24 PIEBALD RATS AND SELECTION. 



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 indi- 

 viduals 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 Fi 

 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 diffi- 

 culty, 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 homo- 

 zygous 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 



