It would be possible to suppose, as the senior author has elsewhere
suggested (Castle, 1912), that the Mendelian unit character involved in
these experiments is subject to quantitative variation and that such
quantitative variations have a tendency to persist from generation to
generation. This would account for the effectiveness and permanency of
selection when brought to bear upon the variations. It might also form a
basis for explaining the increased variability which follows crossing,
this being regarded as due to contamination in the heterozygote, but
there are certain other observed effects of crossing which it seems
impossible to account for on this basis. In particular it is observed
that while crossing the minus series makes it _less_ minus as the
hypothesis of contamination would demand, crossing the plus series makes
it _less plus_, the opposite of what a contamination theory would demand.
For we can readily understand, on the basis of contamination, how a +6
gamete being combined with a -2 gamete might change the latter in a plus
direction; but if the same +6 gamete is associated with a +4 gamete we
should expect it, if it has any influence at all, to make this also more
plus, but the observed effect is the opposite; the extracted gametes are
_less_ plus in character.
This difficulty is met by an alternative explanation, the main feature
of which was first suggested by our colleague, Dr. E. M. East, viz, that
although we seem to be dealing with a single unit character as evidenced
by the monohybrid ratios obtained, nevertheless the modifications
which form a basis for selection are due (in part at least) to agencies
transmitted independently of the hooded pattern (not forming a part of
the same unit character), and which may be present in Irish as well as in
wild rats. By crosses with such rats the supposed modifiers may become
associated with the hooded pattern in extracted recessive individuals and
so increase its extent. Such increase does actually occur in experiment.
The hypothesis of modifiers independent in transmission of the hooded
unit will account for the fact that F₂ is more variable than F₁ when
crosses are made, on the familiar principle of recombination of
independent factors. It will account for the observed effectiveness of
selection on the ground that what selection accomplishes in the plus
series is the isolation of homozygous conditions of modifiers at first
present only in heterozygous form, and that what it accomplishes in
the minus series is the isolation of conditions homozygous for _lack_
of modifiers (or for inhibitors) of pigmentation. This same hypothesis
will account also for the observed reduction of variability during the
progress of selection, for as soon as any particular modifier attains a
homozygous condition in the race it will cease to occasion variability,
and as more and more factors become homozygous the variability should
accordingly diminish and finally disappear altogether, so far as it is
due to internal and heritable causes.
At this point the hypothesis of modifiers encounters serious difficulty,
if one holds the prevalent or “genotype” conception as to the nature of
Mendelian factors, viz, that they are fixed and unchangeable and not
subject to quantitative variation, but only to combination in different
ways with other factors. This conception has been presented very clearly
by Dr. East (1912). Some objections to this view had previously been
stated by Castle (1912) and need not here be repeated.
If we assume that there exists at the outset a definite number of
modifiers and that these possess a definite and unchanging power to
modify, then it is evident that selection can do nothing but secure
homozygous conditions as regards the presence or absence of these
modifiers. When such homozygous conditions are secured, selection will
cease to modify the race. The experiment has progressed far enough to
show that extensive modification through selection is possible without
any marked falling off in variability. No indication is observable that
selection will become ineffective before an all-black rat is obtained
in the plus series and an all-white rat in the minus series. A _nearly_
all-black race of rats has already been secured. We propose to continue
the experiments until demonstrative evidence is obtained.
If the fixed-factor idea as regards modifiers of the hooded pattern is
rejected, there remain still two possible alternative views regarding
them. Either we may consider that the modifiers vary in strength, that
is, in power to modify, or we may consider that new modifiers arise
from time to time, which selection may either add in homozygous form to
the germinal complex or reject altogether from it. If we assume that the
modifiers vary in strength, we shall have to grant also the possibility
that the character modified, the hooded pattern, may itself vary in
strength independently of its modifiers. For evidence see the description
of the “mutant” series, page 30. This assumption, I understand, would be
unacceptable to those who hold a genotype conception of heredity, though
we ourselves can offer no valid objection to it.
If, on the other hand, we admit that new modifiers or inhibitors are from
time to time coming into existence spontaneously, and that selection can
use these to modify the pattern either in a plus or in a minus direction,
then we must admit that selection is an agency of real creative power,
able to modify unit characters indefinitely so long as physiological
limitations are not reached.
Now it seems to us probable that what we call the unit-character for
hooded pattern is itself variable; also that “modifiers” exist—that is,
the extent of the hooded pattern is not controlled exclusively by a
single localized portion of the germ-cell; otherwise we should be at a
loss for an explanation of the peculiar results from crossing plus series
hooded rats with those which are still more extensively pigmented; for by
such crosses the pigmentation is rendered not _more_ extensive but _less_
so. This result we can explain on the supposition that the selected
plus series has accumulated _more_ modifiers of the hooded pattern than
the wild race contains, so that a cross tends to reduce the number of
modifiers in the extracted hooded individuals. No other explanation
at present offers itself for this wholly unexpected but indubitable
result. If a different one can be found we are quite ready to discard the
hypothetical modifiers as a needless complication, contenting ourselves
with the supposition that the unit character for hooded pattern is itself
variable, and that for this reason racial change in either plus or minus
directions may be secured at will through repeated selection.
We have been led to adopt tentatively an hypothesis that modifying
factors exist independent of the single factor for hooded pattern (though
both the factor for hooded pattern and its modifiers may, so far as we
can see, be quantitatively variable) by another series of observations,
which will now be described.