A second cross of selected animals of the minus series was made between a wild male and four females of grade -2 derived respectively from generations 5¼, 5½, 6½, and 7. As a group these mothers are more nearly comparable with generation 6, [Table 21], than with any other uncrossed group. As the F₁ progeny of these four mothers by a wild male were mated inter se, it is possible to deal with their hooded grandchildren only as a group. The character of these is indicated in the second row of [Table 42]. They number 48 individuals and have a mean grade of +0.25, showing a modification in a plus direction similar to that observed in the previous case.
A third cross in which the minus series is concerned was made between females of grade -2 and -2.25, generation 10, and wild males. The F₂ offspring include 91 hooded individuals classified as to grade in the third row of [Table 42]. Their mean grade is +0.24, confirming fully the results obtained in the two previous experiments.
With these three cases we may compare three cases in which animals of the plus selection series were crossed with a wild male. (See the last three rows of [Table 42].) Females of grade +3.00, generation 3, were crossed with a wild male. From this mating resulted 21 hooded grandchildren, ranging in grade from +1.75 to +3.50, mean +2.56. These grandchildren, it will be observed, in no case are of minus grade, as are about half the grandchildren when the grandparent is of minus grade. There is also no clear evidence of modification of the hooded character by the cross in this case. The grade of the extracted hooded individual is just about what uncrossed individuals of grade +3.00 produced in the corresponding generation of the plus series.
In the next case two females of the plus series, belonging to generations 5 and 6, respectively, were crossed with a wild male and their children were bred inter se. There resulted 38 hooded grandchildren, as shown in the next to the last row of [Table 42]. The range of the grades of these hooded grandchildren was similar to that of the grandchildren in the foregoing case, but their mean was somewhat higher, as we should expect, since they are descended from more highly selected individuals; for the hooded grandparents in this case were of grade +3.25 (generation 5) and +3.50 (generation 6), whereas the grandparent in the foregoing case was of grade +3.00 (generation 3).
It is a noteworthy fact that in both these cases the wild cross does not seem to have increased the pigmentation in extracted hooded individuals, as it did when the minus series was crossed, but rather to have diminished it; yet the difference between observed and expected is not great. We might disregard it altogether, if a similar but more striking result were not observable in the third case as well as in another series of crosses presently to be described.
The third case (last row of [Table 42]) involves a cross between a female of grade +4.25, generation 10, plus series, and a wild male. The F₂ offspring include 16 hooded individuals of mean grade +3.15. Animals of this grade in the uncrossed race in this generation produced young of mean grade +3.84.
Before leaving this subject it is important to observe the considerable difference between the extracted hooded grandchildren of the minus-series rats, as a group, and those of the plus series. The latter is unmistakably a plus-series group; the former is on the border line between the two series. (Cf. Plates [2] and [3].)
CROSSES WITH BLACK “IRISH” RATS.
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].)