(ECOLOGY. 207 



combinations, to oxidize : they are oxidizing organisms. 

 This explains why the favorable influence of plants upon 

 animals ceases immediately, when they change the charac- 

 ter of their metabolism. \Vith the disappearance of their 

 chlorophyl moulds and bacteria lose the power of reducing 

 carbonic acid; they derive their food from other organisms 

 and decompose this into carbonic acid, water, etc. ; like 

 animals, they are oxidizing organisms, and consequently 

 dangerous competitors. When they establish themselves 

 upon the animal body, they almost always work injury to 

 it ; hence in animals they are the cause of many extremely 

 dangerous ailments. 



IV. ANIMAL AND PLANT. 



Distinction between Animal and Plant. The con- 

 sideration of symbiosis has led us up to the fact that be- 

 tween plants and animals a distinction exists in the mode 

 of metabolism, which may be expressed thus: plants usually 

 take in carbonic acid and breathe out oxygen, while ani- 

 mals breathe in oxygen and give out carbonic acid. Hence 

 it may be concluded that it must be easy to discover differ- 

 ences which generally obtain between plants and animals, 

 for, as a matter of fact, the laity are never in doubt in 

 deciding to which realm of nature belong the more highly 

 organized animals and plants, which are the only ones 

 known to them. 



Doubtful Cases. But the more one studies this ques- 

 tion, the more difficult becomes the solution of it. The 

 old zoologists indeed formed the conception that there are 

 organisms which stand on the limits between the animal king- 

 dom and the vegetable, and the Englishman Wotton named 

 these directly plant-animals or zoopJiytcs. No\v we know that 

 Wotton's plant-animals are true animals with but a super- 

 ficial similarity to plants; but, by means of the micro- 

 scope, we have become acquainted with numerous lower 

 organisms, and it is still doubtful in which of the two 



