44 OUR NATURAL FERNS AND THEIR ALLIES. 



115. Synonymy. — It may also be remarked in this con- 

 nection that different authors have described the same fern 

 under widely different generic and specific names, owing (i) to 

 the different conceptions that have prevailed at different times as 

 to what constituted generic characters, and (2) to ignorance of 

 what others had already written on species, redescribed as new. 

 For example, the delicate Woodsia Ilvensis of Robert Brown was 

 described as Acrostichum Ilvense by Linnaeus, Pofypodium II- 

 vense by Swartz, Nephrodium rufidulum by Michaux, Aspidium 

 rufidulum by Willdenow, and Woodsia rufidula by Beck. Many 

 other species have been as variously classified. The oppor- 

 tunities for errors of this character are much less now than for- 

 merly, yet redescription is not unknown in our day. 



1 1 6. Species. — Goethe tells us that nature knows only in- 

 dividuals, and that species exist only in the school-books. From 

 this extreme there has been every grade of opinion respecting 

 species to the one which regards species as invariable, actual 

 existences, types originally ordained and summoned to existence 

 by the Creator. Linnaeus, for example, defined species in these 

 words : " Species tot sunt diversa, quot diversas forinas ab initio 

 creavzt infinitum ens." * Various definitions have been given to 

 species, but none accord with the actual practice of systematists, 

 who seem inclined to make a species what they choose ; and 

 indeed the existence of various connecting forms between 

 many species distinct under normal conditions makes the prac- 

 tical definition of the term almost an impossibility. We may, 

 however, for practical purposes, regard as a species an assem- 

 blage of individuals not differing essentially from each other, 

 and capable of producing like individuals by the ordinary pro- 

 cesses of reproduction. A recent writer defines species as " the 

 present aspect of a line of organic development, destined to 

 become something else in the future, as it was something else 

 in the past," — a definition in accord with the now universally 

 accepted biological doctrine respecting the origin of species. 

 Species among ferns are founded chiefly on differences in the 

 cutting of the fronds and their method of venation. 



* There are as many different species as tlie Infinite Being created in tlie 

 beginning. 



