When animals of the plus selection series are crossed with animals of the minus selection series, an F₁ generation of offspring is obtained which varies about a mean intermediate between those of the respective uncrossed races. Thus, from an examination of Table 50 it will be seen that when -2 animals of generation 6, minus series, were crossed with +3.50 or +3.75 animals of generation 5, plus series, an F₁ generation (Series 1) was obtained consisting of 93 animals of mean grade +0.06. This generation is rather more variable than either uncrossed race, its standard deviation being 0.71. The same is true of a second set (Series 2) of crosses made between a male of grade -3.25, generation 10, minus series, and females of grade +3.75, generation 10, plus series. The 14 F₁ offspring are of mean grade +1 and have a standard deviation of 0.60. (See Table 50, Series 2.) In both the series of crosses summarized in Table 50, the F₂ generation is more variable than F₁. In Series 1, 305 F₂ animals are recorded, having a standard deviation of 1.01 as compared with 0.71, the standard deviation of the F₁ generation. In Series 2, the F₂ offspring number 73 and have a standard deviation of 0.87, that of the previous generation being 0.60. The mean of the F₂ generation is very similar to that of the F₁ generation. In Series 1, the mean of F₁ is 0.06, and that of F₂ is 0.24; in Series 2, the mean of F₁ is 1.00, and that of F₂ is 0.72. It may also be seen from an examination of Table 50 that among the F₁ offspring produced by crossing the plus and the minus series there are differences in transmission, as there are in the expression of the hooded pattern. In general those F₁ individuals which are of high grade produce offspring of higher grade than do their low-grade brothers and sisters. This is exactly what has been observed in both uncrossed races. SUMMARY OF RESULTS. The experiments which have been described in the foregoing pages have shown that: 1. The hooded pattern of rats behaves as a simple Mendelian character in crosses with either the Irish pattern or the wholly pigmented condition of wild rats. 2. Though behaving as a unit, the hooded pattern fluctuates—that is, it is subject to plus and minus variations. 3. Selection, plus or minus, changes the position of the mean and mode about which variation occurs. 4. The results of such plus or minus selections are permanent, for return selection is not more effective than the original selection, and during return selection regression occurs _away from_ the original mode, that is, toward the mode established by selection. 5. During the progress of the original selection (thirteen successive generations) variability as measured by the standard deviation was somewhat diminished.