PREFACE ix 



treat these relations more fundamentally than is usual in 

 physiological publications. All this has contributed to expand 

 the book, perhaps unduly, beyond its natural limits. Again, it 

 may be objected that the whole survey is taken from one definite 

 standpoint, so that individual chapters are perhaps treated in too 

 one-sided a fashion. But I frankly confess to having thought 

 less of avoiding a subjective tinge by the elimination of every 

 partisan consideration, than of showing how the phenomena 

 range themselves under that point of view from which I formerly 

 learnt to judge of them from my venerated master, Hering. 



In dedicating this book to him as an unworthy token of my 

 esteem and gratitude, I am well aware that I am only giving 

 back what I formerly received from him. 



JENA, November 1894. 



