viii PREFACE 



many organisms that are more nearly related to plants than to 

 animals. 



The figures have been drawn for the present work by my wife. 

 Figures not credited to other authors are either new or taken from 

 my own previous works. 



The author is much indebted to the Carnegie Institution of 

 Washington for making possible a year of uninterrupted research, 

 devoted largely to studies preliminary to the preparation of this 

 work and to its actual composition. He is further indebted for 

 the use of a number of figures first published by the Carnegie 

 Institution. 



University of Pennsylvania, 

 December II, 1905. 



