illnesses — able, that is, to restrict the lives of other species. For we know 

 that growing and developing and reproducing are possible for some indi- 

 viduals only on condition that other living things are destroyed; here too 

 life is self-limiting. Now the destruction is going on all the time, just as the 

 production of new protoplasm — and new individuals — is going on all the time. 

 Making of new is limited by destroying of old. Just as the growing body 

 carries on by oxidizing parts of its own protoplasm, life in general continues 

 as individuals die and are replaced by others. 



The Life-and-Death Cycle Since there must be a limit to the various 

 kinds of elements, and since plants and animals make use of the materials in 

 the earth and the air and the waters, will not these materials at last become 

 exhausted? And would not that mean the end of all life? 



The plants and animals in a restricted area, such as a farm, might live for 

 several years without the need for replacing what they removed from the soil. 

 But as the products of a farm are normally carried off to be used elsewhere, 

 the soil must in time be deprived of certain elements essential to further life. 

 But what happens in a balanced aquarium, in which the carbon dioxide ex- 

 haled by the animals is converted by the green plants into food used by the 

 animals, and in which the animals are supplied with oxygen? 



In addition to the balance of carbon and oxygen, the living organisms in 

 this restricted area must have a supply of the materials that become permanent 

 parts of the protoplasm — nitrogen and certain salts. The nitrogen also cir- 

 culates through the organisms, the soil, and the water, as we have seen (see 

 pages 151, 152). But some of the inorganic material remains largely within the 

 living bodies until they die. 



When we consider life in general, maintaining a succession of living things 

 appears to depend upon the circulation of materials. There is no danger 

 that all life will come to an end merely because the materials may become ex- 

 hausted. The same materials enter into a constant succession of new living 

 things. The chain is endless because it includes the remains of plants and ani- 

 mals that have died. The materials, instead of being "locked up" in bodies, 

 whether living or dead, pass on into other cells, other plants and animals. 

 Each particle in the course of years becomes part of many different organ- 

 isms, of many different kinds (see illustrations, pp. 151 and 153). 



How Can Man Regulate Population for His Purposes? 



Distribution of Life^ In the world as a whole there are about 2000 mil- 

 lion human beings. If we should spread out evenly over the land surface, we 

 should be about 33 to each square mile. That would give us plenty of elbow- 

 room. But a very large fraction of us would soon die. For millions of those 



iSee No. 4, p. 538. 

 534 



