xil PREFACE 



in the brain stem, it seems fair to presume that by means of these two sets 

 of calculations a fair degree of accuracy has been attained. 



Even though every method which reasonably could be appHed to the 

 material has been used in controlling these observations, the fact still remains 

 that this study lacks detailed investigation of the cellular elements of the 

 various nuclear components. The addition of this detail must be left for 

 later consideration, as the inclusion of such studies would of necessity too 

 greatly expand the size of the present work. 



In dealing with the brain stem the treatment has not been that of an 

 atlas, but rather that of a critical review with especial emphasis upon the 

 structures having the greatest evolutional significance. In the selection of 

 such structures two groups were recognized : 



First, those more plastic elements of recent acquisition which have been 

 especially susceptible to adaptive influences. 



Second, the more inflexible, archaic components of the brain stem, which 

 because of their great antiquity have acquired fundamental stability and 

 respond but little to the influences of adaptation. Chief attention is directed 

 to the first group from which a surprisingly rich harvest of evidence has been 

 gathered. 



The addition of the chapters dealing with the cultural phases of human 

 development and the brain of prehistoric man was made upon the suggestion 

 and with the assistance of Professor Osborn. 



Prof. William K. Gregory of Columbia University has given me most 

 generous assistance and I am especially grateful to him for his careful read- 

 ing and criticism of the concluding chapters. 



I also wish to express my appreciation to Miss Christine D. Matthews 

 of the American Museum of Natural History for her revisions in the tabu- 

 lation showing the fossil remains of prehistoric man. 



Throughout the entire preparation of the manuscript I have had the 



