IN RATS AND GUINEA-PIGS. 23 



Head spots regularly give higher inheritance coefficients than any other 

 set of spots, a possible reason being that head spots really include two 

 distinct pairs of pigment areas, namely, eye spots and ear spots. Commonly, 

 in the Dutch-marked series, both of these are pigmented, in which case it 

 makes no difference whether the two are dealt with separately or jointly- 

 But sometimes an eye area may be pigmented, while the adjacent ear area 

 is unpigmented, or vice versa; in such cases separate enumerations of eye 

 and of ear spots would indicate a less amount of head pigmentation than 

 the joint reckoning followed, for purposes of convenience, in this paper. 

 Aside from this consideration, however, it is certain that the head region is 

 oftener pigmented than any other part of the body. 



HEAD-SPOT SERIES (SERIES H). 



The data concerning spot inheritance in the head-spot series are contained 

 in tables 15 to 24. The entries are made separately for the different regions 

 of the body in tables 15, 17, 19, and 21, for four different groups of indi- 

 viduals possessing different average amounts of head-spot ancestry. The 

 average amounts of head-spot ancestry in these several groups are a half- 

 generation, one generation, two generations, and three generations, respec- 

 tively, and may be expressed by the symbols //, H\ t H 2t and H 3) as explained 

 in connection with the Dutch-marked series. In the group H\ (table 15), 

 one of the parents of each individual recorded bore certain spots other than 

 head spots, but in the other three groups of Series// (tables 17, 19, and 21), 

 none of the parents bore spots other than head spots. Tables 15, 17, 19, 

 and 21 are combined in table 23, which shows the average degree of in- 

 heritance of particular spots in the series as a whole. 



As in the case of Series D, the entries for the different regions of the body 

 have, in the case of each group of individuals, been combined to get an expres- 

 sion for spot inheritance in general, in the particular group of individuals 

 under consideration. These combination entries are found in tables 16, 18, 

 20, and 22, for the four groups considered separately, and combined into 

 one table for the whole head-spot series, in table 24. 



In this series, rigid selection has been exercised for head spots, and against 

 all other spots. We will now consider what the effects of this two-fold 

 selection are. Paired head spots are present in the following per cents of 

 the young of parents possessing such spots, in the several groups: 79.8, 

 85.4, 77.1, and 82.1 per cent; average for the series (table 23),8r.i per cent.* 

 No change is observed uniformly in the direction either of increase or of 



*In obtaining the average, each table is weighted in proportion to the number of individ- 

 uals recorded in it. These are, for the successive groups, 207, 193, 129, and 58; total, 587. 



