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.
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. The _mean_ of 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.