through their mutual dependence upon this constant stream. Individual plants 

 and animals take from it, but each one also yields to it — at first perhaps only 

 wastes but eventually up to the very last atom of its physical being. The life 

 of a region becomes slowly richer in total life and richer in forms as new 

 species move into it or perhaps evolve in it. These changing inhabitants are 

 capable of operating more efficiently, in the special circumstances, than their 

 predecessors. The total population attains at last an optimum — the climax of 

 plant and animal increase in a balanced system of mutual interdependence. 



In the course of slowly building up a climax population, life and death in- 

 teract. Chlorophyl-bearing plants and some of the simplest species that build 

 up more complex compounds might live indefinitely in the absence of animal 

 species. The animal species, however, could not live in the absence of the 

 former. And by destroying plants and oxidizing organic materials, animals 

 restore to the surroundings raw materials that make possible new plants. 

 There is thus a mutual exchange, a constant give-and-take. Much of this is a 

 quiet, even invisible process — diffusion of gases, diffusion of dissolved sub- 

 stances in water, breathing, absorbing, excreting. But much of it involves 

 activities that are fairly described by the term struggle — the capture of prey, 

 the pursuit, the flight, the direct combat. Every phase of this struggle is, of 

 course, destructive of living individuals; but it is also the condition for pro- 

 longing the lives — of other individuals. 



During this struggle of living beings with one another, as well as with the 

 nonliving environment, the total amount of life may steadily increase — up 

 to the time that a cHmax is reached. Then the actual amount and the actual 

 composition of the plant and animal population continue to change from 

 moment to moment, from season to season, from year to year; but there is a 

 balance. The life destroyed is quickly replaced by new growths or new births, 

 and the new life destroys its own equivalent. 



One feature of life that is at once a source of destruction, and also a means 

 for filling in every possible gap, is the fact that each species not only repro- 

 duces, but multiplies. As a result, there is a constant push outward from every 

 single plant, from every group of animals. We might imagine a slower rate of 

 reproduction, a replacement rate, which might permit every individual to 

 live out his own cycle, according to the species. But that would overlook the 

 fact that at each stage every animal species is food for others. The species 

 breeding most slowly could attain an optimum of survivals only as it managed 

 to get food without itself being eaten. 



The pressure of population is constantly disturbing the balance in any life- 

 community. When human beings come into a situation that they find favor- 

 able, they are disposed to work it intensively. As a result, they often destroy 

 its capacity to maintain human life further. Migration has been part of man's 

 history from the beginning. The conditions of mutual aid, division of labor, 



601 



