PREFACE. ix 



plant regarded as a living machine, so to speak, 

 in order that the student may the better appre- 

 hend the point of view taken of the bearings of 

 the matters discussed in Part II. 



With regard to references, it seemed a better 

 plan to give, in the form of notes after each 

 chapter, the titles of the principal books and 

 papers on which a student may base a further 

 course of reading, than to overweight the pages 

 of what is, after all, merely an introductory sketch 

 to a huge subject, with detailed quotations from 

 the numerous sources of information made use of 

 I have freely expressed my own opinions, but the 

 sources for others are, I hope, as freely given. It 

 will, however, be understood that I have not 

 aimed at a complete bibliography, and, particu- 

 larly, I have only given foreign references where 

 it seemed that adequate treatment of the subject 

 could not be found in English. 



My sincere thanks are due to Mr. F. Darwin, 

 F.R.S., who has kindly looked through many of 

 the proofs, and given me the benefit of several 

 suggestions : and to my wife for the very material 

 aid she has afforded me in the preparation of the 

 index. 



H. MARSHALL WARD. 



Cambridge, 



November, 1900. 



