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. ‘[n all 
of which there is a great hope, 
