tance. He has gone on to rearranging the very face of the earth, to rerouting 

 its rivers, to altering the character of its plant and animal life. He distributes 

 species and changes the relative numbers of various species, all to serve his 

 needs and his desires. But in extending his domination, man sets up processes 

 the remote results of which he cannot possibly anticipate. Who could have 

 foreseen that placing a paper factory at one point along a river would ruin 

 the life in the river or the water supplies of cities far away? Who could have 

 guessed that making fine wheat and cotton grow in rich crops in place of the 

 scrubgrass would end by destroying the soil itself? 



When we undertake to change the numbers of any species, it is not enough 

 to know that a particular species is useful or harmful. We ha\'e to proceed 

 cautiously, and seek as thorough a knowledge as possible of all the relation- 

 ships in which each species is involved. Nor is it a simple matter, as many 

 assume, of "interfering with nature's plans". Nature's "plans" include man 

 and Ufe, and Ufe is always interfering. It is a matter of altering certain slowly 

 movmg processes of mutual adjustment, certain balancings, so that we can fit 

 ourselves into them while advancing our own welfare. 



In Brief 



Within a balanced community of living things, the essential ratios and re- 

 lationships of the different species remain fairly constant. 



Each wave of abundance for any species lasts only until the organisms have 

 expanded to the limit of the resources. 



Within any living community there is a point beyond which more food 

 does not mean more growth, for other factors limit the results. 



An epidemic may be considered as an exceptionally favorable opportunity 

 for some plant or animal that finds itself among a crowd of its potential hosts 

 or victims. 



In proportion as man has thrived and grown in population, he has made 

 increasing demands upon the earth and has exerted increasing pressure upon 

 other species. 



By making many plants and animals of the same kind live close together, 

 man has brought on a constant succession of epidemics. 



With every migration, the new individual becomes an interloper; every 

 new arrival competes with plants and animals in an existing balance and threat- 

 ens to drive some away or to destroy them. 



A parasite moving into a new region may find a host that is incapable of 

 defending itself, or it may become the prey of a species against which it has 

 no defense. 



597 



