72 GENETIC STUDIES ON A CAVY SPECIES CROSS. 



were shown to be practically the same; and, hence, a difference of less 

 than 1 mm. in the standard deviation of any measurement would com- 

 pletely obliterate the differences in the coefficients of variability. 

 Although I have undertaken no experiments to ascertain the effect of 

 environment on skeletal dimensions, experience with many hundreds of 

 guinea-pigs and hybrids leads me to believe it would be decidedly 

 strange if environment could not effect a difference of less than 1 mm. 

 in the standard deviation of the guinea-pigs and hybrids. 



Summarizing the facts concerning variability in the guinea-pigs and 

 hybrids, we may say that — 



(1) The variability of all the classes of hybrids and the guinea-pig 

 was very small. 



(2) There were no great differences in variability in the back crosses 

 of hybrids to guinea-pigs which would indicate segregation and recom- 

 bination of factors for size. This is true for the individual measure- 

 ments and for the general average variability of each class. 



(3) The results in no way controvert the possibility that size may 

 be due to factors whfch are inherited in Mendelian fashion; but segrega- 

 tion was not apparent in these classes of matings in this species cross. 

 The dominance of the guinea-pig may well be very nearly complete. 

 Since the hybrids were mated back to the guinea-pig each time, it is 

 simply a case of dominance with little or no evidence of segregation. 

 According to this explanation, the vigorous growth of the first, or ^ wild, 

 hybrids was due to their heterozygosity, but without the effect of 

 heterozygosis they would have been a little smaller than the I wild. 

 Mating the | wild to the guinea-pig raised the mean of the j wild nearly 

 to that of the guinea-pig and a second back-cross raised the mean of 

 I wild right up to the guinea-pig. If the guinea-pig is dominant, or 

 almost so, one would expect little or no evidence of segregation. 



(4) It would be interesting to know whether the small C. rufescens 

 was derived from a larger species such as C. aperea, C. cutleri, or C. 

 porcellus by the loss of size factors, or whether the larger species arose by 

 progressive variations from this small wild species. 



15. THE SKULL SUTURES. 



Among other characters which differentiate the wild C. rufescens 

 from the guinea-pig, the nasal-frontal suture and frontal-parietal suture 

 appear to be prominent. In the wild, the suture between the nasal 

 and premaxillary bones and the frontal bones forms an M. The 

 caudal margin of the nasal bones forms a V, and with the premaxil- 

 laries the whole suture is more or less M-shaped. In the tame, 

 this suture is approximately truncate. The suture between the frontal 

 and parietal bones in the wild is practically a straight line; but in the 

 tame this same suture dips distinctly backward (see figs. 10, 11, 15, 

 16, and 31). 



