DIFFERENCES BETWEEN ANIMALS AND PLANTS. 43 



ment of new branches. Some of the lowest animals, how- 

 ever, the polyps for instance, increase in a somewhat analo- 

 gous manner, ( 329, 330.) 



56. In the effects they produce upon the air by respira- 

 tion, there is an important difference. Animals consume 

 the oxygen, and give out carbonic acid gas, which is de- 

 structive to animal life ; while plants, by respiration, which 

 they in most instances perform by means of the leaves, 

 reverse the process, and thus furnish oxygen, which is so 

 essential to animals. If an animal be confined in a small 

 portion of air, or water containing air, this soon becomes so 

 vitiated by respiration, as to be unfit to sustain life ; but if 

 living plants are enclosed with the animal at the same time, 

 the air is maintained pure, and no difficulty is experienced. 

 The practical effect of this compensation, in the economy of 

 Nature, is obviously most important ; vegetation restoring 

 to the atmosphere what is consumed by animal respiration, 

 combustion, &c., and vice versa. 



57. But there are two things which^. more than all others, 

 distinguish the animal from the plant, namely, the power of 

 moving itself or its parts at will, and the power of perceiv- 

 ing other objects or their influences ; in other words, volun- 

 tary motion and sensation. 



58. All animals are susceptible of undergoing pleasure 

 and pain. Plants have also a certain sensibility. They 

 wither and fade under a burning sun, or when deprived of 

 moisture ; and they die when subjected to too great a de- 

 gree of cold, or to the action of poisons. Put they have no 

 consciousness of these influences, and suffer no pain ; while 

 animals under similar circumstances suffer. Hence- they 

 have been called animate leings, in opposition to plants, 

 which are inanimate beings. 



