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 occursaway fromthe 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.
6. Upon crossing the selected plus and minus races with each other, the variability was somewhat increased in F₁ and was further increased in F₂. The extreme conditions (plus or minus) of the grandparents rarely, if ever, recur in this generation. Only one individual among 378 F₂ young has been recorded in a grade as extreme as either grandparent.
7. Hooded animals extracted in F₂ as recessives from a cross with either Irish or wild rats are as a rule more variable than the selected race used in making the cross. In crosses with an Irish race the minus series was affected in like measure. In crosses with wild rats the variability of the plus series was not appreciably affected (in two experiments it was slightly reduced, and in one experiment it was slightly increased). But the variability of the minus race was more than doubled by crosses with wild rats.
8. Themeanof the minus race was lowered by a cross with either the Irish race or with wild rats, but more extensively by the latter. The mean of the plus race was lowered a very little by a cross with wild rats, but considerably by a cross with the Irish race.