THE CURVE OF LIFE 149 



of some other mammals, especially those with a 

 play-period, there is, as Dr. Groos and Dr. Chal- 

 mers Mitchell have so well shown, an adaptation 

 which secures the breaking down of rigid instincts 

 and their replacement by the remembered results of 

 free and intelligent experiment. (3) While man is 

 a slowly varying creature, changing but little from 

 age to age in the organic punctuation of his life, 

 he is eminently plastic or modifiable, and therefore 

 able, probably to an extent unsuspected, to lengthen 

 out his youth, to prolong his period of cerebral 

 variability, and to shorten his senescence. In all 

 of which there is a great hope, 



