

( L ! B f? A R % 



PREFACE TO SECOND EDITION. 



In writing' this volume the author has made no effort to give a 

 complete account of the Protozoa. As indicated by the title, it is 

 rather a study in biology illustrated by the unicellular animals. 

 The concept of a changing organization brought about by continued 

 metabolism was developed in the first edition. This conception 

 has been amplified in some respects, strengthened and condensed 

 in others, and furnishes the basis for an interpretation not only of 

 life histories but of the significant biological phenomena of cell 

 division, maturity, sex differentiation, fertilization and senescence 

 as well. To strengthen this conception a considerable change in 

 the order of presentation has been introduced. After the first intro- 

 ductory chapter we plunge at once in Chapter II into the sub- 

 stances and structures of the fundamental organization. This is 

 followed in Chapters III and IV by the development of these sub- 

 stances and structures into cytological derivatives (Chapter III) 

 and taxonomic structures (Chapter IV) of the derived organization. 

 In Chapter Y the general physiological activities are considered in 

 anticipation of Chapter VI on reproduction. The problem of gen- 

 eral vitality and its significance in fertilization and the accompany- 

 ing phenomena of sex differentiation, maturation, reorganization, 

 adaptation and variations are treated in Chapters VII, VIII and 

 IX. The special chapters on taxonomy, together with more elab- 

 orate keys to genera, are transferred from the middle of the book 

 to the end in Chapters XI, XII, XIII and XIV. 



Parasitism and disease should be considered in any work on 

 general biology. These topics were omitted in the first edition 

 but are introduced here in Chapter X. Another innovation is the 

 elimination of all references to chlorophyll-forming flagellates, the 

 protozoan flagellates being limited to the Zoomastigophora. 



Reorganization or de-differentiation of the derived taxonomic 

 structures at periods of division, endomixis and fertilization wherebv 



