GENETIC STUDIES OF RABBITS AND RATS. 9 



(180 to 210 days) is also less abrupt and the increase in weight sub- 

 sequent to puberty is greater. The average weight at 360 days of 

 Polish rabbits of both sexes is less than 1,400 grams, for Himalayan 

 rabbits it is 1,800 grams, and for Flemish it is over 3,200 grams. 



In Polish rabbits, as compared with other and larger breeds, the 

 initial weight is less, the growth-rate less, and the completion of 

 growth comes earlier. All these phases of growth combine to make 

 the ultimate weight smaller. Comparison of the growth-curves of 

 Polish and Flemish brings this point out very clearly. The Flemish 

 curve is far above the Polish curve at the outset and diverges from it 

 increasingly up to the age 360 days. The Himalayan growth curve 

 lies everywhere between the Polish and Flemish curves, but is much 

 nearer to the Polish. In form, however, it is more like the Flemish 

 curve, in that growth continues longer after puberty. 



Fi rabbits produced by crossing Polish with Himalayan have a 

 greater initial weight than either pure parent race and grow faster. 

 At puberty (180 to 210 days) the F x rabbits surpass Polish by over 

 500 grams. They are more than 40 per cent heavier. As compared 

 with Himalayans of like age, they are more than 20 per cent heavier. 

 At age 360 days, the FI rabbits are 45 per cent heavier than Polish, 

 but only 9 per cent heavier than Himalayan, since the slower-growing 

 Himalayans have lessened the superiority which the FI rabbits showed 

 at puberty, but have not extinguished it. 



F 2 rabbits from this cross, obtained by mating the FI rabbits, 

 brother with sister or half-sister, show an average adult weight of 

 1,632 grams, which is intermediate between the adult weight of 

 Polish and Himalayan rabbits, the races originally crossed, being 

 20 per cent heavier than Polish and 10 per cent lighter than Hima- 

 layan. The superiority in size of the FI animals is a purely F t 

 phenomenon, as we shall see. It never persists into the F 2 generation. 

 Its existence is nevertheless an important practical consideration, 

 which makes grading and the crossing of breeds highly advantageous 

 under certain circumstances. 



In the cross of Polish with Flemish rabbits, F t is in size well above 

 the intermediate between the parent races, another illustration of 

 FI vigor, although in this case FI does not surpass the larger race in 

 size. The form of the growth-curve is also intermediate. Maturity 

 is attained early, as in the Polish race, but the practical cessation of 

 growth comes later than in the Polish race, at 240 days rather than 

 at 200. The adult weight of the FI rabbits, at the age 360 days, 

 averaged 2,539 grams, as compared with 3,240 for Flemish and 

 1,359 for Polish. The intermediate would be 2,300 grams, which 

 the FI group surpasses by more than 10 per cent. In the Himalayan- 

 Polish cross, FI surpassed the intermediate by over 20 per cent, and 

 Fj surpassed it by 3 per cent. 



