UNIT-CHARACTERS OF RODENTS 129 



that of other rodents. Its variation has probably merely 

 been followed up more closely by selective breeding. Among 

 domesticated rabbits, at least seven of the ten enumerated 

 variations have occurred; all except the pink-eye and the 

 rough-coat variations are reported for rabbits, and most of 

 them are well known. The house mouse has undergone at 

 least six of the ten variations listed in Table 12. Its yellow 

 varieties have apparently not arisen in the same way as yel- 

 low varieties of guinea-pigs and rabbits, but by a peculiar 

 change in the agouti factor, for yellow in mice is a third 

 allelomorph of agouti and non-agouti. Mice also lack long- 

 haired and rough-coated varieties, but in other respects the 

 variations of mice are parallel with those of guinea-pigs. In 

 the Norway rat four of the ten unit-character variations of 

 guinea-pigs find exact equivalents, viz., in albinism, non- 

 agouti, pink-eye and white spotting. A third allelomorph of 

 the color factor (ruby-eye) has been shown by WTiiting and 

 King to occur among wild rats. 



A red-eyed yellow variety of rats is due to a unit-character 

 variation distinct from the yellow variations known in 

 guinea-pigs and in mice respectively. In one and the same 

 linkage system in the Norway rat are found (1) the color 

 factor and its allelomorph, ruby-eye, (2) the factor for pink- 

 eyed yellow and (3) the factor for red-eyed yellow. 



