2, Table 50, the uncrossed races (generation 10) had standard deviations of 0.36 and 0.24; their F₁ offspring had a standard deviation of practically twice this, namely 0.60; F₂ showed a further increase to 0.87. At the time of the mutant-minus race crosses, the minus race (generation 10) had a standard deviation of 0.24, the plus race of 0.36. F₁ (lower group) had a standard deviation of 0.77, and F₂ of 1.17. F₁ mutants (upper group) had a standard deviation of 0.31 which rose in F₂ to 0.44. These various facts will perhaps be better grasped if presented in tabular form: TABLE C.