IN RATS AND GUINEA-PIGS. 1 9 



pigmentation of the two sides of the body, failure of the eye and ear spots 

 to coalesce, or entire absence of one of the two, absence of one or both side 

 spots, or the occurrence of a white gap between side and rump spots. Our 

 attempts to fix this pattern by selection, continued through a series of gen- 

 erations, have not been successful. 



Animals of the head-spot pattern (pi. 2, fig. 2) differ from Dutch-marked 

 chiefly in the entire absence of pigmentation posterior to the girdle, though 

 the pigmentation on the side of the head also is usually less extensive than 

 in Dutch-marked animals. Because of the practical difficulty of distin- 

 guishing eye spots from ear spots, when the two are confluent and similarly 

 pigmented, we have throughout this series of experiments treated the two 

 as forming a single pair, which we call head spots. 



When the head pigmentation is most reduced, only the retina of the eye 

 is pigmented. Then the head-spot type passes over into the black-eyed 

 white type. In tabulating our results, however, we have treated the two 

 types as distinct. The head-spot pattern, like the Dutch-marked, we have 

 been unable to "fix." 



The nose-spot pattern occurs less frequently than the two already de- 

 scribed. The entire body is white, except that part of the head which is 

 situated anterior to the eyes. This part of the body is usually unpigmented 

 in spotted guinea-pigs, which fact makes the occasional occurrence of the 

 nose-spot pattern more striking. Our attempts to fix this pattern have 

 succeeded scarcely better than those with the Dutch-marked and head-spot 

 patterns. 



In addition to our attempts to fix particular color-patterns arbitrarily 

 chosen, we have made records of the pigment distribution on several hun- 

 dred spotted guinea-pigs of known ancestry, with a view to ascertaining 

 whether or not particular spots are inherited. The question for which an 

 answer was sought may be stated thus : Does an animal having, let us say, an 

 eye spot, produce young with eye spots any oftener than animals bearing a 

 like amount of pigmentation but without eye spots? If this question receives 

 an affirmative answer, then we may conclude that eye spots, as such, are 

 inherited, and it will remain to ascertain the law governing their inheritance. 



The evidence bearing on this question can best be presented in the form 

 of tables, showing what areas of the young are pigmented in comparison 

 with those of their parents. Such a table has been constructed for each of 

 the successive generations in which selection was exercised for the fixation 

 of a particular coat-pattern. By a comparison of the tables for the succes- 

 sive generations, we may learn whether selection was having any effect in 

 fixing the desired coat-pattern. For, if it was doing so, later generations 

 should show closer agreement than earlier ones between parents and children 

 as regards distribution of pigment on the body. 



