I()8 HELEN DEAN KING. 



before reaching maturity, and the 114 rats for which n-o.nl- are 

 at hand form a very great majority of all of UK- offspring pro- 

 duced. Of these individuals 61 \\eiv males and 5.^ \\erc femaK -: 

 this gives a sex ratio of 115. 0() male- io 100 female-. 



In the other series of experiments the majority of the indi- 

 viduals belonging to the F 2 generation died while immature, and 

 records were made for only 27 individuals. Since twice as many 

 females as males reached maturity, it is evident that in this 

 instance either the mortality was much greater among the young 

 males than among the- young females or that there was a very 

 unequal distribution of the sexes in the newborn rats. 



Individuals from only one lot of hybrids belonging to the 

 second generation were mated. The animals were paired accord- 

 ing to color, their possible blood relationship being entirely dis- 

 regarded. Dr. Hatai found, as have other investigators who 

 have bred hybrid rodents, that there is increasing infertility 

 among individuals belonging to succeeding generations of hybrids. 

 The total number of hybrid offspring belonging to the F 3 genera- 

 tion was relatively small: many of the rats were stunted in their 

 development, and the majority of them died before reaching 

 maturity. Since records are available for only 23 of these 

 individuals the sex ratio among them can give no idea of the 

 probable ratio in a large number of hybrids belonging to tin- 

 third generation. 



Since in mating individuals belonging to the first and to the 

 second generation of hybrids no attention was paid to their 

 blood relationship, it is very probable that in >ome cases closely 

 related individuals were paired. There is the po ibilitv, there- 

 fore, that inbreeding might have had some influence on the sex 

 of the descendants. According to Diising ('84), a noticeable 

 increase in the number of male offspring i> produced 1>\ inbreed- 

 ing, in man as well as in various mammals. Inbreeding could 

 have had little, if any, influence on the sex ratio- in the-e \ ,u ion- 

 lots of hybrid rats. The sex ratio found among the 114 hybrid.- 

 of the Fo generation which comprise practical!) all of the off- 

 spring produced in the series of experiment- to \\hich they be- 

 longed, is somewhat less than that found among the offspring 

 produced by crossing pure Mock (.Table II.). The data tm 



