8 GENETIC STUDIES ON A CAVY SPECIES CROSS. 



that these wild cavies belonged to the commonly described Cavia aperea 

 Erxleben, but a more careful investigation showed later that they 

 belonged to the less well-known Cavia rufescens Lund (Lund 1841, 

 Waterhouse 1848, Thomas 1901). This cavy is considerably smaller 

 than Cavia aperea or Cavia porcellus, both in total size and in the 

 individual bone measurements. Thomas asserts that Cavia rufescens 

 never reaches the size of Cavia aperea. The color is agouti or ''ticked," 

 as in most wild rodents, but somewhat darker than the agouti of Cavia 

 porcellus, because more black shows in the individual hairs and less 

 yellow on their subapical bands. The belly varies from a light yellow 

 to a slightly ticked condition. The systematists lay great stress on 

 the formation of the last upper molar, in which a deep, narrow inden- 

 tation on the outer surface almost separates the small third lobe from 

 the body of the tooth. Lund describes his specimen from Minas 

 Geraes, Brazil. In all essential points the wild animals in this experi- 

 ment agree with the descriptions, plates, and general locality given by 

 the above-mentioned authors. 



A report of the experimental work does not necessitate an argument 

 on the number of differential characters which would infallibly place 

 two types in those more or less arbitrary categories— "species." It 

 is sufficient for the purposes of this problem to find that the wild cavies 

 used belong to a species more distantly related to the tame guinea-pig 

 than are Cavia aperea or Cavia cutleri, according to the methods of 

 most taxonomists. The taxonomists differ much among themselves. 

 For instance, Waterhouse held that Cavia porcellus, Cavia aperea, and 

 Cavia cutleri might all be placed in the same species. He found forms 

 bridging typical differences. Darwin (1876) held that Cavia aperea 

 was not the ancestor of the guinea-pig, basing his views on the fact 

 that a distinct genus of lice infested each form. As far as his evidence 

 goes, it might be considered decisive, for entomologists have reported 

 that closely related mammals are infested by closely related lice (Osborn 

 1908). Giebel (1855) placed a number of cavy forms in the species 

 aperea, and held that Cavia rufescens was only a variety of the larger 

 Cavia aperea. Nehring (1889) considered Cavia cutleri to be the direct 

 ancestor of our tame guinea-pig, being inclined to such a view on both 

 historical and morphological grounds. He later showed (Nehring 1893, 

 1894) that Cavia aperea may be reciprocally crossed with the guinea- 

 pig and give perfectly fertile offspring — fertile inter se or when mated 

 back to either parent. Thomas (1901) is in doubt as to which of the 

 two wild forms, Cavia aperea or Cavia rufescens, is the real ancestor of 

 the guinea-pig. It would appear, from a comparison of Nehring's 

 experiments and the experiments described in this paper, that Cavia 

 aperea must be more nearly related to the guinea-pig than Cavia 

 rufescens is, for the latter gives sterile male offspring in a cross with 

 the tame guinea-pig, whereas Cavia aperea does not. 



