As a further test of the permanency of the modification effected by
selection in the hooded pattern of rats, crosses have from time to time
been made of the selected races with a pure wild stock, _i. e._, with
ordinary wild animals caught in traps. In all cases the wild animals
used were known to be homozygous as regards gray coat and self pattern,
since when crossed with black-hooded animals they produced only gray
self offspring. In such crosses the hooded pattern is recessive, the F₁
offspring being indistinguishable from ordinary wild gray rats except for
the possession of a white patch of varying size upon the belly, but even
this may be lacking. (See Plate 2, ♂ 8000, 8018, and 8021.)
The grade of the hooded young extracted from a cross with wild animals
corresponds in a general way with the grade of the hooded animal used in
making the cross, as the following cases will show. (Compare also Plates
2 and 3.)
A female of grade -1.87, belonging to generation 2½, minus series
(compare Tables 2 and 3), was crossed with a wild male. (See Plate 2, ♀
6176.) Among her F₂ descendants (cf. Plate 2, 8070 to 8078) occurred 62
hooded individuals, whose grade distribution is shown in Table 42, first
row. Their mean grade is +0.31, although the uncrossed race of the same
grade and generation gave offspring of mean grade about -1.20. The cross,
therefore, had apparently increased the pigmentation of the extracted
hooded recessives. This idea is supported by the result of a control
mating of the particular female used in making the cross. When she was
mated with a hooded male of the same grade as herself, she produced
three young, all of grade -1.00. The extracted recessive grandchildren,
as a group, show greatly increased pigmentation as compared with this,
but vary greatly in the extent of the increase. Some show very little
modification, others very much, the most extreme individual being of
grade +3.50. It was undoubtedly out of just such modified recessive
individuals as this that the material for our initial plus selections
arose; to this point we shall return later.
The F₂ (or second generation) offspring, however, include about 1 hooded
individual in 4. In a total of 962 F₂ young; 230 were hooded, or 24 per
cent. This summary includes only those litters in which dominants as well
as recessives were recorded. In many litters only the hooded young were
recorded, as the special object of the investigation was to ascertain
whether the extracted recessives were like the pure hooded race in grade
or not. In the above summary also the hooded grandparent was in every
case a female. The reciprocal cross is more difficult to obtain, but one
wild female rat, caught in 1911, has bred quite regularly in captivity,
though each time she has murdered her hooded mate prior to the birth of
the young. Her F₂ grandchildren derived from matings with males of the
minus series include 32 hooded and 96 non-hooded individuals, exactly 25
per cent hooded.
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.)