58 Heredity and Eugenics 



was now directed toward two ends: (i) to secure animals 

 which were free from spots of red or white, a condition 

 which was present in the original stock; and (2) to secure 

 extensive and uniform silvering on a black background. 

 In both these objects good progress has been made. We 

 have animals which are silvered all over the body except 

 on a part of the head, and the percentage of such well- 

 silvered individuals is relatively high. 



A more extensive selection experiment is one in which 

 I have been assisted by Dr. John C. Phillips (Figs. 31 and32). 



-3. -1 l+l 



FIG. 31. Diagram showing variation in the color pattern of hooded rats. 

 Numerals indicate arbitrary grades used. 



Selection in this case has been directed toward a modifica- 

 tion of the color pattern of hooded rats, a pattern which is 

 known to behave as a recessive Mendelian character in 

 crosses with either the self (totally pigmented) condition 

 or the so-called Irish (white bellied) condition found in 

 some other rats. The extreme range of variation among 

 our hooded rats at the outset of this experiment is indicated 

 by the grades 2 and +3 of Fig. 31. Selection was now 

 made of the extreme variates in either direction and these 

 were bred separately. Two series of animals were thus 



