EFFECTS OF INBREEDING ON FERTILITY AND VIGOR 337 



The number and average size of the Htters produced in each of 

 the first twenty-five generations of the A series of inbreds are 

 given in table 1. Similar data for the litter production in the B 

 series of inbreds are shown in table 2. 



These tables are inserted chiefly for reference, but a com- 

 parison between corresponding data indicates clearly that the 

 fertility of the animals in one inbred series was about the same as 

 that in the other series. The summary of the data for the two 

 series shows that the 1752 litters in the A series contained an 

 average of 7.5 young, while the 1656 litters in the B series had an 

 average of 7.4 young. This close agreement in the records for 

 two such large groups of animals is doubtless due to the fact that 

 all of the inbred rats were descended from the same ancestral 

 stock and that individuals in corresponding generations of the two 

 series were reared simultaneously under similar environmental 

 conditions. 



The data in table 1 and in table 2 have been combined in table 

 3, which thus shows the number and average size of the litters 

 produced in the first twenty-five generations of the inbred strain. 

 The data given comprise the records for 3408 litters containing 

 25,452 individuals. 



To facilitate the discussion of the effects of inbreeding on 

 fertility the data given in table 1 to table 3 were combined by 

 generation groups. There were relatively few individuals in the 

 first six generations of inbreds and their data were united to form 

 the first group, since the character of the experiment was changed 

 at this point. Data for subsequent generations were divided into 

 five groups, each of which, with the exception of the last, com- 

 prised the records for four successive generations. Such a 

 division of the data was, of course, purely arbitrary, but it seemed 

 the most satisfactory arrangement possible. A group of four 

 generations covers approximately the litter production for one 

 year, and as the number and size of the litters vary considerably 

 at different times of the year, this grouping assured a uniform 

 distribution of the seasonal variations in litter size among all of the 

 various groups. 



Litter data for the A series of inbreds, arranged according to 

 generation groups, are given in table 4. 



