Not the least interesting feature of these plant societies is that we must view them as associations of plants, often of widely differing origin due to the vicissitudes of plant distribution, but all taking their part in the society to which they belong and often, as individuals, losing their life that the society may live. Upon such a conception a wood or prairie, or river bank, or salt marsh or alpine garden upon a mountain summit are, with many other plant societies, places of intense conflict. More cruel than any human society, these plant communities exist under conditions where only the individually strong survive, and only those societies are destined to reach their climax which can take advantage of every aid, quite without regard for severe losses or even death to the individual members of it. It is as if we poured into a crucible molten metals from many different sources, and after the incredible and relentless forces of manufacture had worked their magic upon them there resulted a product, purified and cleared of all dross. So the inexorable and relentless processes of nature work over the materials found in these plant communities, the results of which are the dominant types of vegetation in the world to-day. With this understanding of the part they play in plant distribution we may now consider a few of the most widely recognized plant societies and see how they have affected the vegetation, sometimes even the history of the regions in which they are found. 4. SOME OF THE CHIEF PLANT SOCIETIES Forests No one who has ever seen both our temperate forests and those in the tropics can fail to be impressed with the difference between them. Not only for the different plants in them, but for their wholly different aspect, tropical and temperate forests stand far apart as an expression of the forest covering the earth. Not all of us realize, however, that the heat of the tropics is not the deciding factor in the luxuriance of those dim jungles, and that a rainfall far above anything occurring in the United States is even more important. Upon the distribution of rainfall depends the occurrence not only of the two forest types that will be mentioned here, but of most of the other chief plant societies. TROPICAL RAIN FOREST