CHILD'S SENESCENCE AND REJUVENESCENCE 425 



whose tissues possess a large amount of highly differentiated and 

 relatively stable substance of low metabolic rate (the ' old ' 

 type of tissue) and in addition a more labile substance which can 

 be rapidly mobilized in functional activity without disturbing 

 the pattern of the more stable framework. This second type 

 is characteristic of the brains of higher vertebrates, where there 

 is (1) a very definite stable and heritable organization which 

 expresses itself functionally in the reflex and instinctive life, and 

 (2) pervading this organization the more labile substance which 

 provides for the individual modifiability of behavior, including 

 the capacity of learning by experience and all higher conscious 

 functions. 



The preceding summary includes a few only of the important 

 conclusions which Child draws from his experimental studies 

 upon lower forms of life. It should be emphasized that these 

 conclusions are not speculative, but rest upon a very large body 

 of carefully controlled experimental work, only a small part of 

 which is presented in detail in the two books here reviewed. 

 The application of these fundamental principles of organic 

 growth, reproduction, and regulation to the higher forms of 

 behavior is not attempted in these works, though a few hints 

 are dropped which give promise of possible applications in the 

 realms of comparative neurology, psychology and sociology (see 

 Individuality, pp. 173, 178, 188, 205). It is hoped that the author 

 may himself elaborate these suggestions in more detail in subse- 

 quent publications 



