Once a plant reaches a new and for it a strange country it is remarkable how quickly it will often capture the new territory. In the United States over six hundred of our commonest weeds have come from Europe and Asia. The daisy, dandelion, wild carrot, many hawkweeds, dozens of wild mustards, and many others are among the somewhat undesirable immigrants that now reach over a great part of the country--all brought over by the early settlers of America. From their home in subtropical Asia the lemon, lime, and orange have invaded every part of the tropical world. Once, in the most remote part of the Sierra Maestra Mountains in eastern Cuba, where only by the most arduous cutting could a passage through the dense tropical forest be forced, the writer found within a few square rods an orange and a tree from tropical India. Such cases could be multiplied, and all over the world we see this endless struggle of plants to conquer new territory, often at the expense of existing vegetation. On Long Island, New York, the introduced locust tree, brought from the southeastern United States about a hundred years ago, has completely routed the native trees in many places along the north shore of the island. And in Hawaii, some seeds of a screw pine, washed up among refuse along the beach or brought in by early aborigines, have made this Malayan plant a common tree thousands of miles from its home. 3. HOME ECONOMY OF PLANTS, OR ECOLOGY The geographic distribution of species of plants may be, as we have seen, the result of the geological changes of the past, of bird migrations, of more or less fortuitously water-borne seeds, or more usually of the slow spreading by invasion of those species apparently not so well supplied with external helps to dispersal. But no matter where plants grow, nor how they got there, they must fit the particular environment in which they find themselves or perish. This home economy of plants, or how they meet the environment and each other, is called Plant Ecology, a phase of botany now much studied, for it tells us more directly than most other plant research what the actual response to various factors of the environment may be. Just as plant distribution is the reflection of many, usually widely operating forces, so ecology narrows down to individual plants or groups of them the impact of the immediately surrounding conditions upon vegetation.