It has been suggested that the original material out of which the plus series came consisted of _modified_ individuals produced by a cross with the wild race. This was not known positively to be so, because part of the original stock (with which MacCurdy worked) consisted of hooded black and hooded gray rats captured in company with gray self and black Irish rats and albinos. Subsequent experiments showed that ordinary albino rats, if crossed with wild gray ones, will produce in F₂ all these classes of individuals. This indicated pretty clearly that the particular colony which had fallen into our hands had probably arisen by the crossing of an escaped albino rat with wild ones. But it still remained uncertain what sort of hooded pattern the escaped albino had transmitted and whether or not this had been influenced by the wild cross. We therefore determined to ascertain whether out of our minus series crossed with wild a plus series could be derived. To this end certain of the F₂ extracted hooded individuals (entered as grandchildren in Table 42, row 1, and descended from a single hooded individual of grade -1.87, generation 2½) were mated _inter se_, thus producing an F₃ generation, Table 44, second row. The selected individuals were the aberrant male of grade +3.50 and females of grade +1.50, so that the mean grade of the chosen parents (extracted from the crossed minus series) was +2.50. They had 34 young ranging in grade from 0 to +3.50, mean +2.06, a regression of 0.44 toward 0, repeating the phenomenon regularly found in both selection series. In this same experiment some F₂ parents of mean grade -0.75 had 19 young (first row of Table 44), whose mean grade was -0.04, a regression of 0.71 toward 0. We should expect the regression of the offspring of such parents to be less than that of the offspring of the -2.50 parents, and so it would be if it were not for one aberrant individual. Larger numbers of offspring would undoubtedly have given the expected result. From among the F₃ offspring were chosen parents for the next generation (F₄). The chosen parents ranged in mean grade from +2.25 to +3.12 (Table 45), average +2.52. They produced 205 young ranging in grade from -0.25 to +3.50, mean +1.86, a regression of 0.66. The parents for the next generation (Table 46) ranged in mean grade from +2.00 to +3.00, the mean being +2.27. They produced 119 offspring of mean grade +2.06, a regression of only 0.21. TABLE B.—_Comparison of the present series with the more general plus selection series._