I. LIFE PROCESSES 19 



Although the average age at which the females of this 

 group began breeding was 252 days (table 6), it is probable 

 that the great majority of them had become sexually mature 

 by the time they were six months old. At this age, which 

 marks approximately the merging of the first growth phase 

 into the second, the average body weight of the males \va> 

 59 grams greater than that of the females. This difference is 

 25.3 per cent, if the body weight of the males is taken as tin- 

 standard in computation. In adult life both males and fe- 

 males became increasingly heavier, but the percentage differ- 

 ences in their weights changed but little. The maximum dif- 

 ference, 27.7 per cent, was reached when the rats were 578 

 days old. 



As shown in table 1, no female in the first group attained 

 a maximum weight exceeding that of a male of the same age, 

 nor was the lowest weight recorded for any male below that 

 for a female of like age. 



Growth data are given in table 2 for the second group of 

 captive Grays, comprising individuals in the fifth to the 

 seventh generations, inclusive. 



Differences between the average body weights of males and 

 females during early postnatal life were about the same in 

 the second group as in the first one (compare table 1 and 

 table 2). At 182 days, however, males in the second group 

 exceeded the females in body weight by only 47 grams, -i 

 19.6 per cent. The maximum male excess in weight, L'l.fi per 

 cent, came at the age period of 578 days. 



Maximum body weights for both sexes were higher in tin- 

 second group than were those in the first group, and tin- 

 average weights were greater at all age periods. 



Data for the third group, including individuals in the eighth 

 to the tenth generations, inclusive, are shown in table 3. 



In the third group, as in the two preceding ones, sex dif- 

 ferences in the average body weights were small during tin- 

 early growth period, and increased as the animals grew older. 

 The male excess in weight at 182 days was 4'J grams (17.4 

 per cent), and its maximum (21.3 per cent) came at 578 

 as >hown in table 3. 



