10 GENETIC STUDIES OF RABBITS AND RATS. 



In the Flemish-Polish cross, as in the Himalayan-Polish cross 

 Fs falls below Fi in size. It is found to be 2,128 grams as compared 

 with 2,539 for Fi. It is even less than the intermediate, 2,300 grams, 

 by about 8 per cent. 



In the Flemish-Himalayan cross, the growth-curve for Fi is very 

 similar in form to the curve for pure Flemish. There is a gradual 

 falling in the growth-rate at about 210 days, which indicates the 

 usual slowing-up influence of puberty, coming about a month later 

 than in PoHsh rabbits. But growth continues even after this retard- 

 ing influence sets in, just as it does in Flemish rabbits. The weight 

 at 360 days is 2,781 grams as compared with a strict intermediate 

 between the parent races of 2,523 grams, which it surpasses by 10 

 per cent, about the same relation found in the Pohsh-Flemish cross. 

 Again F2 shows a falling off as compared with Fi to 2,466 grams, 

 which is 2 per cent less than the intermediate. 



To summarize the foregoing observations: 



(1) In races of small-sized rabbits the initial weight is small, the 

 growth-rate is low, and growth terminates early. Conversely, 

 rabbits of large-sized races have a large initial weight and growth 

 energy, which not only makes them grow faster, but also makes 

 them grow a longer time than do rabbits of small-sized races. 



(2) When races of small rabbits are crossed with a race of large 

 rabbits, the initial weight, growth energy, and duration of growth 

 are all intermediate in character, but are greater than the strict 

 intermediate in the Fi generation and approximate it or fall slightly 

 below it in F2. 



(3) When two races of rabbits (such as Polish and Himalayan) are 

 crossed, which do not differ greatly in size, Fi may surpass either 

 parental race in vigor of growth, though not in duration of growth. 

 Consequently the maximum advantage in size of Fi over the parent 

 races will be attained at puberty. F2 will approximate the interme- 

 diate in size between the parent races originally crossed. 



WEIGHT AND SEX. 



In the foregoing pages the average weight of groups of individuals 

 has been under discussion without reference to individual variation 

 within each group or the effect of crossing on such variation, or the 

 relation of weight to sex. In any discussion of the inheritance of 

 weight, these questions must be taken into consideration. First 

 let us consider the relation of weight to sex. The ''standards" of 

 the various breeds of rabbits formulated by breeders in England and 

 the United States make no specification as to the relative sizes of 

 the two sexes in the small breeds of rabbits, but in the large breeds 

 they regularly specify a larger size for females than for males. Thus, 

 the English standard for the variety Steel Gray Flemish Giant 



