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.
In this same experiment some F₂ parents of mean grade -0.75 had 19 young
(first row of Table 44), whose mean grade was -0.04, a regression of
0.71 toward 0. We should expect the regression of the offspring of such
parents to be less than that of the offspring of the -2.50 parents, and
so it would be if it were not for one aberrant individual. Larger numbers
of offspring would undoubtedly have given the expected result.
From among the F₃ offspring were chosen parents for the next generation
(F₄). The chosen parents ranged in mean grade from +2.25 to +3.12 (Table
45), average +2.52. They produced 205 young ranging in grade from -0.25
to +3.50, mean +1.86, a regression of 0.66.
The parents for the next generation (Table 46) ranged in mean grade from
+2.00 to +3.00, the mean being +2.27. They produced 119 offspring of mean
grade +2.06, a regression of only 0.21.
TABLE B.—_Comparison of the present series with the more general plus
selection series._
+----------+------------------------------------------+
| | Present series. |
| +--------+----------+-----------+----------+
|Selection.| Mean, | Mean, |Regression.| No. of |
| |parents.|offspring.| |offspring.|
+----------+--------+----------+-----------+----------+
| 1 | 2.50 | 2.06 | 0.44 | 34 |
| 2 | 2.52 | 1.86 | .66 | 205 |
| 3 | 2.27 | 2.06 | .21 | 119 |
| 4 | 2.69 | 2.41 | .28 | 194 |
| 5 | 2.77 | 2.32 | .45 | 97 |
| 6 | 3.08 | 2.67 | .41 | 45 |
+----------+--------+----------+-----------+----------+
+----------+------------------------------------------+
| | General (plus) series. |
| +--------+----------+-----------+----------+
|Selection.| Mean, | Mean, |Regression.| No. of |
| |parents.|offspring.| |offspring.|
+----------+--------+----------+-----------+----------+
| 1 | 2.51 | 2.05 | 0.46 | 150 |
| 2 | 2.52 | 1.92 | .60 | 471 |
| 3 | 2.73 | 2.51 | .22 | 341 |
| 4 | 3.09 | 2.73 | .36 | 444 |
| 5 | 3.33 | 2.90 | .43 | 610 |
| 6 | 3.52 | 3.11 | .41 | 861 |
+----------+--------+----------+-----------+----------+
The parents chosen from among these offspring ranged in mean grade from
+2.37 to +3.25, average +2.69. They produced 194 offspring of grade +0.50
or higher (F₆, Table 47), the range for the first time lying wholly
in the plus direction. The mean grade of the offspring was +2.41, a
regression of 0.28.
The parents of the next generation (F₇, Table 48) range in mean grade
from +2.62 to +3.37, their average being +2.80. Their 154 offspring range
from +0.75 to +3.75, mean +2.46, a regression of 0.34.
The parents of the last generation in this experiment (F₈, Table 49) were
of mean grade +3.08. They produced 45 offspring of mean grade +2.67, a
regression of 0.41.
As a result of a single cross with a wild race followed by six successive
selections, a narrow-striped or minus family has thus been converted
into a wide-striped or plus family. Considering the smaller number
of offspring from which selections could be made, progress was quite
as rapid in this series as in the larger plus selection series. The
regression is surprisingly similar, generation by generation, in the
two series. (See Table B.) But it seems improbable that the closeness
of the agreement has any significance. _This series has the theoretical
advantage of being derived from a single individual of the minus
selection series._