508 
PHYTOGRAPHY. 
BOOK IV. 
living things, a strict definition in Natural History becomes 
impracticable, for which reason the term differential must be 
admitted instead. 
Differential characters express, in the least possible space, 
the distinctions between plants : they should contain nothing 
superfluous, nor any thing which can be considered implied 
by the contrasted characters of those with which they are to 
be compared. By this means the distinctions of species are 
brought into the least possible compass ; and the analysis of 
their characters becomes so effectual, that a botanist is ex- 
pected to be able, without difficulty, to determine the exact 
station and name of any one of the 100,000 species supposed 
to exist. Nothing can sound better than this ; but, unfortu- 
nately, the advantages of differential characters are not quite 
so great as would appear. In sacrificing every thing to 
brevity, it is found in practice that doubts and ambiguities 
are continually created ; and for this especial reason, among 
others, that differential characters must necessarily be framed 
upon a consideration of what we know, and not with reference 
to what we do not know : on this account, a differential cha- 
racter, constructed in the most unexceptionable manner by 
one botanist, may be unintelligible to another who possesses 
more knowledge, or a greater number of species. For ex- 
ample, when Linnaeus framed the differential character of 
Rosa indica, “ germinibus ovatis pedunculisque glabris, caule 
subinermi, petiolis aculeatis,” it probably distinguished that 
species from all others that he knew : but our acquaintance 
with Roses is so much more extensive than that of Linnaeus, 
that we have many Roses to which his character is equally 
applicable. A differential character, moreover, conveys no 
information beyond that of the differences between one thing 
and another, and can be viewed in no other light than as a 
convenient method of analysis. For this reason, the essential 
character is more generally adopted at the present day, 
either to the exclusion of the differential character, or in 
union with it. 
The essential character of a plant expresses, as its name 
implies, those peculiarities which are known by experience to 
be most essential to it ; but admits nothing unimportant or 
