21 



pear later. In many cases tlie actual tracings were kept for 

 reference. 



EXIIHIMEIITAL liliJULTo. 



It was planned to work v/ith five groups of rats, twenty- 

 five, si>:ty -five, two hundred, three hundred and five hundred 

 days old res} ectively, since it was thought that these ages 

 represented fairly well the successive stages in the growth 

 and development of the animal; twenty-five days for youth, 

 aixty-five days for sexual maturity, two hundred days for ma- 

 turity, three hundred days for age, and five hundred days 

 for old age. The attempt was made to have thirty rats in each 

 groui , hilt sickness and unavoidable accidents among the animals 

 have brought the number 3omev;hat lower. it has been found ex- 

 tremely difficult to obtain rats for the last group (500 days). 

 Although Slonaker finds the average length of life of the v.hite 

 rat to be thirty -four months, and Donaldson gives it as three 

 years, from tl^rea hundred to four hundred days is the ma>:imum 

 longevity for most of the rats used in this laboratory, and up 

 to this tire only ten have lived to work at the five hundred 

 day age, one of these dying ap.iarently of old age before the 

 problem was learned. 



The groups used, v.ith the number of rats in each group 



1. op. cit. Jourl. An. Behav. jp. 37-38, tables. 



2. Dr. r/atson has informed the writer that his exj crience was 

 quite similar, very few of his rats living to be more than 

 500 to 600 daya old. 



