X 
INTRODUCTION. 
duals ; and this affords us now the only means of ap- 
proximating to a knowledge of what constituted a typical 
species, — not, however, what is generally called the type 
of a species, such being merely that form or variety 
which the author knows best, has seen most of, or is most 
common. We have no way of ascertaining directly 
what was the primitive form of each, but we may infer 
it by combining individuals together, according to the 
number and importance of the organic characters common 
to them. Some of the descendants of each aboriginal 
species may occasionally appear to us to vjiry so much 
from the general characters exhibited, as to create doubts 
as to which of two allied nuclei they belong to. On the 
other hand, some of the forms or races connecting mere 
varieties may have become extinct from accidental 
causes. The doctrine of intermediate forms rested on 
by some appears, therefore, to be of little value for ascer- 
taining what ought to be considered species, for we may 
find specimens exactly intermediate, as far as the human 
eye can judge, between two which are essentially dis- 
tinct; and of others we may have varieties or races* 
without connecting forms. Our knowledge of natural 
species is thus the result obtained from a consideration 
of several important characters, after a careful com- 
parison of foreign as well as British allied forms, rather 
than the discovery of one only, however constant, which 
latter may frequently indicate merely an artificial species, 
or, in other words, a variety. 
Linnaeus took nearly all his specific characters from 
conspicuous parts, especially from the stem and foliage ; 
these organs are, however, liable to great variation, 
and therefore at the present day we are apt to fall into 
the opposite extreme, and to select minute ones: of these 
some are of trifling value, while others, principally de- 
rived from the flower or fruit, and sufficient to constitute 
