LIFE PROCESSES IN CAPTIVE GRAY RATS 45 



this excess being most marked in offspring produced near the 

 close of reproductive life (table 14). The change in the sex 

 ratio as the age of the mother advanced is shown by the graph 

 in figure 13, which was constructed from data in table 14. 



Beginning relatively high, the graph in figure 13 falls gradu- 

 ally for a time, rises again at the middle of its course, and 

 then declines until its end. This cyclic change in the sex ratio 

 with the advancing age of the mothers is very similar to that 

 found in the four strains of rats, mentioned above, where data 

 were arranged by parity. The accord in these various series 

 of ratios can hardly be a matter of chance, although in none 

 of them are differences between the ratios in various groups 

 important when judged solely by their probable errors. 



Fig. 13 Age of the mothers and the sex ratio in the young. 



Few of the studies on lower mammals in which data have 

 been analyzed with reference to the relation between the age 

 of the mothers and the sex ratios in the young have given 

 consistent results, since the data were obtained, for the most 

 part, by the method of 'random sampling,' and therefore do 

 not cover the complete breeding records for a given group of 

 females. Investigations on the guinea pig, made by Ibsen 

 ('22), showed a difference of 6.06 times the probable error 

 between the sex ratio in young from mothers 8 months old 

 and that in offspring of females 15 months of age. In later 

 data obtained from this animal, Ibsen and Burhoe ( '28) found 

 no relation between the age of the mother and the sex ratio 

 in the young, "thus furnishing proof that results biometrically 



