34 JAMES ROLLIN SLONAKER 



lifetime at which certain arbitrary fractional parts of the total 

 amount of work was performed. The males had accomplished one- 

 half of their total activity at the age of 10 months, which rep- 

 resented 36.4 per cent of the whole life time. When the female 

 had performed half of its total activity it had reached the age 

 of 13.8 months' or 41 per cent of its lifetime. This was at a 

 trifle later age than when this rat accomplished its greatest 

 amount of daily activity. 



This table shows us some very interesting things. It shows 

 that the males turn off their work at an earlier age in life, rep- 

 resenting a lower percentage of lifetime than the female. This 

 is true in the case of each of the arbitrary fractional parts of 

 .their total work which has been selected. One-half of the total 

 work has been accomplished when but 36.4 per cent of the 

 male and 41 per cent of the female's life has been passed. When 

 seven-eighths of the total work was done they still had about 

 30 per cent of their lives to live. In other words only one-eighth 

 of their work w T as still left for them to do during this compara- 

 tively long time. This shows a marked contrast in the activity 

 of youth and old age. In the young males for example it re- 

 quired but 1 6. 1 per cent of their lifetime to do one-eighth of 

 their life work, while in old age it required 32.1 per cent of 

 their lifespan to perform the same amount. At this latter per- 

 centage of lifespan they had accomplished in their youth three- 

 eighths of the total work. 



Since these animals were in no way compelled to work but 

 did it of their own volition we can readily conclude that we 

 might expect a greater amount of work during youth up to 

 middle age than in the latter half of life. I think one is justified 

 in saying also that such animals could be driven to do a much 

 greater amount of work during the first half of their life than 

 the last half. From general observations and these data I 

 believe one is warranted in saying that similar results would 

 be obtained from other animals including man. 



THE RATE OF GROWTH AND LENGTH OF LIFE 



Owing to the fact that the mothers sometimes destroy their 



young when they are disturbed no weights were made of the 



young until they were weaned. Other investigators have studied 



the growth of other animals from birth. They have come to 



