PREFACE. 



THE following lectures are the natural outgrowth 

 of professional study and instruction in College. 

 Portions of them have already appeared in Reviews 

 and in my published addresses. In fact, several of 

 them are but the unfolding of the fourth lecture on 

 "Tin: RELATIONS OF NATURAL HISTORY," deli- 

 vered at the Smithsonian Institution in 1859. 



When I received the invitation to lecture before 

 the Lowell Institute, the press of other duties left 

 no time to do more than to arrange the materials 

 already on hand. In the text, I have endeavored to 

 indicate the authors from whom special aid was 

 derived. All who have studied any subject for 

 years, without thought of publication, know how 

 difficult a task it is to tell all the sources of their 

 knowledge. Much of Natural Theology, instead of 

 commencing with Paley, or with Nieuwentyt, to 



