(ECOLOGY. 171 



formed from becoming food material for the animal. Thus there is on a 

 small scale that cycle of matter which exists on a grand scale in Nature 

 between the animal and vegetable kingdoms. By aid of chlorophyl and 

 of the chemical influence of sunlight the plants decompose water and 

 carbon dioxide and form from them oxygen, which they breathe out, and 

 compounds rich in carbon, which they store up in their tissues : they 

 are reducing organisms. On the contrary, animals give off carbon 

 dioxide and water, but take their oxygen from the air, and carbon com- 

 pounds in their food ; they use oxygen to break down the chemical 

 combinations, to oxidize : they are oxidizing organisms. This explains 

 why the favorable influence of plants upon animals ceases immediately 

 when they change the character of their metabolism. With the disap- 

 pearance of their chlorophyl moulds and bacteria lose the power of reduc- 

 ing carbon dioxide ; they derive their food from other organisms and 

 decompose this into carbon dioxide, 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 dan- 

 gerous ailments. 



IV. ANIMAL AND PLANT. 



Distinction between Animal and Plant. The consideration of 

 symbiosis has led us up to the fact that a distinction exists between 

 plants and animals in the mode of metabolism, which may be 

 expressed thus : plants usually take in carbon dioxide and give off 

 oxygen, while animals breathe in oxygen and give out carbon 

 dioxide. Hence it might be concluded that it is easy to discover 

 differences 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 the more highly organized animals and 

 plants, which are the only ones known to them, belong. 



Doubtful Cases. But the more one studies this question, the 

 more difficult becomes its solution. The old zoologists indeed 

 formed the conception that there are organisms which stand on the 

 limits between the animal kingdom and the vegetable, and Wotton 

 named these directly zoophytes or plant-animals. Now we know 

 that Wotton's plant-animals are true animals with but a superficial 

 similarity to plants; but, by means of the microscope, we have 

 become acquainted with numerous lower organisms, and it is still 

 doubtful in which of the two realms of nature these belong. As 

 such may be mentioned the Myxomycetes and many Flagellata. 



