507 
CHAPTER L 
OF diagnoses; or, of generic and specific characters. 
We have seen that plants are distinguished from each other 
by their characters : of the application of these characters we 
must now speak. Were each species to be characterised 
independently of other species, and to be described with all 
the minute circumstances of structure that belong to it, the 
progress of investigation would be too slow, and the length 
of time requisite to acquire information much too great: for 
this reason, the process of enquiry has been simplified, by 
collecting in groups all those species which have certain great 
characters in common, and abstracting those characters, which 
then become the distinctions of classes : the species of a class 
are again collected into other groups, agreeing in some other 
common peculiarities, which are in like manner abstracted, 
and form the characters of orders. Thus reduced in extent, 
the species of each order are submitted to the same process 
of combination ; the characters by which they are combined 
become distinctive of genera ; and the species are, finally, left 
shorn of the greatest part of their characters, which are thus 
reduced within a very narrow compass. Each plant has, 
therefore, four characters; or, if sub-classes, sub-orders, or 
other modes of division are adopted, as many separate cha- 
racters as there may be divisions^ 
These characters are of two sorts ; the one called essential, 
the other differential. The former are the most commonly 
employed for orders and genera ; the latter are chiefly used 
in discriminating species : the former are the most valuable, 
and will probably, in time, supersede the others, which convey 
little information, and are only useful in aiding us in our 
analysis of large bodies of species : the latter are often called 
definitions ; but, as no definite limits can be traced between 
