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.)
Six individuals of the minus selection series, of generation 3½, and of
mean grade -1.50, were crossed with Irish rats producing Irish offspring
which were mated _inter se_. Among the grandchildren appeared the usual
proportion of recessives (hooded), 90 in number. The distribution of
these as regards grade of pigmentation is shown in the first row of Table
43. Their mean grade is -0.62, that of uncrossed hooded rats of the same
grade as the hooded grandparents being -1.31 in generations 3 and 4. In
other words, the cross has considerably increased the pigmentation in
the hooded grandchildren over what was to be expected had the cross not
taken place. Nevertheless the increase in this case is _less_ than in the
similar cross with wild rats. (Compare Table 42.)
In the second row of Table 43 is shown the grade distribution of
extracted hooded grandchildren of two mothers of grade -1.87 and
generation 4. The mean of the 53 hooded grandchildren is in this case
-0.73, that of uncrossed hooded parents of the same grade and generation
being 1.18. This average is probably too low. An examination of the means
of adjacent classes (Table 19) indicates that it should be about 1.35.
In the third row of Table 43 is shown the grade distribution of the
extracted recessive grandchildren of a -2.00 male, minus-series rat, of
generation 7½. The 66 grandchildren are of mean grade -0.94, expected
-1.75.
Comparing the three experiments (first three rows of Table 43), we
see that the more advanced grandparents, in grade and generations of
selection, have the more advanced grandchildren; but in every case these
are less advanced than grandparents of the same sort would have given
had they not been crossed. Hence crossing with Irish has clearly had the
effect of increasing the pigmentation in the minus series in the same way
(but in lesser degree) as did crossing with wild animals.
The results of crossing hooded rats of the plus series with Irish ones
are shown in the last two rows of Table 43. Several rats of mean grade
+2.25 and of generation 2 were crossed with Irish, and their Irish young
were then bred _inter se_, producing 239 hooded grandchildren. These
ranged in grade from -1.00 to +3.25, their mean being +1.27. The grade of
uncrossed rats of like grade and generation to the hooded grandparents
is +1.80. Hence here, as in the cross with wild rats, the pigmentation
has not been increased, but _decreased_ by the cross, contrary to what
we should expect. Further, the departure from expectation is greater in
this cross than in the wild cross. These conclusions are supported by
the results shown in the last row of Table 43. In the experiment here
recorded a +3.00 rat of generation 3 was mated with an Irish rat. The
hooded grandchildren derived from this cross were, as shown in the table,
of mean grade +0.95, expected about +2.50. Since the number of animals
recorded in this experiment is comparatively small, the quantitative
result is less important than that of the foregoing experiment, but
qualitatively the two are in entire agreement.
The various crosses of the selected minus and plus series with wild rats
and with Irish rats respectively are consistent with each other. _In
every case the cross increases the pigmentation of the minus series and
decreases that of the plus series_; in other words, _it undoes the work
of selection to some extent_. Does this mean that the condition created
by selection was in reality an unstable one, so that an outcross tends
to do away with it? We do not think so, but to this question we shall
return again.
The question might be asked whether the modifications produced in the
selected races by a cross with wild or Irish stock are likely to be
more or less permanent than those produced in unselected races by the
same means. A single experiment was made which bears on this question
in relation to the Irish cross. One of the -2.00 grandchildren recorded
in the third row of Table 43 was mated with -2.00 individuals of the
uncrossed stock of the minus series and produced nine young of mean
grade -0.63, the expectation for the uncrossed race of the same grade
and generation being about -1.90. In other words, this extracted -2.00
individual regressed (in breeding) as if it really had been affected by
the cross, even though it did not show it, but the number of young is so
small that no emphasis should be placed upon this result.
From the experiment recorded in the last row of Table 43 were obtained
extracted individuals of mean grade +1.37, which as parents produced
16 young of mean grade +1.68, or, in other words, offspring about like
themselves. Hence the changes effected by a cross are permanent, like
those effected by selection.