Everywhere man is altering the balance of nature. He is facilitating the spread of plants and animals into new regions, sometimes deliberately, sometimes unconsciously. He is covering huge areas with new kinds of plants, or with houses, factories, slag-heaps and other products of his civilization. He exterminates some species on a large scale, but favours the multiplication of others. In brief, he has done more in five thousand years to alter the biological aspect of the planet than has nature in five million.
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