ANIMAL PRODUCTION. 465 



la later generations the hybrids show the incomplete dominance of uniformity 

 over spotting, which is characteristic of the guinea pig. Any color variety 

 known in guinea pigs can be produced in the hybrids. Combinations of tame 

 and wild characters can be made, even bringing in such a moi-phological char- 

 acter as polydactylism from a tame race, together with the peculiar agouti 

 of the wild race. The inheritance of coat and color characters throughout this 

 species cross is in accordance with Mendel's law. It is equally true of matings 

 of hybrids inter se and of matings of hybrids of either sex with guinea pigs." 



In part 2, which deals with growth and morphological characters, the follow- 

 ing general conclusions were reached : " The wild C. rufescens used in these 

 crosses were about half as large as the guinea pigs C. porcellus. They were 

 not only less in weight, but tbeir bones were also shorter and more slender. 

 The one-half wild hybrids were usually heavier at all ages, had larger skeletal 

 dimensions, and gave every indication of being more vigorous than either 

 parent species. The one-quarter wild hybrids lacked this vigor, for they were 

 smaller than the one-half wild hybrids in every way. They were very nearly 

 the equal of the guinea pig in average size and skeletal dimensions. Possibly 

 the males were a little smaller than the guinea pig. The one-eighth wild 

 hybrids averaged about the same as the guinea pig in weight and skeletal di- 

 mensions. Two black crosses were sufficient to render the Fs hybrids and 

 guinea pigs practically indistinguishable in size and skeletal dimensions. The 

 number of adult wild available was too small to give a satisfactory index of 

 their variability. The same was true of the one-half wild hybrids. The guinea 

 pigs were remarkably uniform. The variability of all hybrids in both sexes 

 was very low and gave no clear Indication of segregation. The M -shaped nasal- 

 frontal suture of the wild appeared to be dominant. Crossing back to the tame 

 species gave a wide range of variability In the F:, F3, and F4 generations. 

 The truncate nasal-frontal suture of the tame species was recovered in the Fa 

 generation or one-quarter wild, but did not breed true. The differences in skull 

 shape between the wild and tame were blended in the Fi generation. In later 

 generations all traces of the pointed, wild skull shape were gradually lost. 

 The deep narrow indentation on the outer surface of the last upper molar, 

 almost separating the small third lobe from the body of the tooth, was reduced 

 in the Fi generation ; and all traces of it were lost in later generations. The 

 taxonomists lay great stress on this character. There was no apparent effect 

 of sterility on size in the male hybrids. The unusual frequency of an inter- 

 parietal bone, the occurrence of a 5-toed individual, and other anomalies were 

 observed in the hybrids but not in the guinea pig." 



In part 3, which deals with the fertility of parent species and hybrids, it 

 was found that "crosses between C rufescens males and C. porcellus females 

 gave completely sterile male hybrids and fertile female hybrids. By crossing 

 the female hybrids back to guinea pig males, one-quarter wild hybrids were 

 obtained, which were again sterile males and fertile females. A few males of 

 this second hybrid generation, however, showed some degenerate nonmotile 

 sperm. By repeated back crosses of female hybrids to guinea pigs, increasing 

 signs of fertility appeared. Fertility seemed to act like a very complex reces- 

 sive character ; for the results obtained were what one would expect if a num- 

 ber of dominant factors for sterility were involved, the elimination of which 

 would give a recessive fertile type. There was an enormous range of forms 

 between hybrids with no sperm and fertile hybrids with many motile sperm. 

 The results indicated that a completely fertile hybrid male could be bred to 

 female hybrids or to guinea pigs, giving about the same results as a normal 

 guinea pig male in such matings. 



