M 



'W 



K^y-f 





^M 



•New lurk Uotamciil Garden 



MIGRATING FROM A CENTER 



This "fairy ring" of mushrooms (Lepiota) on a ranch in Colorado suggests how a 

 vegetation or population, fixed to the earth, moves outward as it exhausts the food 

 available 



often be seen moving away from a center in all directions, and in ever- widening 

 circles (see illustration, above). Man, along with other species, has pushed out 

 into new regions not only to find more food, but to escape enemies. Indeed, 

 many of us today move from one place to another for our health. The partic- 

 ular climate, the presence of particular plants or animals, may make our present 

 location unsuitable — for some of us. Again and again people have moved in 

 hordes from regions considered unwholesome and regions invaded by pests. 



But in moving away the individual or horde becomes an interloper. Every 

 new arrival disturbs the existing "balance" and threatens to drive some of the 

 plants and animals away or to destroy them. Men moving in large numbers 

 are like a swarm of locusts moving across the land and destroying every scrap 

 of vegetation. In a comparatively short time European man has driven from 

 their former habitations the Indians who had lived in North and South 

 America for centuries. He has reduced to a small fraction of their former 

 numbers many species of wild mammals, birds and fishes. He has destroyed 

 the trees on millions of acres, practically all the grasslands, and the fish in 

 hundreds of miles of stream. 



To offset the destruction, man has made millions of acres bear vastly greater 

 quantities of particular kinds of vegetation than would have been possible 

 under natural conditions. The corn, the potato, the tomato, the tobacco, the 

 peanut, the strawberry, had inhabited this continent long before the white 

 man came. But never had any of these species thrived so luxuriantly and so 



587 



