As a control on the results given by the wild crosses, we may examine the results obtained by crossing the plus and minus selected races with a black Irish race. The Irish race used for this purpose consisted of animals black everywhere except on the belly. On the system of grading used in this paper they would range from +4½ to +5¾, +6 being an all-black rat. Crosses of minus-series hooded rats with Irish produced Irish F₁ offspring with rather more white on the belly than the Irish parents possessed. In the F₂ generation hooded individuals reappeared in approximately the expected 25 per cent. In a total of 764 second generation young, 171, or 22.4 per cent, were hooded. The grade of pigmentation of these extracted recessives as compared with that of their hooded grandparents we may now consider, as was done in the case of the wild crosses. (See Table 43.) Six individuals of the minus selection series, of generation 3½, and of mean grade -1.50, were crossed with Irish rats producing Irish offspring which were mated _inter se_. Among the grandchildren appeared the usual proportion of recessives (hooded), 90 in number. The distribution of these as regards grade of pigmentation is shown in the first row of Table 43. Their mean grade is -0.62, that of uncrossed hooded rats of the same grade as the hooded grandparents being -1.31 in generations 3 and 4. In other words, the cross has considerably increased the pigmentation in the hooded grandchildren over what was to be expected had the cross not taken place. Nevertheless the increase in this case is _less_ than in the similar cross with wild rats. (Compare Table 42.) In the second row of Table 43 is shown the grade distribution of extracted hooded grandchildren of two mothers of grade -1.87 and generation 4. The mean of the 53 hooded grandchildren is in this case -0.73, that of uncrossed hooded parents of the same grade and generation being 1.18. This average is probably too low. An examination of the means of adjacent classes (Table 19) indicates that it should be about 1.35. In the third row of Table 43 is shown the grade distribution of the extracted recessive grandchildren of a -2.00 male, minus-series rat, of generation 7½. The 66 grandchildren are of mean grade -0.94, expected -1.75. Comparing the three experiments (first three rows of Table 43), we see that the more advanced grandparents, in grade and generations of selection, have the more advanced grandchildren; but in every case these are less advanced than grandparents of the same sort would have given had they not been crossed. Hence crossing with Irish has clearly had the effect of increasing the pigmentation in the minus series in the same way (but in lesser degree) as did crossing with wild animals.