54 HELEN DEAN KING 



to "move the racial mode and mean either in a plus or in a 

 minus direction without encountering impassible limits short 

 of an all white or an all black condition" (Castle, '31). 



In light of Castle's work, it seems probable that a hooded 

 mutation occurred at some period in the ancestry of the wild 

 rats from which gray hooded rats were derived by selective 

 breeding. Evidently breeding under natural conditions, and 

 the action of plus modifiers, had kept recessive the hooded 

 pattern in the wild stock, but not effaced the gene for spotting 

 which manifested its presence only in the coat of the female, 

 as the male with which this female was mated had no white 

 in his hair. By selective breeding through a number of gen- 

 erations plus modifiers were gradually eliminated, or rendered 

 inactive, and the white area increased until the typical hooded 

 pattern was restored. 



Aside from the hood strain, developed among the descend- 

 ants of one pair of wild rats, captive grays bred true to type, 

 as far as known, for a period of 7 years, during which time 

 over 10,000 young were born and many hundreds of them 

 reared to an age when coat characteristics were well defined. 



In the eleventh generation one of four females, mated to a 

 brother, cast a total of forty-five young of which eleven died 

 at or a few days after birth so their coat color could not be 

 determined. The remaining thirty-four individuals comprised 

 twenty-four normal grays and ten mutant blacks. The first 

 mutant, a male, appeared in a litter of four, born when the 

 mother was 15 months old. Although sisters of the female 

 that cast black young were mated to the same sire, no mutants 

 appeared among their progeny, nor has this mutation ap- 

 peared a second time in the strain. These mutants were a 

 very intense black on the dorsal side of the body, and a dark 

 slate color on the ventral side. There was no change in their 

 color for a period of about 4 years. Then individuals ap- 

 peared in which white hairs were scattered through the coat 

 and among the vibrissae. These white hairs were especially 

 numerous on the sides of the body, and in some cases were 

 found in groups of three to six. Possibly a 'silver' mutation 



