Part 4

To show that, in the plus selection series, a return selection has a result similar to that just described, two experiments may be cited:

The sixth generation of the plus selection series was produced by parents of mean grade 3.52, and their offspring were of mean grade 3.11, a regression toward 0 amounting to 0.41. Certain of these offspring of mean grade 2.00, when chosen as parents, produced 17 young of mean grade 2.36, a regression away from 0 amounting to 0.36. (See Table 40.)

The eleventh generation of the plus selection series (Table 11) was produced by parents of mean grade -3.97; their offspring were of mean grade -3.78, a regression of 0.19 toward 0. Certain of these offspring, ranging in grade from -2.62 to -3.25 (Table 41), mean -2.79, produced 53 young of mean grade -3.32, a regression away from 0 amounting to 0.53. The regression in this case, as in all those previously described, was toward the racial mean of the previous generation, which, however, it has in no case reached.

This can have but one meaning. The genetic character of the hooded rat is in a general way correctly indicated by its somatic character. Selection is therefore immediately effective, whether plus or minus in character, and whether or not preceded by selection in the same direction or in an opposite direction. But regression may be expected from the character of aberrant parents back toward the normal of the previous generation, yet this regression will in general be less than the departure of the aberrant parents from the normal of their generation. If one desires in such a case to obtain continuous and progressive departure from the normal in either a plus or a minus direction, he need only select continuously in the desired direction.

CROSSES WITH WILD RATS.

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.)

CROSSES WITH BLACK “IRISH” RATS.

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.

PLUS SELECTION OF “EXTRACTED HOODED” RATS.