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 w^ell- 

 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 and 3 2). 



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 



