viii PREFACE TO THE FIRST EDITION 



are to be avoided, and the science is to be allowed free develop- 

 ment, it appears to me indispensable to return to Muller's method. 

 For this reason I have dedicated the following pages to the memory 

 of that great physiologist. 



The plan of the present book first assumed fixed form during a 

 journey which I made in the year 1890 to different points on the 

 Mediterranean Sea and the Red Sea for the purpose of making 

 comparative-physiological researches. After my return my uni- 

 versity lectures in Jena gave me an opportunity to present the 

 collected material in connected form. But the greater part of the 

 labour remained to be performed, and in the summer of 1892 I 

 began the writing of the book. Although for nearly ten years I 

 have been busy with the problems of general physiology and have 

 endeavoured to contribute something to their solution, so much 

 labour has been associated with the collection, examination, selec- 

 tion, completion, and arrangement of the much scattered material, 

 that the book has progressed slowly and with varied feelings on 

 my part. I have often wondered whether the result would accord 

 with the enthusiasm and love with which the task was undertaken. 

 Only the criticism of my colleagues can decide this. It is not to 

 be expected that a book which brings together for the first time 

 in a unified form a mass of material hitherto regarded as hetero- 

 geneous, shall upon its first appearance pretend to completeness. 

 I cherish no illusions that I have succeeded more than approxi- 

 mately. I am fully aware that many faults and errors must have 

 crept in, and these I beg my colleagues in friendliness to correct. 



It has afforded me especial satisfaction that one of my American 

 colleagues, Professor Frederic S. Lee, of New York, in an address 

 before the New York Academy of Sciences ('94), has developed 

 simultaneously and independently the same ideas regarding the 

 claims of modern physiology as are presented in detail by myself 

 in the first chapter of this book. These ideas have also been 

 expressed by me elsewhere, especially in an article in the Monist 

 (Chicago, '94). 



If a book is to reach a wide circle of readers, its language must 

 be neither too technical nor too prosaic. I have endeavoured to 

 comply with this requirement. I wished to write something that 

 would appeal first to my fellow physiologists, and offer them, 

 besides certain new facts and ideas, a summary of our scattered 

 knowledge. But at the same time I wished the work to give to 

 any interested scientific reader, whether a student of medicine, 



