If, on the other hand, what may be called the prairie type of climate has resulted in the formation of grassland, which has happened in our own West, in the steppes of Russia and less extensively in many other places in the world, forests can hardly ever get a foothold. Where, as in river bottoms, they sometimes flourish, the line between forest and grassland is sharp and apparently an impassable barrier for trees. This invasion of immediately adjoining territory is going on constantly not only by different types of vegetation but by the units of it. The frequency of different plants in different years, their final ascendancy or extinction, all point to the struggle for expansion which in a score of ways the plant world is constantly waging. In many cases we are not yet able to see the struggle, but only its results, while in some places the bitterness of it may be gauged by the dead and dying that strew these silent battle fields. The dominance of certain species of plants, such as grasses on a prairie, the fir and spruce in the coniferous forests of our North, the blue-gum trees in parts of Australia and the giant dipterocarp forests in the Philippines, are all based on the ability of individuals to spread from their point of origin. All species of plants must have one day been of very local distribution and confined to the region where they were born, but from their often very modest beginnings some of them at any rate have gone to the ends of the earth. The common bracken fern is found in nearly every country in the world from far northward through the tropics to the antipodes, and yet no one knows where its original home might have been. Somewhere up in the northern Andes it is supposed that the first ancestor of the huge daisy family had its origin. From there its many descendants, now ramified into hundreds of genera and over eleven thousand species, have spread to the very limits of plant growth. And the daisy family is one of the most recent of all the families of plants. The distribution of plants is of many kinds depending on local conditions of climate and soil, on individual and racial competition, on methods of fertilization or other means of propagation or seed dispersal, and particularly upon the distribution of the plant or its ancestors in past ages. While it is often difficult or impossible to determine upon which of these factors, or upon what combination of them the distribution of any particular species is based, certain facts of plant dispersal appear to be indicated by a study of existing floras. A few instances must suffice here to illustrate the principles by which many plants are scattered over the earth, or else restricted to localized regions, and which, without knowledge of the factors involved, seem merely the wayward caprice of nature.