PREFACE TO SECOND EDITION. 



IN preparing this edition we have made no change in the 

 original plan of the book. The text, however, has been 

 carefully revised, and the results of the more recent re- 

 searches have been incorporated. Some parts have been 

 condensed, but, in consequence of the introduction of new 

 subject-matter and of additional illustrations, the size of 

 the book as a whole has been considerably increased. We 

 trust that these alterations will be found to be in the 

 direction of improvement. 



May 1899. 



