16 GENETIC STUDIES OF RABBITS AND RATS. 



to 72.5 millimeters. The skull-lengths of 5 Flemish rabbits studied 

 range from 82.5 to 88 millimeters (see table 6). 



The cross between Polish and Himalayan rabbits produced off- 

 spring surpassing both parent races in skull-length, as in body-weight, 

 a manifestation, no doubt, of heterosis or cross-bred vigor. The 

 FI averaged 70.2 millimeters in skull-length, the larger parent race 

 averaging 68.9 millimeters, the intermediate between the parents 

 being 67.3 millimeters. The average skull-length of F 2 in this cross 

 was exactly equal to that of the larger parent. It would seem that 

 the pure Polish and Himalayan races may have been below their 

 genetic possibilities in skull-length, owing perhaps to inbreeding. 



The Himalayan-Flemish cross (table 6, FI, H.XF.) produced FI 

 animals averaging a little larger than the intermediate between the 

 parent races, and F 2 animals averaging a little less. But the F 2 

 animals were more variable than the F t animals, their standard 

 deviation in skull-length being 2.85 millimeters as compared with 

 1.79 millimeters for FI. The Polish-Flemish cross produced an F! 

 generation close to the intermediate between the parent races as 

 regards skull-length, but F 2 fell more than 2 millimeters below the 

 intermediate. Again F 2 was more variable than FI, the standard 

 deviations being 3.15 and 1.18 respectively. 



Tables 7 and 8 show the variation of the several groups of rabbits 

 in skull-width measurements taken anteriorly and posteriorly respec- 

 tively to the orbit. It appears that Polish rabbits, though weighing 

 less than Himalayan rabbits and having shorter skulls, nevertheless 

 have skulls slightly broader. The FI rabbits from the Polish-Hima- 

 layan cross have skulls broader than those of either parent (as well 

 as longer, see table 6), and this superiority is retained in part in the 

 F 2 generation. These statements apply both to the anterior and 

 to the posterior skull-width measurements. Although the Polish 

 rabbits have slightly broader skulls than the Himalayans, neverthe- 

 less they do not transmit as much skull-width in crosses with Flemish 

 as do the Himalayans, for in every case the skull-width of the Hima- 

 layan-Flemish cross-breds is slightly greater than that of the 

 Polish-Flemish cross-breds. The explanation probably is that the 

 Himalayans transmit greater general body-size (weight) in crosses 

 with Flemish than the Polish do. Skull-width is sufficiently involved 

 in the general increase of all bodily dimensions to more than offset 

 the specific tendency of Polish to transmit a broad skull; for in 

 skull-length (table 6), the Himalayan-Flemish cross-breds exceed 

 the Polish-Flemish cross-breds even more than in skull-width. 



The amount of variability in skull-width, as indicated by the 

 standard deviation (tables 7 and 8), is too erratic to have any par- 

 ticular significance. F 2 is not uniformly more variable than FI in 

 these cases. 



