330 THE STUDY OF ANIMAL LIFE CHAP, xvi 



to prolong the juvenile period, when the young animals 

 are well cared for and given freedom to experiment 

 without too serious responsibilities. Groos in his Play 

 of Animals has worked out the idea that this play- period 

 is eminently educative of powers which are essential in 

 after-life. It also affords opportunity for the emergence 

 of variations before too stern and rigorous selection 

 begins. Animals, Groos says, do not simply play because 

 they are young ; they continue young that they may play. 



' For play is the young form of work, and the animals 

 who played best when young, worked best, lived best, 

 perhaps loved best, when they grew up." * In his Child- 

 hood of Animals Dr. Chalmers Mitchell has expounded 

 the important thesis that the purpose of youth is to give 

 time for the breaking down of rigid instincts, and their 

 replacement by action controlled by experience and 

 memory, by remembered results of experiment. To put 

 it in another way, youth is the time when co-ordinations 

 are established between the instinctive processes of the 

 lower brain-centres and the intelligent processes of the 

 cerebral cortex. Thus has youth especially prolonged 

 y out h been justified in the past ; so is it justified every 

 day. 



Adolescence. In the higher reaches of life it is possible 

 to distinguish between youth and mature strength a 

 period of adolescence. " This is an arc on the up-grade, 

 when juvenile characters are shed and adult characters 

 put on. There is a final acceleration of growth (which 

 suggests the value of a correlated abundance of rest and 

 play and food) ; there are internal rearrangements and 

 readjustments ; there is a sifting of idiosyncrasies, to wit 

 variations ; there is a selective criticism of that veneer 

 which we call modifications or individually acquired 

 characters ; and there is more than a beginning of sex- 

 impulses." 2 



Senescence and Rejuvenescence. Many reasons have 



been given to account for an organism growing old. 



1 The author's Biology of the Seasons, p. 227. (Melrose, London, 

 1911.) 



The author's Wonder of Life, p. 410. (Melrose, London, 1914.) 



