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.
PLUS SELECTION OF “EXTRACTED HOODED” RATS.
It has been suggested that the original material out of which the plus series came consisted of modified individuals produced by a cross with the wild race. This was not known positively to be so, because part of the original stock (with which MacCurdy worked) consisted of hooded black and hooded gray rats captured in company with gray self and black Irish rats and albinos. Subsequent experiments showed that ordinary albino rats, if crossed with wild gray ones, will produce in F₂ all these classes of individuals. This indicated pretty clearly that the particular colony which had fallen into our hands had probably arisen by the crossing of an escaped albino rat with wild ones. But it still remained uncertain what sort of hooded pattern the escaped albino had transmitted and whether or not this had been influenced by the wild cross. We therefore determined to ascertain whether out of our minus series crossed with wild a plus series could be derived. To this end certain of the F₂ extracted hooded individuals (entered as grandchildren in [Table 42], row 1, and descended from a single hooded individual of grade -1.87, generation 2½) were mated inter se, thus producing an F₃ generation, [Table 44], second row. The selected individuals were the aberrant male of grade +3.50 and females of grade +1.50, so that the mean grade of the chosen parents (extracted from the crossed minus series) was +2.50. They had 34 young ranging in grade from 0 to +3.50, mean +2.06, a regression of 0.44 toward 0, repeating the phenomenon regularly found in both selection series.