66 



GENETIC STUDIES ON A CAVY SPECIES CROSS. 



father (cf 1) were used as the wild parent in the crosses that produced 

 the hybrids. They were fully adult, healthy animals, and in all prob- 

 ability as large or larger than most members of their species. The 

 tables indicate that C. rufescens is smaller than the tame parent species 

 in all measurements considered. This was also found to be true of the 

 scapula, radius, ulna, innominate bone, fibula, and the different verte- 

 brse. (See figs. 10, 11, 15, 16, 20, 21, 25, 26, 30, 31, and 34 to 41.) 

 The long bones of the wild were likewise more slender than those of 

 the tame. The average skeletal dimensions of the tame were found 

 to be higher than those of the wild in every case in both sexes. It is 

 appropriate to say, briefly, at this point that all the figures of the skulls 

 and bones given in the plates are of natural size and represent as nearly 

 as possible the averages given in tables 63 and 64. The skulls and 

 bones shown in these plates were chosen because each one represents 

 the average of its class. In all cases the figures are not visibly different 

 from the computed average and any actual difference is generally much 

 less than 1 mm. 



It may seem that the differences between the averages of individual 

 measurements are too small to separate the two species distinctly; 

 but if, for example, an average guinea-pig skull is compared with a 

 wild skull (figs. 10, 11, 15, and 16), it will be seen readily that the total 

 effect of all these differences in the eleven skull dimensions is enough 

 to separate the wild from the tame distinctly. Furthermore, there is 

 a minimum amount overlapping between individuals of the two species. 

 Although the 2,250 individual measurements for 78 male and 63 female 

 guinea-pigs are not presented, there were very few cases in which any 

 guinea-pig was found to be as small in any of its dimensions as the 

 longest wild. The exact number of guinea-pigs overlapping the wild 

 is as shown in the accompanying table. 



Therefore, out of a total of 2,250 guinea-pig measurements, there 

 were only 77 which overlapped the corresponding wild measurements. 

 This means that no guinea-pig of either sex was as small as the wild 

 in the case of 10 of the 16 dimensions. In the 6 dimensions given 

 above, there were a few cases in which some guinea-pigs equaled or 

 were smaller than the wild, but when they were smaller it rarely 



