Vlll PRKFACE TO THE SECOND EDITION. 



The suggestion of one of the reviewers of the first edi- 

 tion, that the bibliography should be chronologically in- 

 stead of alphabetically arranged was carefully considered, 

 and at one time I had concluded to adopt it, but when 

 I attempted to do so, I found that the inconvenience 

 resulting from the wide separation of several papers by 

 a single author, more than oflfeet the advantages of a 

 chronological arrangement. I therefore adhered to the 

 original plan. 



For valuable assistance in collecting references and 

 examination of papers I am greatly indebted to my as- 

 sistant. Dr. H. F. Formad. 



1506 SPBrcE Street, 

 October 1st, 1878. 



