GEXEBAL PKIXCIPLES OF CLASSIFICATION. 389 



There is not the slightest foundation for the theory, which has 

 been advocated by some naturalists, of a transmutation of species. 

 All such statements therefore, that have been made, of the 

 conversion of Oats into Eye, or of any species whatever into 

 another, are entirely without foundation, and have arisen from 

 imperfect observation. 



In practice it is ver\- important that we should distinguish all 

 the above varieties from true species, for nothing is so calculated 

 to lead to confusion in descriptive botany as the raising of mere 

 varieties to the condition of species. No individuals should be 

 considered as constituting a species unless they exhibit important 

 and permanent distinctive characters in a wild state, and which 

 can be perpetuated by seed. G-reat imcertainty still prevails in 

 our systematic works as to what is a species and what is a 

 variety ; and hence we find different authors, who have written 

 on British and other plants, estimate the number of species con- 

 tained in such genera as Eosa, Eubus, Saxifraga, Hieracium, 

 Salix, and Smilax, verj- differently. 



2. GrEXEEA. — The most superficial observer of plants will have 

 noticed that certain species are more nearly allied to one another 

 than to other species. Thus, the different kinds of Eoses, 

 Brambles, Heaths, Willows, may be cited as familiar examples 

 of such assemblages of species ; for, although the plants com- 

 prehended under these names present certain well-marked dis- 

 tinctive characters, yet there are at the same time also, striking 

 resemblances between them. Such assemblages of species are 

 called genera. A genus, therefore, is a collection of species 

 which resemble one another in general structure and appearance 

 more than they resemble any other species. Thus, the various 

 kinds of Brambles constitute one genus, the Eoses another, the 

 "Willows, Heaths, Clovers, Oaks, &c., form also, in like manner, 

 as many different genera. The characters of a genus are taken 

 exclusively from the organs of reproduction, whi'e those of a 

 species are derived generally from all parts of the plant. Hence 

 a genus might be defined as a collection of species which re- 

 semble one another in the structure and general characters of 

 their organs of reproduction. It does not necessarily happen 

 that a genus should contain a number of species, for, if a single 

 species presents peculiarities of a marked kind, it may of itself 

 constitute a genus. 



It frequently happens that two or more species of a genus 

 have a more striking resemblance to one another in certain im- 

 portant characters than to other species of the same genus ; in 

 which case they are grouped together under the name of a sub- 

 genus. 



3. Ordees or Eamilies. — If we regard collections of genera 

 from the same point of view as we have just done those of species, 

 — that is, as to their resemblance or family likeness, — we shall 



