INTRODUCTION. 23 
not then understood. Nevertheless, with that sagacity which so remark. 
ably characterized him, Linnzeus succeeded in grouping together genera 
into orders, which are even now regarded as, for the most part, very 
natural assemblages; that is, as containing plants really allied to each 
other in their most important characters, and differing from those of other 
orders in the same. But of the best mode of arranging these orders he 
was necessarily ignorant, since the most important characters were not 
then understood. The great progress which has been made since his 
time, in the structural and physiological departments of Botanical science, 
has done much to place classification on a more certain basis; yet there is 
still much wanting before Botanists shall be generally agreed on the prin- 
ciples which shall regulate the division and subdivision of the vegetable 
kingdom. In the following outline, it has been deemed advisable to 
adopt the classification of De Candolle, being the one which is most in 
use at the present time; and the principles upon which it is founded 
will therefore now be explained. 
PRINCIPLES OF NATURAL ARRANGEMENT, 
25. It may be remarked, however, in the first place, as a principle 
common to all systems of classification which profess to be natural, that 
the different values which are attached to the various characters 
furnished by the several organs of plants should be estimated by the 
degree in which they respectively indicate important similarities or 
differences of general conformation. It often happens that attention to 
one or two characters may afford a considerable amount of knowledge 
of the whole; because those characters are found to be inseparably 
connected with others. An instance of this has been already given in 
regard to the primary division between Exogens and Endogens {§ 65) 
and it may be useful to illustrate it further by reference to the animal 
kingdom. If, for example, we meet with an animal covered with 
feathers, we at once know a great deal of its internal structure and 
economy. It is a vertebrated animal, possessing a jointed back bone and 
complete internal skeleton: it has all five senses, its blood is red, it 
breathes air, its temperature is high, its young are produced from eggs, _ 
it walks upon two legs, &c. Here we are at once informed that this 
unknown animal possesses all the characters peculiar to the class of 
birds ; since no other animals than birds possess a covering of feathers, 
which is inseparably connected with the whole plan of their structure 
and economy. In the same manner the classification of the Mammalia, 
(Quadrupeds) according to their teeth, proposed by Linnzus, proves 
to be a very natural one, although founded upon a single set of 
characters; because the form and number of the teeth vary with the 
nature of the food on which the animal is intended to live; and to make 
