40 PHILOSOPHY OF ZOOLOGY. 
the subject. Part of this difficulty, however, may be re- 
ferred to the employment of ambiguous phrases, and to m- 
attention to the number and character of those properties 
which are common equally to plants and animals. But the 
greatest share may be traced to the practice of examining the 
doubtful objects, without attending to all their relations, —by 
comparing the less perfect animals with some of the quali- 
ties of the most perfect plants, and by allowing our opinions 
to be influenced by circumstances connected with mere size 
and form. 
When we compare together those animals and plants, 
which are considered as occupying the highest stations in 
each kingdom, we perceive that the characters by which 
they may be distinguished, are obvious and well defined. 
But when we descend to the animals and plants which oc- 
cupy the lowest stations, and perceive that they are less 
complicated in their structure; exercise few functions be- 
sides those which are essential to living bodies; and, in 
consequence, present only obscure points of difference, we 
may be led into the supposition that, at a certain link of 
the chain, the two kingdoms coalesce. When, however, 
we examine all the characters which the imperfect plants 
and animals exhibit, we shall be able to trace the relations 
which connect these minute and obscure species, with those 
in which the characteristics of the kingdom to which they 
belong are more fully developed. In order to illustrate 
this subject still farther, and attempting to guard against 
the errors into which others have been betrayed, we have 
already enumerated those characters which are essential to 
the existence of organized bodies ; and propose now to con- 
sider the marks by which animals may be distinguished 
from plants. 
I. Animals differ from Plants in Composition.—The es- 
sential elements of organized matter appear to be carbon, 
