24 INTRODUCTION. 
use of this, a certain form of digestive apparatus is adapted ; as well as 
a certain kind of general structure, furnishing the instruments by which 
the food is obtained: so that these may be known to a great extent 
from the inspection of the teeth alone. In like manner, the Botanist, 
whilst founding his arrangement upon the whole group of characters 
which each plant exhibits, endeavors to select those, as marks for dis- 
tinguishing the several divisions, which are at once easily recognized, 
and which serve as the best key (so to speak) to those which are seated 
within. Such characters are natural, then, in proportion as they indicate 
general conformity or difference of structure; thus the distribution of 
the veins of the leaves,—a character easily recognized,—will in general 
serve to distinguish Exogens and Dicotyledons from Endogens and 
Monocotyledons ; and it is therefore a very natural character, serving 
as a key to all those which are indicated by these terms. On the other 
hand, the number of stamens and pistils in a flower is a purely artificial 
character, since it gives no further certain information of the general 
structure of the plant. 
26. Another general principle of Natural classification must next be 
pointed out. When a number of Plants or Animals are associated, on 
account of their general resemblance to each other, into a Natural group, 
it will be found that the characters in which they agree, are presented by 
some members of the group much more prominently than by others; and 
that in some they are occasionally so much wanting, that these can 
seem to be more easily included in any other groups. Now, those 
members of a natural group which most strikingly present a union of all 
the characters by which it is distinguished, are spoken of as its types ; 
and those in which these characters are less obvious are termed aberrant 
members of the group. It is by these, in fact, that natural groups are 
connected with one another ; for it will generally be found that in the 
aberrant members of one group, its characters become (as it were) 
gradually shaded off, until they almost blend with those of the next. To 
revert to an illustration; where the countries occupied by two nations 
are not separated by any marked natural boundary, (as a broad river 
or high chain of mountains,) the peculiar characters of these nations, 
which may be regarded as most strongly exhibited in their respective 
chief towns become gradually blended towards the border where they 
meet, so that the transition from one to the other is by no means so 
abrupt, as if the traveller were conveyed at once from the metropolis of 
each to that of the other. Every natural group then, may be regarded 
as a sphere, surrounded by other spheres—each representing another 
group,—which touch it at certain points, the type of each will occupy 
its centre, and the aberrant members will be disposed in various posi- 
