PREFACE 



TEXT-BOOKS of science may be placed in two classes. 

 There are those which aim at fullness of statement and 

 seek to acquaint their readers with experimental methods 

 and original sources. Such books must give a large place 

 to controverted matters, weighing conflicting evidence 

 and comparing the views of various workers. The mak- 

 ing of them is properly in the hands of great masters of 

 the several branches. 



Other books have a more modest scope. Their pur- 

 pose is to present concisely the accepted facts with only a 

 limited description of the experiments by which these 

 facts have been established. They contain compara- 

 tively little about unsettled questions though they are at 

 fault if they do not make it plain that these confront the 

 investigator at every turn. They may be written by 

 teachers who have not lost the point of view of elementary 

 students or ceased to sympathize with them in their 

 perplexities. 



The present book belongs definitely to the second class. 

 An extreme course of action has been adopted with regard 

 to the accrediting of discoveries. It is certainly a source 

 of irritation and bewilderment to the beginner to have 

 the pages he reads sprinkled thickly with the names of 

 men of whom he has never heard before. In the chapters 

 that follow no mention is made of living experimenters 

 though a few eminent physiologists of earlier times are 

 referred to. It has been hard not to make exceptions 

 and the use without acknowledgment of illuminating 

 ideas and happy teaching devices which I owe to my con- 

 temporaries has aroused a feeling akin to guilt. Some 

 atonement may be found in the list of collateral readings 

 at the end of the book, 



P. G. S. 



