40 PHILOSOPHY or ZOOLOGY. 



the subject. Part of this difficulty, however, may be re- 

 ferred to the employment of ambiguous phrases, and to in- 

 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, 



