DISCUSSION OF EXPERIMENTS. 



MATERIAL. 

 SYSTEMATIC POSITION. 



Guinea-pigs belong to the family Caviidae of the hystricomorph 

 division of rodents. There are three living genera of Caviidse: Doli- 

 chotis Desm., which contains the large Patagonian cavies; Hydrochcerus 

 Brisson, to which belongs the capybara; and Cavia Pallas, containing 

 the small cavies. Genus Cavia is divided into two subgenera, Cavia 

 proper and Cerodon F. Cuv., distinguished most conspicuously by the 

 greater complexity of the molars in the former. Seven living species 

 are listed under Cavia proper by Trouessart (1904) : 



C. rufescens Lund, a small dark Brazilian cavy with subspecies in Guiana and 



Argentina. 

 C. fulgida Wagler, a Brazilian cavy probably closely allied to rufescens (Thomas, 



1901). 



C. aperea Erxl., a large pale-colored Brazilian cavy. 

 C. azarcB Wagner, a cavy of Paraguay probably closely allied to aperea (Thomas. 



1901). 



C. cutleri Bennett, a small pale-colored cavy of Peru. 

 C. tschudii Fitzinger, a large, richly colored cavy described from lea, Peru. 

 C. porcellus Linn., the tame guinea-pig, much larger than at least rufescens and 



cutleri. 



DESCRIPTION OF STOCKS. 



Four of these species are dealt with in the experiments to be described, 

 viz, Cavia rufescens, C. cutleri, C. porcellus, and a type which is quite 

 certainly that described as C. tschudii, although it is also quite certain 

 that it is simply feral porcellus. Breeding experiments have been 

 carried on with a fifth species, C. aperea, by Nehring (1894). 



The C. rufescens stock was derived from 3 individuals received from 

 Mr. Adolph Hempel, of Campinas, Brazil, in 1903. The history of 

 this stock is fully described by Detlefsen (1914). When received by 

 the writer, most of the stock consisted of hybrids containing only from 

 yV to -5^ rufescens blood. There were a few | and \ bloods and one % 

 blood, 9 A68, which is still alive (August 1915) at the remarkable age of 

 8 years 1 month, 1 a good illustration of the vigor of the first-generation 

 hybrids. All of the pure rufescens stock has died out. The rufescens 

 hybrids have been crossed with nearly all of the guinea-pig stocks to 

 be described, and most of the color varieties may be found among them. 

 The ticked-bellied type of agouti has been found only among them and 

 in pure rufescens. C. rufescens was not completely fertile with the 

 guinea-pigs (Detlefsen, 1914). Detlefsen found that while the female 

 hybrids were fertile, all of the male hybrids obtained were sterile. In 

 the \ rufescens, derived by crossing the females with guinea-pigs, the 

 males were again all sterile. Not until the | bloods were obtained did 



'Died October 1915, aged 8 years, 3 months. TT. E. C. 



74 



