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 advan- 

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

 Walter Jessop, and Dr. F. Semon, in the subjects with which 

 they are respectively familiar, for which I tender them my 

 best thanks. I also desire to thank my friends, Mr. James 

 Ward and Prof. Postgate, of Trinity College, for valuable 

 advice. 



August, 1891. 



