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.

THE “MUTANT” SERIES.

In the tenth generation of the plus selection series there appeared two individuals, a male and a female, of considerably higher grade than any previously recorded in this series. They are not included in Table 10 because we have been and still are in doubt as to their exact nature and think it best to give a separate account of them. If entered in Table 10 one would appear as a 5½ individual born of 3⅞ parents (mean grade), the other as a 5¾ individual born of 3¾ parents (mean grade). The nearest individuals in grade to these two produced by the same group of parents are of grade 4½, but some 4⅛ parents of the same generation produced two offspring of grade 5. (See [Table 10].) Because of the marked advance in grade of these individuals beyond the ordinary range of variation in the series we called them “mutants,” without wishing then or now to commit ourselves to any particular theory as to their nature or origin. We have used the term and now use it as one of convenience merely. The two “mutant” individuals had the same father and their mothers were sisters. Their pedigree for two generations is as follows:

Mutants. Parents. Grandparents.
♂ 4763, +5½ { ♀ 2956, +3¾ } { ♀ 1939, +3¾
{ ♀ 2957, +3½ } { ♂ 1817, +3¾
{
♀ 5153, +5¾ { { ♀ 1162, +3½
{ ♂ 2963, +4 { ♂ 1810, +3¾

The mutant male was mated with the mutant female and also with other females of the plus series, with the results shown in [Table 51]. In every case the young fall into two distinct groups, one of which varies about the general mean of the plus series (approximately 3¾), while the other varies about the father’s grade as a mean (approximately 5½).