EFFECTS OF INBREEDING ON FERTILITY AND VIGOR 349 



in the human race (Powys, '05) and also in poultry (Pearl, '17). 

 Most of the litters recorded were cast by young females that had 

 not reached the height of their reproductive power; such litters 

 tend to be larger than those cast after this time (King, '16b). 

 Data for litters cast by females of unknown age, however exten- 

 sive they may be, cannot, therefore, properly be used to furnish 

 the norm for litter size in the albino rat. 



In order to obtain standards for litter size with which the data 

 in the inbred strain might justly be compared, the complete 

 breeding history of a considerable number of stock Albino females 

 was recorded during the past three years. Data for the first four 

 litters produced by 116 females belonging to this group are given 

 in table 7. All of the stock rats from which these litters were 

 obtained were reared under the same environmental conditions 

 as the inbred strain. 



In table 7, as has already been noted, the litters of the series 

 bear the same size relation to each other as that found in the litter 

 series of the inbred rats. The first litter was the smallest, aver- 

 aging 6.2 young; the second litter, with an average of 7.3 mem- 

 bers, was the largest of the series; while the third and fourth 

 litters were somewhat smaller than the second. The entire 

 series of 424 litters gave an average of 6.7 young per litter. This 

 average is 0.3 less than that in the random collection of stock 

 litters previously recorded (7.0), and 0.4 more than the norm as 

 given by Crampe (6.3), so it is probably a fair standard for litter 

 size in any similar series of Albino litters. There is no reason to 

 believe that the stock females from which the litters recorded in 

 table 7 were obtained were, as a group, inferior in reproductive 

 power to other stock females, and presumably their fertility at 

 any given age is fairly representative of that in the general run 

 of stock Albinos. 



Each litter of the stock series, shown in table 7, contained a 

 smaller average number of young than the corresponding litter 

 in either of the two inbred series when the data were arranged 

 according to generation groups (tables 4 and 5), and, omitting the 

 records for the first five generations where the fertility was les- 

 sened by malnutrition, there was not a single generation in either 



