IN RATS AND GUINEA-PIGS. 5 



applies only to black-pigmented individuals, but for lack of a better descrip- 

 tive term, we shall apply it to all animals having the color-pattern of 

 "Irish" rats, whether gray or black pigmented. A rat of the Irish pattern 

 has pigmented sides and dorsal stirface, but bears more or less white fur 

 upon its belly, varying in extent from a few white hairs midway between 

 the front legs to a wholly white ventral surface. 



Doncaster (:o6) has distinguished two types of Irish rats, in one of which 

 the white is more extensive than in the other. He has observed that the 

 rats with larger white areas are regularly heterozygous, producing hooded 

 as well as Irish offspring when mated inter se. Our experiments in the main 

 confirm this idea. It is not possible to determine with certainty, from the 

 size of the ventral patch alone, whether a particular Irish rat does or does 

 not contain the hooded pattern in a recessive condition, but in a lot of Irish 

 rats of similar ancestry those with the larger white patch oftener transmit 

 the hooded condition, while those which do not transmit the hooded condi- 

 tion oftener have a small white patch. 



In hooded rats (pi. i, figs. 1-4) the white areas are more extensive than 

 in Irish ones ; pigment occurs only on the head, shoulders, and forelegs (con- 

 stituting the "hood" of fanciers), and as a median dorsal stripe extending 

 back on to the tail, the stripe being sometimes continuous, sometimes inter- 

 rupted, and of varying width. According to the nature of the pigment 

 which they bear, hooded rats may be distinguished as gray hooded or as 

 black hooded, precisely as animals bearing the Irish pattern are designated 

 either gray Irish or black Irish. 



When crosses are made between rats differing in color-pattern, the more 

 extensively pigmented pattern tends to dominate in the offspring, a fact 

 recognized by Crampe ('77-84, '85), Bateson (103) and Doncaster (:o6). 

 The dominance, however, is not complete, so that the result might be de- 

 scribed as "goneoclinic," i. e., intermediate between the parental forms but 

 approximating one much more nearly than the other, in this case always 

 the more heavily pigmented one. Thus, a cross between a wild gray male 

 rat and a black hooded female, known to be homozygous, produced a litter 

 of seven young, all gray, but with a small patch of white on the chest, vary- 

 ing in extent from merely a few white hairs to an area of 4 to 5 sq. cm. The 

 same wild male, mated with an albino female, produced a litter of young 

 similar in character, all with some white below. Bateson (:o3, p. 78, foot- 

 note) mentions a similar result obtained by Miss Douglas. 



Again, a cross between an Irish and a hooded individual produces Irish 

 offspring. An example will be found on page 11 in the matings of black 

 Irish 9 141 with black-hooded males, producing nineteen offspring, all Irish. 

 Most matings of Irish with hooded rats have in our experiments produced 

 offspring of both sorts, not because of reversal in the nature of the domi- 



