PREFACE. 



WE are told that this age is one of nervous strain. 

 Probably in their own time the monsters of cretaceous 

 days would have expressed a similar opinion, had the 

 opportunity been granted them. From the beginning, 

 the outer world has modified all animals possessed of 

 a nervous system mainly by its aid, and as far as this 

 system could alter its reactions, just so far could the 

 animal adapt itself to the changed environment. 



In this generation, to be sure, our cephalic centres are 

 sometimes overworked, whereas in the remote past 

 the stress fell more on other parts ; but we are rather 

 allied to the rock-bound dead by an inherited power to 

 respond than separated from them by a recent capacity 

 for nerve exhaustion. 



As the reader will perceive, these remarks might 

 serve to magnify the office of this book by suggesting 

 how fundamental to the welfare of all higher animals 

 are the powers of the nervous system, and therefore 

 how important it will be to search out the growth 

 changes which produce them. 



It seemed desirable to bring together in a compre- 

 hensive way the facts bearing on this portion of the 

 problem. In discussions upon growth, the valuable 

 records on brain-weight are sometimes alone brought 

 forward, but there exist a vast number of other facts, 

 which, when joined with these, illuminate not only them, 

 but the entire field of view, and indicate the unworkecl 

 areas within it. I have therefore sought especially to 



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