60 HELEN DEAN KING AND HENRY H. DONALDSON 



The mortality among the young during the suckling period 

 was considerable, as the mothers frequently destroyed or 

 neglected their young if the nest was disturbed or the young 

 removed for examination. Mortality among the young at this 

 period was still further increased by the fact that the adults 

 in the cage usually crowded into the nest when frightened, 

 and in consequence some of the young were often crushed 

 or suffocated. As the young rats were able to look out for 

 themselves after their eyes were opened, at the seventeenth 

 day of postnatal life, the mortality among them from this 

 time until the end of the suckling period at thirty days was 

 negligible. Litters finally selected for weighing and as 

 parents of the succeeding generation were those in which 

 there were no deaths until the animals had passed the age 

 of sixty days. 



Table 10 shows, for each generation, the number of indi- 

 viduals of each sex living at the age of twelve months, the 

 time when reproductive activity has passed its height and 

 the animals have not yet become seriously affected by lung 

 disease. This table also gives the number of animals still 

 living at twenty months of age, together with the percentages 

 of mortality that had occurred at the two ages specified. A 

 summary of the data by generation groups is also included. 



In all generations, as the data in table 10 show, mortality 

 was very low during the first year of life, averaging less than 

 4 per cent in both sexes. Deaths during this period were 

 due, in the main, to accident or to the savageness of some of 

 the rats that caused them to kill other individuals in the cage. 



The mortality increased greatly after the rats were a year 

 old, being due chiefly to lung infection. In the first genera- 

 tion 80 per cent of the individuals of both sexes died before 

 reaching the age of twenty months. The high mortality 

 among the older rats of this generation was probably due, 

 in part, to lack of adjustment to conditions of captivity which 

 rendered them very susceptible to disease. 



The change in mortality with the advance of the genera- 

 tions is shown in the lower part of table 10, where the data 



