Now it seems to us probable that what we call the unit-character for hooded pattern is itself variable; also that “modifiers” exist—that is, the extent of the hooded pattern is not controlled exclusively by a single localized portion of the germ-cell; otherwise we should be at a loss for an explanation of the peculiar results from crossing plus series hooded rats with those which are still more extensively pigmented; for by such crosses the pigmentation is rendered not _more_ extensive but _less_ so. This result we can explain on the supposition that the selected plus series has accumulated _more_ modifiers of the hooded pattern than the wild race contains, so that a cross tends to reduce the number of modifiers in the extracted hooded individuals. No other explanation at present offers itself for this wholly unexpected but indubitable result. If a different one can be found we are quite ready to discard the hypothetical modifiers as a needless complication, contenting ourselves with the supposition that the unit character for hooded pattern is itself variable, and that for this reason racial change in either plus or minus directions may be secured at will through repeated selection. We have been led to adopt tentatively an hypothesis that modifying factors exist independent of the single factor for hooded pattern (though both the factor for hooded pattern and its modifiers may, so far as we can see, be quantitatively variable) by another series of observations, which will now be described. THE “MUTANT” SERIES. In the tenth generation of the plus selection series there appeared two individuals, a male and a female, of considerably higher grade than any previously recorded in this series. They are not included in Table 10 because we have been and still are in doubt as to their exact nature and think it best to give a separate account of them. If entered in Table 10 one would appear as a 5½ individual born of 3⅞ parents (mean grade), the other as a 5¾ individual born of 3¾ parents (mean grade). The nearest individuals in grade to these two produced by the same group of parents are of grade 4½, but some 4⅛ parents of the same generation produced two offspring of grade 5. (See Table 10.) Because of the marked advance in grade of these individuals beyond the ordinary range of variation in the series we called them “mutants,” without wishing then or now to commit ourselves to any particular theory as to their nature or origin. We have used the term and now use it as one of convenience merely. The two “mutant” individuals had the same father and their mothers were sisters. Their pedigree for two generations is as follows: Mutants. Parents. Grandparents. { ♀ 2956, +3¾ } { ♀ 1939, +3¾ ♂ 4763, +5½ { ♀ 2957, +3½ } { ♂ 1817, +3¾ { ♀ 5153, +5¾ { { ♀ 1162, +3½ { ♂ 2963, +4 { ♂ 1810, +3¾