PREFACE 



The present volume is a somewhat extended form of the 

 Joseph Leidy Memorial Lecture in Science delivered at 

 the University of Pennsylvania March 3, 1936. In ad- 

 dition to the historical summaries it consists in large 

 part of the recent studies by my students and myself 

 on the means of activating color-cells in the higher ani- 

 mals and on the significance of these processes for the 

 workings of the nervous system. It is believed that the 

 idea of the neurohumors, set forth in numerous earlier 

 publications and rather fully elucidated in this volume, 

 has a measure of truth in it for general nervous func- 

 tions, and it is one of the objects of this essay to point 

 out some of the reasons for accepting this idea and for 

 testing its further applications. The whole proposal is 

 quite obviously in a formative stage and, as every inves- 

 tigator knows, its outcome must await further study. 



The invitation to deliver the Leidy lecture came to 

 me from a committee consisting of Dr. Josiah H. Penni- 

 man, Provost of the University of Pennsylvania, Dr. 

 Eliot Clark, Dr. Milton Greenman, and Dr. C. E. 

 McClung. To these gentlemen I wish to express my 

 keen appreciation of the honor of this invitation and the 

 great pleasure I take in accepting it. A certain personal 

 gratification that I feel in this acceptance I have at- 

 tempted to indicate in the Foreword. 



I cannot conclude this brief preface without acknowl- 

 edging with sincere thanks the aid in preparing the 

 manuscript for this volume received from my wife, 

 Louise Merritt Parker, and from my assistant, Helen 

 Porter Brower. I am also greatly indebted to Dr. F. M. 



