178 SYSTEMATIC BOTANY. 
636. Before proceeding to examine in detail the Natural 
and Artificial Systems, we must attend to the division of 
plants, into genera, species, and varieties. _ A genus includes 
many species, and a species many varieties. The Systems of 
Linnzus and Jussieu do not differ about the arrangement of 
varieties or species—with respect to these they are both the 
same, always placing the same variety under the same species, 
and the same species under the same genus. _ It is in the at- 
rangement of the genera that these two methods are at va 
riance. 3 
637. A species (626) includes so many plants of the same 
structure, form, and general appearance, and the seeds of 
which always produce plants similar to themselves. _Differ- 
ence in species is the only precise and constant distinction 
established by nature. The arrangement of species into ge- — 
nera is an artificial classification ; the occurrence of varieties 
is not constant nor regular ; but a species is a group of plants 
formed by nature distinct from all other kinds of plants. The 
distinctive characters of species are taken generally from the 
root, stem, or leaves. A variety of a species differs from it 
in some trifling character, such as colour of the flowers, size, 
"pubescence of the leaves, &e. A genus consists of several 
species differing from each other by what are called the spe — 
cifie characters, taken from the leaves, &c., but agreeing t0- 
gether in the more important characters taken from the fruc- 
tification. The following generic and specific characters a 
the genus Pyrus, and the species which it includes, will illus 
trate the distinction between genera and species. 
PYRUS (PEAR, APPLE, AND SERVICE. ) 
_. Generie Character.—Calyx superior, of five segments 
Petals five. Styles two to five. Fruit fleshy (a pome, °F 
_ apple), with five cartilaginous two-seeded cells. 
_ Pyrus communis (Wild Pear-tree) ; leaves simple, ovate, 
_ serrated, peduncles corymbose, fruit turbinate. ere 
Pyrus Malus (Crab-apple) ; leaves ovate, acute, serrated, — 
