The plus and minus selection series already described make it clear
that one can, in a race of hooded rats, either increase or decrease the
average pigmentation at will, and at the same time secure more advanced
stages either of pigmentation or of depigmentation than those previously
occurring in the race. The question now arises, are these changes
permanent; will these displaced means retain their new position, if the
race is left to itself; or will the newly obtained stages vanish as soon
as selection is suspended? A presumption that the changes will prove
permanent is afforded by the gradual decrease of regression and its final
reversal in the case of offspring of a particular grade, upon repeated
selection made in the same direction. (See page 12.) But in order to
test the matter more directly and thoroughly, the experiment has been
repeatedly made of reversing the course of selection, after it had been
in progress for several generations, with a view of ascertaining whether
the return toward the former condition of the race would be made more
speedily and easily than the original departure from it had been.
The first experiment of this sort was a return selection from generation
6 (and 6½) of the minus selection series. The parents of generation 6
(Table 21) averaged -1.86 in grade; the average grade of their offspring
was -1.56, a regression of 0.30. The range of the offspring extended
from 0 to -2.50. Some low-grade offspring were chosen for a return
selection series (Table 31). The mean grade of the selected pairs ranged
from -0.37 to -0.87, their mean being -0.60. These parents produced 118
offspring, whose average grade was -1.28, a regression of 0.68 _in a
direction contrary to that of the regression in the minus selection
series_. The large amount of the regression might seem to imply that it
was even more difficult to return toward the former state of the race (in
the neighborhood of 0) than it had been to depart from it, but this can
not be insisted on, because the number of individuals under observation
is not sufficiently large. To test the reality and permanency of the
reversed regression, the selection was repeated five additional times,
altogether six successive return selections being made with the idea of
undoing what had been effected by six original selections in an opposite
direction. The result of the second successive return selection is shown
in Table 32. The parents here were of grade -0.50 and they produced 19
offspring of the average grade -0.95, a regression of 0.45 _away from_ 0
as before.
Table 33 shows the result of the third return selection. Individuals
entered in Table 32 as offspring appear here as parents. Only those pairs
which were of mean grade, -0.25 or -0.37, should really be regarded as a
_third return selection_. They gave offspring with mean grades of -0.63
and -0.86 respectively, which show regression of 0.38 and 0.49 _away
from_ 0.
But Table 33 shows also the character of young produced by -1.12 and
-1.25 parents in this same third return-selection generation, _i. e._,
by _unselected_ parents of the generation in question. Their young
also regress _away from_ 0—that is, in the direction of the original
selection. The -1.12 parents produced -1.61 offspring, a regression of
0.49, while the -1.25 parents produced -1.35 offspring, a regression of
0.10. For Table 33 as a whole the regression _away from_ 0 averages 0.31.
A fourth generation in the return-selection series is summarized in Table
34. The parents are of mean grade -0.63; their 50 offspring are of mean
grade -1.17, a regression amounting to 0.54 _away from_ 0 and in the
direction of the six generations of original selection.
Table 35 contains the results of the fifth generation of the series. The
parents are here of mean grade -0.65. The number of offspring is very
small (13), but they nevertheless show the reversed regression which
characterized the four preceding generations. Their mean was -0.75, a
regression of 0.10 away from 0.
A sixth and final generation in this return-selection experiment is
summarized in Table 36. It includes 36 offspring of mean grade -0.39, the
mean of the parents being -0.26, a regression of 0.13 away from 0. It
will be seen, therefore, that the effect of the six original selections
had not been entirely overcome by an equal number of return selections.
The reason for this is obvious. Much smaller numbers are concerned in
the return selections than in the original minus selections. The return
selections are accordingly less efficient. Nevertheless, after the sixth
return selection we find that 1 in 6 of the offspring have plus grades
and their average is lower (that is, _less minus_) than the offspring in
the minus series after a single generation of selection. (Cf. Tables 16
and 36.)
The amount and persistency of the reversed regression in this series
show clearly that return selection is not easier or more rapid than the
original modification of the race by selection, but that selection in
either a plus or minus direction has cumulative and permanent effects.
Further support for this conclusion is furnished by return selections
(one each) made from the seventh generation, from the eighth generation,
and from the eleventh generation of the minus selection series. (See
Tables 37, 38, and 39.) Generation 7 (Table 22) was produced by parents
of average grade -2.01. Their offspring were of average grade -1.73, a
regression (toward 0) amounting to 0.28. Certain pairs of these offspring
of grade -0.75 and -0.87 (mean -0.78) constitute the return selection
from generation 7 (Table 37). They had 33 offspring of average grade
-1.15, a regression _away from_ 0 amounting to 0.37.
Generation 8 of the minus-selection series (Table 23) was produced by
parents of mean grade -2.05. Their offspring were of mean grade -1.80, a
regression (toward 0) of 0.25. Certain pairs of these offspring of grades
-0.50, -0.62, and -1.00 (mean -0.72), when chosen as parents, produced 41
young of mean grade -1.51, a regression _away from_ 0 amounting to 0.79.
(See Table 38.)
Generation 11 of the minus series (Table 26) was produced by parents of
mean grade -2.30. The offspring were of mean grade -2.15, a regression
of 0.15 toward 0. A pair of the offspring of mean grade -1.62 (Table
39) produced 16 young of mean grade -1.95, a regression of 0.32 away
from 0. This result shows that the selected race had now passed the
point represented by the grade of the parents (-1.62) and the offspring
regressed toward a racial mean as advanced as the most extreme
individuals obtained previous to selection.
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.