1903.] 



Observations on the Sex of Mice. 



35 







o • 







produced 



9 



17 



Q1.2 



)> 



3 



11 





?j 



Z 





01-2-2 



5) 



2 



5 



01. 2-2.1 





5 



10 







21 



46 



(31 per cent.) (69 per cent.) 



On the other hand C 1 ' 3 had 17 male and 6 female young, but she 

 was a daughter of e not of a. There is, however, an influence at work 

 in this family which may possibly account for the excess of females 

 over males, and it is that the same buck a was largely responsible for 

 the results, as each of the does was put to him; thus 1 * 2 - 2 * 1 - was his 

 daughter on the male side, and his great, great granddaughter on the 

 female. It has already been noted that cc was a buck who produced 

 practically the normal proportion of young, so that his influence alone 

 is not likely to account for the excess of females. But the practice of 

 inbreeding a buck with his daughter, granddaughter, &c, for several 

 generations may perhaps account for an excess of female offspring; 

 this theory being strengthened by the fact that 1 - 2 -2-i- was the only 

 doe with which the male-producing buck v had more females than 

 males in a litter. The clue is one which we are now following up and 

 which we would suggest to other breeders as worthy of further 

 investigation. On the whole, our statistics seem to point to the fact 

 that certain bucks and does tend to produce a preponderance of one 

 sex, but that the influence is greater in the male parent ; also that a 

 doe which is the result of prolonged inbreeding is more likely to 

 produce female than male offspring. 



The next point inquired into refers to the possibility of the number 

 of young in a litter exerting any influence on the proportion of the 

 sexes. If there is any basis of fact in the theory that the amount of 

 nourishment an embryo receives affects the determination of its sex, we 

 should expect that large litters would show a predominance of one sex and 

 small litters a predominance of the other. Up to the present we have 

 neglected the young which were eaten by their mothers before their sex 

 had been determined, because we have no reason to believe that the 

 mother preferred to eat male or female young, and, in taking large 

 numbers, we have presumed that as many male as female young would be 

 eaten in this way. We have, however, kept a record of the total number 

 in each litter when first seen ; often an hour or two, and never more 

 than 12 hours after the birth. A doe hardly ever eats the whole of a 

 young one at once, as she apparently prefers to first eat the viscera 

 and brains of several, leaving their carcases for a future meal. So 

 that, even after 12 hours from the birth, it is easy to see, by the 



D 2 



