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WHAT KEEPS A SPECIES FROM SPREADING 



The distribution of a species away from a center is influenced by the pressure of 

 population and by the physical conditions in surrounding regions. But the limiting 

 factors always include other species — possible food, possible enemies — as well 

 as soil and climate 



Hindrances to Human Life^ Human population can increase only 

 where the soil and the climate are suitable for those species that we depend 

 upon for food and for other materials. But suitable soil and climate are not 

 enough to make a region secure for human habitation. Other animals and 

 plants may have established themselves ahead of us, and they may succeed in 

 keeping us out. Breaking new territory has often meant fighting wild animals 

 and driving out inhabitants already there. When early settlers cleared forests 

 to make their homes and farms, they removed not only trees, but a vast amount 

 of animal life — birds, mammals large and small, insects of many species. And 

 they created conditions in which many species of plants could no longer keep 

 going. 



The expansion of human population would seem to be a simple problem of 

 replacing the native population with cultivated plants and domestic animals. 

 When this process was repeated over and over again, and more and more 

 rapidly, other things began to happen. Sometimes the attempts to cultivate 

 crops in a new region succeed from the first: the soil and the climate happen 

 to be right. Sometimes a species succeeds even better than it did in the old 

 home from which the settlers came, for the insect pests or the parasitic fungi 

 of the old home were not brought along. In other cases, however, the best 



iSee No. 5, p. 538. 

 536 



