PREFACE ix 



the problem of the nervous system in its physiological 

 aspects, it has seemed a task worth undertaking, even 

 though our knowledge is still fragmentary in many 

 respects, to attempt to point out the relations between 

 the gradients and the nervous system. As regards 

 many points, of course, only hypotheses or suggestions 

 are possible, but if these shall stimulate further investi- 

 gation and so aid, even indirectly, in advancing our 

 knowledge, their chief purpose will have been attained, 

 whether they are confirmed or shown to be incorrect. 



Since many different lines of investigation concerned 

 in the discussion are of necessity mentioned only very 

 briefly, it has seemed advisable to give references, so 

 far as it could be done without unduly breaking the 

 continuity of the text. The list of titles following the 

 text makes no pretensions to completeness, but includes 

 merely the publications to which reference is made. 



I am deeply indebted to certain of my colleagues and 

 co-workers in various ways: first of all to Dr. C. J. 

 Herrick, who has played no small part in the develop- 

 ment of the book. While it is by no means my intention 

 to shift to his shoulders any of the responsibility for 

 conclusions drawn or suggestions advanced, I feel it a 

 great pleasure, as well as a duty to acknowledge my 

 obligations to him for many fruitful suggestions made 

 in the course of personal discussions during the years of 

 our association in Chicago, for information on many 

 questions of fact, for his kindness in reading and criti- 

 cizing the entire manuscript, and finally for permission 

 to reproduce certain figures. I am also under great 

 obligation to Dr. L. H. Hyman of the Department of 

 Zoology for reading and criticism of the manuscript 



