DISCUSSION. 25 



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 pos- 

 sibility that the character modified, the hooded pattern, may itself vary 

 in strength independently of its modifiers. For evidence see the de- 

 scription of the ''mutant" series, page 30. This assumption, I under- 

 stand, 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 selec- 

 tion 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 physio- 

 logical 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 hypotheti- 

 cal 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 5| individual born of 3| parents (mean grade), 

 the other as a 5f individual born of 3f parents (mean grade). The 



