VI PEEFACE. 



most part entirely avoided, as being calculated to perplex 

 rather than instruct those who are but acquiring the rudi- 

 ments of the subject. 



Abstruse questions of identity or of specific distinctions 

 have also been regarded as foreign to the purposes of this 

 ' History/ On these points we have been content to follow 

 the generally received opinions of Pteridologists. In one 

 or two instances, in which perhaps this course has been 

 departed from, the reason has been made sufficiently ob- 

 vious. 



These explanations may serve to acquaint more advanced 

 students why so little of novelty has been prominently 

 introduced, and why several recently described plants have 

 been rather treated as varieties than as species. The 

 consideration of the specific distinctness of these plants 

 opens up questions involving much doubt and difficulty, 

 and leading different inquirers to widely different conclu- 



