ROUGH FUR. 103 



character, dominant over smooth, and is thus an example of the com- 

 paratively rare class of dominant mutations. He found that the grade 

 of roughness, while fairly constant in some stocks, could be reduced by 

 crossing with smooth guinea-pigs of a particular stock (tricolor) , which 

 he described as prepotent smooth. Detlefsen (1914) found that rough 

 fur in hybrids between rough guinea-pigs and the wild species Cavia 

 rufescens continued to be inherited in Mendelian fashion, but that domi- 

 nance ceased to be complete. The writer began experiments in 1913 at 

 Professor Castle's suggestion, to investigate further the heredity of 

 variations in the rough character. 



MATERIAL. 



Several stocks have been used as material: 



(1) 4- toe stock. A full-rough was crossed with members of the 4- toe 

 stock, and by repeated back-crosses into the latter a stock has been 

 produced which is practically pure 4-toe. No partial-roughs have ever 

 appeared in these crosses. Pure 4-toe smooth animals have been very 

 useful in the experiments, since it has been amply proved that when 

 crossed with full-roughs they never reduce the grade of rough. 



(2) Tricolor stock. Most of the partial-roughs experimented with are 

 of very mongrel stock, with, however, more or less tricolor ancestry. 

 In this section on rough fur the term tricolor stock will be used for 

 convenience for these animals, without implying that all of them 

 actually were tricolors. 



(3) Lima stock. This stock as has been described was derived en- 

 tirely from 8 guinea-pigs (2 nearly full-rough (rough B) and 6 smooth) 

 brought from Peru in 1913. 



(4) Rufescens hybrids. The writer has worked with a few rough 

 animals descended from Detlefsen's hybrids and containing from to 



Cavia rufescens ancestry. 



(5) Cutleri hybrids. Crossing with Cavia cutleri has been found by 

 the writer to have a similar effect on the rough character to that 

 described by Nehring (1894) for C. aperea and by Detlefsen (1914) for 

 C. rufescens. The behavior of the roughness in these cutleri hybrids 

 has been investigated. 



(6) Miscellaneous smooth guinea-pig stocks have been used in some 

 of the experiments. All used resemble the 4-toe smooths in giving no 

 partial-roughs when crossed with full-roughs. 



(7) BW, lea, and Arequipa stock occasionally have shown a slight 

 roughness which is distinct from the usual type and is discussed later 

 under series II. The BW stock has been used in a few cases as a 

 source of smooth guinea-pigs. Aside from the slight roughness of 

 series II all of those used have behaved like 4-toe smooths. Smooth 



