128 GENETICS AND EUGENICS 



backward as in wild mammals, may become rough or rosetted 

 with the hair radiating out from centers located in various 

 parts of the body. (See Fig. 33.) Rough coat is dominant 

 over smooth coat, for w^hich reason we may consider a unit- 

 character, rough coat, R, to be responsible for it, the recessive 

 phase of which, r, is found in smooth-coated guinea-pigs. 



It should be noted that both rough coat and short coat, 

 like the uniformity factors affecting pigmentation, obviously 

 vary quantitatively. For some rough guinea-pigs are rougher 

 than others and some long-haired guinea-pigs have longer, 

 silkier hair than others. Selection has undoubtedly been 

 concerned in producing the present high standard long-haired 

 and rough-coated guinea-pigs respectively. Dr. Sewall 

 Wright has show^n (Castle and Wright, 1916) that an inde- 

 pendent Mendelizing factor found in many wild cavies inter- 

 feres with or partially inhibits the development of the rough 

 coat in hybrid guinea-pigs. We may designate this factor 

 rough modifier (M), its recessive phase which permits full 

 development of the rough coat may be expressed by m. Aside 

 from this striking modifier of rough, it is probable that nu- 

 merous other factors act as slight modifiers of rough and that 

 the apparently continuous variation in the development of 

 the roughness may thus be accounted for. Continuous varia- 

 tion in the expression of the angora character, as regards 

 length of hair, may be accounted for on similar grounds. 



Leaving out of consideration such quantitative variations, 

 it is possible to obtain by crosses a large number of different 

 unit-character combinations of the ten independent varia- 

 tions which have been mentioned as occurring in guinea-pigs. 

 Theoretically one thousand and twenty -four are possible, or 

 if we count separately homozygous and heterozygous com- 

 binations, fifty-nine thousand and forty-nine are possible. 

 Needless to say there have been produced thus far only a small 

 part of the varieties of guinea-pigs theoretically possible as 

 unit-character combinations of the ten factorial variations 

 known to have occurred in this species. And the variation 

 of the guinea-pig is not different in kind or degree from 



