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.
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.
| Selection. | Present series. | General (plus) series. | ||||||
|---|---|---|---|---|---|---|---|---|
| Mean, parents. | Mean, offspring. | Regression. | No. of offspring. | Mean, parents. | Mean, offspring. | Regression. | No. of offspring. | |
| 1 | 2.50 | 2.06 | 0.44 | 34 | 2.51 | 2.05 | 0.46 | 150 |
| 2 | 2.52 | 1.86 | .66 | 205 | 2.52 | 1.92 | .60 | 471 |
| 3 | 2.27 | 2.06 | .21 | 119 | 2.73 | 2.51 | .22 | 341 |
| 4 | 2.69 | 2.41 | .28 | 194 | 3.09 | 2.73 | .36 | 444 |
| 5 | 2.77 | 2.32 | .45 | 97 | 3.33 | 2.90 | .43 | 610 |
| 6 | 3.08 | 2.67 | .41 | 45 | 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.