406 



C. M. CHILD 



occur with increase in size in Planaria as in other animals gener- 

 ally, and size therefore serves as a criterion of physiological con- 

 dition so far as it concerns these changes. Whether we call these 

 changes physiological senescence or something else is purely a 

 matter of definition. They certainly consist in a decrease in 

 rate of the fundamental activities of life, and if they go far enough 

 either death or a break-up of the individual with reorganization 

 and regressive changes occurs. For the present there seems to 

 be'^ample'"justification for designating as physiological senescence 



B 



I 



the progressive changes in condition associated with increase of 

 size in Planaria and so for using size as a criterion of physiological 

 age. 



Table 1 shows the effect of this factor of size or physiological 

 age upon head-frequency. Each pair of lots, la, Ila, etc., gives 

 the percentages of the different types of head in corresponding 

 pieces from animals of two different sizes. In every case the 

 head-frequency is distinctly greater, usually much greater in 

 the pieces from the larger (older) animals. The experiment has 

 been performed many times with various sizes of animals and 



