PREFACE. 



THE present Part IV. completes the work with the exception 

 of the Appendix which differs so widely in character from the 

 rest of the book that it seemed desirable to issue it separately ; 

 it will be published very shortly. 



Besides receiving assistance from the friends who have helped 

 me in former parts I have in this part had the advantage of 

 the criticisms of my friends Dr W. S. Griffith, Mr Walter Jessop 

 and Dr F. Semon, in the subjects with which they arc respect- 

 ively familiar, for which I tender them my best thanks. I also 

 desire to thank my friends Mr James Ward arid Prof. Postgate 

 of Trinity College for valuable advice. 



August, 1891. 



