GENERA.— ORDERS. 408 
In practice it is important that we should distinguish 
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. Great uncertainty 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 
contained in such genera as Rosa, Rubus, Saxifraga, Hieracium, 
Salix, Snvilax, and others, very differently. 
2. GENERA.—The most superficial observer of plants will 
have noticed that certain species are more nearly allied to each 
other than to other species. Thus, the different kinds of Roses, 
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 each other in general structure and appearance 
more than they resemble any other species. Thus, the various 
kinds of Brambles constitute one genus, the Roses another, the 
Willows, Heaths, Clovers, and Oaks form also, in like manner, 
as many different genera. The characters of a genus are taken 
exclusively from the organs of reproduction, while those of a 
species are derived generally from all parts of the plant ; hence 
a genus is defined as a collection of species which resemble each 
other in the structure and general characters of their organs of 
reproduction. Itis not necessary, however, 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 each other in certain im- 
portant characters than to other species of the same genus, in 
which case they are grouped together into what is termed a sub- 
genus, and further subdivisions of more nearly allied species, 
such as sections, swb-sections, &c., may be made. 
3. ORDERS OR NatTuRAL OrDERS.—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 close resemblances,-— 
work ‘ On the Origin of Species, and in other volumes by the same gifted 
observer. This author contends that species, so far from being immutable, 
are liable to change of almost any extent—in fact, that plants, by the 
operation of causes acting over a long period of time, may become so altered, 
that they preserve scarcely any apparent resemblance to those from which 
-they were originally derived. 
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