68 GENETIC STUDIES ON A CAVY SPECIES CROSS. 



We can not dispatch the whole situation by a simple statement that 

 the guinea-pig is dominant in size. Possibly it is somewhat so, but 

 we do not know how much of this vigor and size was due to heterozy- 

 gosis. Furthermore, since the female was the large parent, it may be 

 that the reciprocal cross, with C. rufescens as the female parent, would 

 not have given the hybrids such a good start. It is conceivable that 

 two fetuses in the guinea-pig uterus would have a greater chance for 

 initial development than the same two in the uterus of C. rufescens. 

 That the guinea-pig is in all probability not completely dominant one 

 can conclude from the size of the next generation. 



The one-quarter wild hybrids were produced by mating the | wild 

 females back to guinea-pig males (see figs. 13, 18, 23, 28, 33, and 34 

 to 41). They showed a striking loss of the vigor which characterized 

 the i wild, for both sexes averaged smaller than these in all dimensions, 

 except measurement 3 in table 64. The single exception was the length 

 of the zygomatic arch in the female sex, in which dimension the ^ wild 

 and I wild females averaged exactly the same. The i wild not only 

 averaged less than the | wild, but no one of the 36 individuals was as 

 large in any measurement as the largest ^ wild, and very few were as 

 large as the smallest ^ wild. Comparing the average of the I wild 

 males with their male parent, the guinea-pig, it was found that there 

 was a general tendency for the hybrids to be smaller, in which respect 

 the growth curves and skeletal dimensions again agree. The averages 

 of the male I wild were less in all measurements except 8 and 9. The 

 female I wild averaged smaller in all measurements except 3, 5, 6, 8, 

 and 9. Although the growth curves and skeletal dimensions of the 

 I wild were in general consistently lower than those of the guinea-pig. 

 the differences were not great. What seems to be a general tendency 

 must be cautiously considered, in view of the small differences, which 

 were often not much larger than the probable error of the averages. 



The one-eighth wild hybrids, or F3 generation, were produced by 

 mating tlie \ wild females back to the guinea-pig males (see figs. 14, 

 19, 24, 29, and 34 to 41). The males of this generation were larger 

 than the \ wild in 14 of the 16 dimensions; and the females were larger 

 in 7 dimensions, and exactly equal in 3. Comparing the | wild males 

 with the guinea-pig, it was found that they were slightly larger in 13 

 of the 16 averages, w^hereas the females were slightly smaller in 15 of 

 the 16. Here again, the differences must be cautiously interpreted, for 

 they were small in comparison with the probable errors and especially 

 in comparison with four times the probable error. The differences 

 between the | wild and the guinea-pig were extremely small, and 

 not apparent to the naked eye, as the figures of average dimensions 

 show. Irrespective of whether or not we consider the \ wild smaller 

 than the guinea-pig, it is quite certain that two back-crosses made 

 the I wild the equal of the guinea-pig in size. 



