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 
