GROWTH AND MORPHOLOGICAL CHARACTERS. 77 



was greatly enlarged and possessed the two lateral horns at the distal 

 end wliich characterize the penis. Their sexual propensities are dis- 

 cussed in Part III. A female ^\ wild hybrid had large caudal vertebrae 

 which, although normal in number and shape, formed a small tail about 

 half an inch in length. 



In the absence of more data relating to these and other anomalies, 

 one can only speculate as to their cause and significance. 



17. GENERAL CONCLUSIONS AS TO GROWTH AND MORPHOLOGICAL 



CHARACTERS. 



(1) The wild C. rufescens used in these crosses were about half as 

 large as the guinea-pig, C porcellus. They were not only less in weight, 

 but their bones were also shorter and more slender. The ^ 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 i wild hybrids lacked this vigor, for they were smaller than the 

 I 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 | wild hybrids averaged 

 about the same as the guinea-pig in weight and skeletal dimensions. 

 Two back-crosses were sufficient to render the F3 hybrids and guinea- 

 pigs practically indistinguishable in size and skeletal dimensions. 



(2) The number of adult wild available was too small to give a 

 satisfactory index of their variability. The same was true of the | 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. 



(3) 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 F2, F3, and F4 generations. The truncate nasal- 

 frontal suture of the tame species was recovered in the F2 generation 

 or i wild, but did not breed true. 



(4) 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 inden- 

 tation 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. 



(5) There was no apparent effect of sterility on size in the male 

 hybrids. 



(6) The unusual frequency of an interparietal bone, the occurrence 

 of a 5-toed individual, and other anomalies were observed in the hybrids 

 but not in the guinea-pig. 



