We have emphasised above (p. 137) the luxuriance of the forests of the west coast of the States, but it should be noticed that luxuriant as its conifers are, there is a remarkable poverty in broad-leaved forms, as compared with these eastern forests, and this even in the warmer parts of the west coast. The reason is probably the same as in the Mediterranean region in Europe. The existence of a belt of desert to the south of the present "Mediterranean" region of western America made it difficult for the trees to migrate southwards at the onset of cold conditions in the glacial period, and thus many forms, which are known to have existed in California in Tertiary times, have now completely disappeared from the region, while they persist in the eastern forests to this day. The third type of forest which occurs in the eastern half of North America is the "rain forest" of Florida and parts of the adjacent states. Here the rainfall is abundant all the year round, with a summer maximum, and the temperature is high. There is thus no need to economise water, and where the soil permits there is a luxuriant type of forest, which recalls that of the tropics, although it is poorer. Where soil conditions are unfavourable we have pine woods, conifers throughout the eastern United States always taking advantage of conditions relatively unfavourable to the broad-leafed trees. Thus if we follow the eastern seaboard of the United States from Labrador to Florida we pass through the following floral regions:-- (1) Coniferous forest, with relatively few species, (2) mixed coniferous and deciduous forest with chiefly the harder types of deciduous trees, (3) predominantly deciduous forest with many of the larger-leafed and more delicate forms, and finally (4) forest of the sub-tropical rainy type, intermixed with coniferous woods on the barren sandy soil and in the swamps. The western coast shows more uniformity, the western type of coniferous forest stretching from Alaska to California, though it is richer, and more luxuriant in the warmer regions when moisture is still obtainable. As the moisture diminishes the forest dies away and desert or semi-desert conditions supervene. CHAPTER VI THE DISTRIBUTION OF ANIMAL LIFE