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One important natural check is a limit on the amount of suitable habitat for each species. Insect problems arose with the intensification of agriculture-the devotion of immense acreage to a single crop. Such a system set the stage for explosive increases in specific insect populations. Obviously then, an insect that lives on wheat can build up its population to much higher levels on a farm devoted to wheat than on one in which wheat is intermingled with other crops to which the insect is not adapted.
一个重要的自然控制就是对于适合于每个物种的栖息地的限制。农业生产高度集中，大量的土地被用来种某一种庄稼，昆虫问题也就接踵而至。这样的系统使某种特定的昆虫数量激增。很明显，以小麦为生的昆虫，如果是在仅种植小麦的地里，它的数量会很多，而如果是在那些小麦和不适合这种昆虫生长的其他农作物混种的地里，它的数量就会少很多。
Another factor in the modern insect problem is the spreading of thousands of different kinds of organisms from their native homes. Some hundred million years ago, flooding seas cut many land bridges between continents and living things found themselves confined in what an ecologist calls "colossal separate nature reserves." There, isolated from others of their kind, they developed many new species. When some of the land masses were joined again, about 15 million years ago, these species began to move out into new territories—a movement that is not only still in progress but is now receiving considerable assistance from man.
导致现代昆虫问题的另外一个因素是成千上万种不同种类的生物从它们原来的栖息地向新的区域蔓延入侵。几亿年前，泛滥的大海切断了许多大陆之间的联系，生物只能被限制在如同一位生态学家所说的“巨大的独立的一然保护区”内。在那里，它们被与其他同类隔绝开，进化出了许多新的物种。大约1500万年前，当一些大陆重新连通之时，这些物种开始迁移到新的地区——这个运动现在仍在进行中，而且正在得到人类的大力帮助。
The importation of plants is the primary agent in the modern spread of species, for animals have almost invariably gone along with the plants.
植物的进口是当代昆虫种类传播的主要原因，因为动物几乎总是跟着植物迁移的。
