106 HELEN DEAN KIN',. 



to each 100 females ranges from 100.77 to 105.55 in various 

 cases. Among these races, tl KMT I ore, the same relation of tlic 

 sexes exist as is normally found \\ hen any large number of human 

 births are examined. When the parent^ \\ere of different racial 

 stock there is a marked increase in the relative' number of male-; 

 the sex ratio showing from 105.72 to 106.74 males for each 100 

 females. The general conclusion reached by these investigators 

 i- that "there is a definite tendency towards an excessive pro- 

 duction of male offspring in cross as compared with pure matin- - 

 in the data considered. Further, it appears that this tendency 

 i- uniformly exhibited in all the mating." 



The records given above comprise all the sex data for consider- 

 able numbers of hybrid offspring that I have been able to find. 

 All of these records are in complete accord, since in each case 

 there is an excess of males that is much greater than that which 

 i- probably normal for the parent stock. 



For several years past Dr. S. Hatai, of the Wistar Institute, has 

 been crossing the wild Norway rat (Mus noroegicus] with the 

 albino rat (Mus noroegicus albinits] and breeding their descend- 

 ants, in order to obtain material fora study of the central nervous 

 -\~tvm in hybrid rats. The records for the many hybrid off- 

 spring that have been examined include the sex data, and Dr. 

 Hatai has generously offered me the use of these records for the 

 study of the sex ratios that is given in the present paper. A- 

 far as I am aware, there are no published statistics regarding 

 ilif proportion of the sexes among hybrid rats, although three 

 investigators, von Fischer ('74), Crampe ('^4) and Bos ('1)4), 

 have carried on extensive hybridizing and inbreeding experiment.^ 

 with these animals. 



Cuenot ('99) examined 30 litters of voting albino rats containing 

 a total of 255 individuals, and found among them a sex ratio of 

 105.6 males to loo females. Records which I have made of the 

 sex of 452 young albino rats belonging \<> NO litter-- uive a sex 

 ratio of 107.33 males to 100 females. The sex ratio in the 

 albino rat, therefore, agrees with that for man and variou- other 

 mammals, since the number of the male-, i- ^li^htK greater than 

 that of the females. 



No statistics have as yet been collected re-anlin- the normal 



