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, F,, H.XF.) produced F t 

 animals averaging a little larger than the intermediate between the 

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

 animals were more variable than the F! 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 Fj 

 generation close to the intermediate between the parent races as 

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

 intermediate. Again Fj 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 F! 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 

 FI 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, is not uniformly more variable than F! in 

 these cases. 



