GROWTH AND MORPHOLOGICAL CHARACTERS. 67 



amounted to more than 0.3 mm. In measurement 8, the width of 

 the skull immediately craniad to the external acoustic pore, the males 

 of both species were more nearly equal, and 49 out of 7.8 male guinea- 

 pigs were actually as small as or smaller than the largest wild. The 

 reason the wild are so large in tliis measurement is due to the large 

 bulla, possibly associated with the organs of hearing. Many other 

 guinea-pig skeletons were examined at a later date, but none could 

 be mistaken for the wild species. 



The wild C. rufescens in these experiments were, therefore, distinctly 

 smaller than the tame C. porcellus. The skeletal dimensions corrobo- 

 rate the data presented in the composite growth curves. The number 

 of wild in tables 63 and 64 is too small to give significant averages; 

 but the known facts regarding C. rufescens and our own observations 

 on irmnature animals indicate clearly that it is specifically smaller than 

 the tame species. Furthermore, since the number of tame is large 

 enough to be significant, it is noteworthy that their lower extremes 

 rarely overlapped with the measurements of our largest, healthy, adult 

 wild animals. 



The one-half ivild hybrids, obtained by crossing the wild males to 

 guinea-pig females, were larger and more vigorous than either parent 

 species. The males averaged larger in all measurements taken, and 

 the females averaged larger in all but two (see figs. 12, 17, 22, 27, 

 32, and 34 to 41). In these latter two exceptional cases (measurements 

 10 and 13) the females were really as large as the guinea-pig, for the 

 difference was hardly significant, considering the probable errors. This 

 increased size and vigor was not only true of the | wild as a whole, 

 but every individual male and female was larger than the average 

 guinea-pig in all its measurements, except two ^ wild hybrids. These 

 two exceptions (crll7 and 9 118), a brother and sister, were fully as 

 large as the average guinea-pig. The individual measurements and 

 the averages of the progeny in this first cross thus attested the remark- 

 able vigor of the | wild hybrids. The skeletal dimensions, therefore, 

 corroborate the data presented in the composite growth curves. This 

 was not only true of size but also of endurance; for, although they 

 were very wild in disposition and difficult to keep in captivity, when 

 successfully reared they showed their physical strength. They lived 

 through winters when ordinary guinea-pigs succumbed to disease. 

 One female had 15 litters of young and is still breeding at the age of 7 

 years. Alezais (1903), quoting Metschnikoff, states that this age 

 would be remarkable for a guinea-pig. None of the several thousand 

 guinea-pigs in this laboratory have ever been as long-lived; neverthe- 

 less, it must be stated that there has been no close study of their 

 longevity. Other ^ wild females were equally vigorous and fertile, 

 but were killed for the purpose of study. 



