28 GENETIC STUDIES OF RABBITS AND RATS. 



individuals appeared in those crosses. Therefore cf3 can not have 

 transmitted angora coat. Accordingly, Flemish 9 7 must have 

 done so. She was not employed in the Flemish-Himalayan cross, 

 otherwise we should have expected to see angora individuals in the 

 F2 of that cross also. 



SUMMARY. 



1. Studies have been made of the weight, ear-length, and several 

 bone-dimensions of three races of rabbits, and of the first and second 

 generation hybrids between these races. 



2. Two of the races (Polish and Himalayan) are of small size, like 

 the wild rabbit of Europe, the ancestral species. The third race 

 (Flemish Giant) is very large, a racially new condition. 



3. Crosses between the pure races produce in general individuals 

 of intermediate size both in Fi and F2, so that the inheritance is 

 correctly described as blending. But when the parent races do not 

 differ greatly in size (as, for example, the Polish and Himalayan races), 

 the size of Fi individuals may be increased by heterosis beyond what it 

 would be through inheritance alone, so that the size of the larger pure 

 race is approximated or even surpassed. Nevertheless, with the 

 disappearance of the heterosis effect in F2 the average size of the 

 cross-breds sinks to a strictly intermediate position. 



4. The heterosis effect is seen in the Fi generation produced by 

 crossing a large with a small race, no less than in the cross between 

 two small races, but when the difference in size between the races 

 crossed is large, the heterosis effect is not sufficient to obscure the 

 essentially blending or intermediate character of the inheritance in 

 Fi. It merely produces a rise in the Fi average size above the strictly 

 intermediate position, which Fo in every cross closely approaches. 



5. The variability in size of F2 is regularly greater than that of Fi, 

 which in accordance with the multiple-factor hypothesis is regarded 

 as indicating the occurrence of genetic factors affecting size in several 

 different chromosomes or linkage systems. 



6. An attempt has been made by statistical methods to estimate 

 how many different chromosomes (or linkage systems) are concerned 

 in the inheritance of the size-differences found in the three race 

 crosses, with the following average results : for the Polish-Himalayan 

 cross, at least two chromosomes; for the Himalayan-Flemish cross, 

 about eight chromosomes; for the Polish-Flemish cross, ten or more 

 chromosomes. As the total number of chromosomes in the rabbit is 

 estimated at 10 to 12 pairs, it seems probable that all chromosomes 

 are concerned in size-inheritance. 



7. A study has been made of the correlation between weight, ear- 

 length, and the several bone-dimensions studied, with a view to dis- 

 covering whether the same genetic agencies influence size in different 



