20 



BIOLOGY 



panied by increasing activity up to the age period of 87-120 

 days, after which the activity declined. 



On the assumption that the span of life in man is thirty times 

 that of the albino rat (Donaldson, '08) this age of greatest ac- 

 tivity would correspond to the age of 7.5-10 years in man. 



As shown by the records of activity (Slonaker, '12) the albino 

 rat is nocturnal. This habit can be modified more or less by 

 feeding or by disturbance during the day time. 



The measure of activity in the cases observed by Slonaker 

 was the number of turns of the revolving cage in which the ani- 

 mal was kept, the cage being set in motion by the voluntary run- 

 ning or other movements of the animal, and the revolutions be- 

 ing automatically recorded. In the case of four rats kept in 

 separate revolving cages from 30 days of age until natural death, 

 the following record of activity was obtained (Slonaker, '12). 



TABLE 2 

 Total number of miles run during life 



This table shows not only great variability in the total per- 

 formances, but also for the one female a record of over five thou- 

 sand miles in a little less than three years. On the average, three- 

 fourths of the total distance is run before the rat has reached 

 middle life, and the last months of old age are always marked 

 by greatly lessened activity. 



2. Span of life. The assumption has been made (Donald- 

 son, '08) that dating from birth, the span of life of the albino 

 rat is three years. A rat three years old therefore may be re- 

 garded as corresponding to a man ninety years old. So far 

 as this assumption has been tested, it appears to be a useful 

 approximation. 



