EFFECTS OF INBREEDING ON FERTILITY AND VIGOR 345 



was the smallest in the litter series, as a rule; the second litter 

 was the largest ; the third and fourth litters were intermediate in 

 size between the first and the second ; a similar relation in the size 

 of litters has been found, also, in two groups of stock Albinos. 

 In both inbred series the first two litters cast in each generation 

 were the offspring of brother and sister matings; the third and 

 fourth litters were produced by the mating of an inbred female 

 with an unrelated stock male. In the first of the stock series 

 noted (King and Stotsenburg, '15, table 7) all of the litters were 

 produced by the pairing of unrelated stock animals ; in the second 

 stock series, for which data are given in table 7 of the present 

 paper, all of the litters obtained were the offspring of brother 



TABLE 7 



Shoiving the average size of each of the first four litters produced by a series of stock 



Albino females 



and sister matings. Since in all three groups the average size 

 of the litters in the litter series varied in a similar way, it is evi- 

 dent the litter size does not depend at all on the relatedness or 

 the unrelatedness of the parents, but chiefly on the age of the 

 female. Young females tend to be somewhat less prolific than 

 older ones, as Crampe ('83) noted. The litters reach their 

 maximum size when the females are about five months old, but 

 the number of young does not decrease appreciably in the various 

 litters cast until the females have passed the height of their repro- 

 ductive power at about seven months of age (King, '16b). 



As shown in the first paper of this series (King, '18), the rats 

 in the seventh to the ninth inbred generations were considerably 

 heavier, at any given age, than the individuals belonging to sub- 

 sequent generations. The cause for this unusually vigorous 



