If we take the trees as representatives of the flora of North America, and this seems to be the only practicable method in a general treatise, we find them growing most densely and presenting at the same time the greatest variety where the temperature is uniformly high throughout the year and the rainfall heavy and long-continued. From the torrid lowlands the forests in general decrease in the variety and number of trees on a given area, both towards the north, where temperature becomes the controlling factor, and towards regions of small rainfall, where the leading adverse condition is deficiency of moisture. In the most highly favoured localities the struggle for existence between species and species and individual with individual is intense, exposure to the life-giving sunlight being the dominant aim of every one of the contending hosts. As drier or colder regions are approached, but few species can survive and the forests are characterized by their monotony. Where the conditions of heat and moisture are such that the existence of a species is precarious, the balance of power, so to speak, passes to the secondary conditions; and the texture and composition of the soil, slight differences in the relief of the land, and consequently in drainage or in the degree of exposure to light, prevalence of fires, etc., make themselves prominent and limit distribution. [Illustration: FIG. 28.--Map showing the distribution of forests in North America.] From a geographical point of view, the broadest features in the flora of North America are the forested and unforested areas. The distribution of the forests, prairies, and treeless plains as they existed previous to the coming of Europeans is shown on the accompanying map. For the portion of the continent to the northward of Mexico the data on which this map is based are much more abundant than for the southern portion. THE FORESTS