EXPLANATORY. 



THIS Series is intended to meet the requirement of 

 brief text-books both for schools and for adult readers 

 who wish to review or expand their knowledge. 



The grade of the books is intermediate between the 

 so-called "primers" and the larger works professing 

 to present quite detailed views of the respective sub- 

 jects. 



Such a notion as a person beyond childhood re- 

 quires of some subjects, it is difficult and perhaps 

 impossible to convey in one such volume. Therefore, 

 occasionally a volume is given to each of the main 

 departments' into which a subject naturally falls for 

 instance, a volume to the Zoology of the vertebrates, 

 and one to that of the invertebrates. While this ar- 

 rangement supplies a compendious treatment for those 

 who wish, it will also sometimes enable the reader 

 interested in only a portion of the field covered by a 

 science, to study the part he is interested in, without 

 getting a book covering the whole. 



Care is taken to bring out whatever educational 

 value may be extracted from each subject without im- 



