GENETIC STUDIES OF RABBITS AND RATS. 15 



extend into the range of either parental race. A gap of 8 millimeters 

 separates the shortest-eared F 2 from the longest-eared Himalayan 

 rabbit, and a gap of 13 millimeters separates the longest-eared F 2 from 

 the shortest-eared Flemish. Hence it is clear that neither parental 

 combination is recovered in the total of 70 F 2 individuals studied. 

 Clearly, many independent factors must differentiate the parent races 

 as regards ear-length. 



The cross of Polish with Flemish is a still wider one than that 

 just described and F x is somewhat less uniform. It is, indeed, about 

 as variable as the F 2 obtained from the Himalayan-Flemish cross, 

 which fact indicates a greater genetic variability in the Polish than 

 in the Himalayan race as regards ear-length. Again, in this cross 

 FI falls below the intermediate between the means of the parent 

 races by about 5 millimeters. F 2 shows greatly increased variability 

 as compared with FI. The range has risen from 14 to 30 millimeters, 

 and the standard deviation from 3.76 to 6.05 millimeters (see table 4). 

 Nevertheless, the range of F 2 does not extend into that of either 

 parental race. Four vacant classes separate it from the range of 

 pure Polish and 20 vacant classes separate it from the range of pure 

 Flemish. It accordingly approaches the smaller race more nearly 

 than the large one (as in weight), but neither parental condition 

 reappears in F 2 , although this consists of 131 individuals. Again we 

 are forced to conclude that . numerous independent genetic factors 

 affecting ear-length differentiate the parent races. 



The question may be raised whether there is a difference between 

 the sexes as regards ear-length, as there appears to be in regard to 

 weight. Table 5 answers this question in the negative. The sexes 

 differ very little in ear-length and neither is consistently greater than 

 the other throughout the groups. 



BONE MEASUREMENTS. 



The length of the skull has been measured, in these studies, from 

 the notch in the ventral margin of the occipital foramen to the 

 anterior surface of the incisor teeth in a straight line, measurement 

 being made with a caliper rule and readings taken in tenths of milli- 

 meters. Differences in skull-length between the races of rabbits 

 studied are less in amount than those which distinguish these same 

 races as regards ear-length. But it is desirable to ascertain whether 

 skull-length differences are inherited in a similar way and whether 

 they are correlated with differences in ear-length and body-weight. 

 Hence the study of skull-length will have value, even though its 

 results are not as clear-cut as those derived from other studies. 



Polish rabbits have a skull-length of from 63 to 69 millimeters. 

 Himalayan rabbits have somewhat longer skulls, ranging from 66 



