v {[[ P R E F A C E. 



In the preparation of this volume, the greatest advantage has been 

 derived from the works of Sir William Hooker, particularly his Genera 

 Filicum, wherein many of the genera of Dr. Presl and Mr. John Smith 

 are characterized and beautifully illustrated; also from his Species 

 Filicum (of which we have seen only the first volume, with two parts 

 of the second) ; a work which, when completed, will have accom- 

 plished more in reducing the vast array of nominal species to an in- 

 telligible state, than any work which has yet appeared. M. Spring's 

 admirable monograph of the Lycopodiacem has facilitated the determi- 

 nation of our species of Lycopodium and Selaginella. 



It has become unnecessary here to discuss those interesting ques- 

 tions which relate to the geographical distribution of the genera and 

 species of Ferns, and the numerical proportions they bear to the rest 

 of the vegetation, in the regions where our collections were principally 

 made; since this portion of the work has devolved upon a naturalist 

 of the Expedition more capable of doing justice to the subject. It 

 will be noticed in the body of this volume, however, that the same 

 Fern has very frequently been met with in two or more remote parts 

 of the globe ; — clearly showing that species of this family have a more 

 extensive geographical range than has been generally supposed. 



I cannot close these prefatory remarks without tendering my grate- 

 ful acknowledgments to Dr. John Torrey, of New York, and Dr. Asa 

 Gray, of Cambridge, for the liberality with which books from their 

 libraries have been supplied or consulted, and especially for assistance 

 afforded in translating my English specific characters into Latin. Dr. 

 Gray has also done me the favour to look over the proofs as the work 

 was passing through the press. My thanks are also due to Mr. J. 

 Drayton, for the care bestowed by him in superintending the engrav- 

 ing of the plates. 



Washington, August, 1854. 



