GENETIC STUDIES OF RABBITS AND RATS. 



11 



(plate 1, F) specifies " Bucks shall be not less than 11 pounds and 

 does not less than 13 pounds." An American publication gives the 

 standard weight for Japanese rabbits as "bucks 7 pounds, does 8 

 pounds," that of French Silvers as "bucks, 9 pounds; does, 10 pounds 

 or over." 



The existence of such specifications in breed standards implies 

 that, at least in large races of rabbits, the female either is naturally 

 larger than the male of like genetic constitution, except as to sex, or 

 else that the breeders have some expectation of making it so. 



The relative weight of the two sexes in the groups of rabbits which 

 I have studied is shown in table 2. In the Polish breed, males were 

 found to be slightly heavier than females. The single male Himalayan 

 studied differed very little from the average of the 5 females; but in 

 the cross-bred rabbits of all three combinations, viz, Polish-Hima- 

 layan, Polish-Flemish, and Himalayan-Flemish, females were heavier 

 by from 50 to 200 grams (2.5 to 7.5 per cent) than were males derived 

 from the same cross. It seems clear, therefore, that the breed stan- 

 dards are correct in recognizing the greater weight of females than 

 of males, at least in the heavier breeds of rabbits. The difference is a 

 natural one, not arbitrary, but the amount of the difference has 

 probably been exaggerated in the standards, which call for a difference 

 of about 15 per cent. This is double the greatest difference observed 

 by me. 



INHERITANCE OF WEIGHT. 



Individual variation in weight can 

 methods. Maximal first-year weight 

 the few cases of rabbits received as 

 adults. For the present, the slight 

 difference in size of the two sexes 

 will be disregarded. The general 

 character of the variation in weight 

 is presented graphically in figures 4 

 to 6. The data on which these graphs 

 are based will be found in table 3. 



Figure 4 shows that the modal 

 weight of Polish rabbits is about 

 1,400 grams, while Himalayan rab- 

 bits are about 400 or 500 grams 

 heavier. The Fi cross-breds are 

 heavier than either pure race, as 

 shown also in the growth-curves (fig. 

 3) . They vary symmetrically around 

 the class whose center is 1,949 grams, 

 designated class 19 in figure 4. The 

 Fj generation, which contained just 



best be studied by statistical 

 will be dealt with, except in 



NO. 



F^.PXH 



F..PXH 



H 



nri P^3 



10 II 12 13 UVI5 16 17 18 I9ZOZI ZZ 23 24 

 WEIGHT, IN HEKTOGRAMS 



Fia. 4. Polygons showing variation in 

 weight of pure Polish (P) and pure 

 Himalayan (H) rabbits, and of their 

 Fi and F t hybrid offspring. 



twice as many individuals as 



