GENETIC STUDIES OF RABBITS AND RATS. 23 



The fact is appreciated that these are at best rough estimates, 

 since (1) it is improbable that all genetic factors affecting size have 

 individually the same amount of influence, and (2) it is improbable 

 that the FI generation is in each case devoid of genetic variability, 

 both of which assumptions are made in the formula employed. 

 Nevertheless, the results may have some value as indicating whether 

 many or few chromosomes are concerned in the inheritance of size in 

 these crosses, if we adopt the chromosome hypothesis. Any addi- 

 tional test which we can apply to that hypothesis will have cumula- 

 tive value. 



The number of chromosomes in the rabbit, according to the sum- 

 mary of Miss Harvey (1920), is estimated by the most recent ob- 

 servers at 10 to 12, an earlier estimate being 14 to 18. It would 

 accordingly seem probable, on the chromosome hypothesis, that all 

 the chromosomes are concerned in size inheritance in such a wide cross 

 as that between Polish and Flemish Giant rabbits, while in the other 

 crosses part only of the chromosomes are concerned. 



We should by this method not expect to find a number of factors 

 indicated greater than the total number of chromosomes, since even 

 if several or many genes in a single chromosome influenced size, 

 these would not appear in the general result as independent agencies, 

 but as a single agency consisting of a linked system. The result given 

 by this formula must accordingly be interpreted, not as indicating 

 the total number of genes affecting size, but as the probable number of 

 chromosomes (or linkage systems) containing such genes. 



SIZE AND SEX. 



The relation of size to sex in rabbits has already been discussed 

 briefly in connection with weight; we may now discuss this question 

 further, and in relation to bone-dimensions and ear-length as well 

 as weight. For this purpose, the groups of cross-bred rabbits afford 

 the best material, because their numbers are largest and they are 

 free from possible effects on size of different degrees of inbreeding. 

 The pertinent facts are brought together in table 32. It will be 

 observed that males are consistently larger in all bone measurements, 

 but females surpass in weight. Yet none of the differences is very 

 great. It is evident that the male is a bigger-framed animal than 

 the female (as in mammals generally), though the female puts on 

 more flesh. But the size-differences of the sexes are less than in most 

 mammals. In length of leg-bones the male exceeds the female by 

 percentages ranging from 0.5 to 1.5. In skull-length the difference 

 is very slight, 0.1 or 0.2 per cent. But in males the skull-width, 

 between the outer edges of the zygomatic arches, is greater by 2 or 

 2.5 per cent. Wright (1918), on the basis of MacDowell's observa- 

 tions, assumed that sex is a specific differential factor affecting the 



