from the south through the desert and Great Basin regions following increased aridity. The great Mexican Plateau was the original home of most of the strictly American genera now found throughout arid and semiarid western America. On this plateau a drought-resisting flora existed in the Miocene age, when the greater part of the United States from the Atlantic to the Pacific was covered with a rich deciduous forest, comprising such trees as the beach, elm and magnolia—a type of flora that still persists in the southern Atlantic States.
The rôle played by climate in California has augmented that of isolation. Without its peculiarities and diversities the rich and varied California flora would never have been evolved. California climate is lauded the world over. Yet the term means little and is misleading as it carries the impression of uniform climate. Naturally within a state extending through more than nine degrees of latitude, 769 miles, one would expect to find considerable difference in the temperature of the northern and southern sections, with a corresponding difference in vegetation. But add to this range of latitude diversity of topography with its marked influence on rainfall, temperature and atmospheric humidity, and we have a complexity of climates and climatic influences that are astounding—literally scores of climates sufficiently distinct to influence profoundly the character of the vegetation.
Temperature, one of the most important factors governing plant distribution, ranges from the perpetual snow fields of the mountains to subtropical valleys where killing frosts are scarcely known. Bordering the snows of the high Sierra such boreal plants as the dwarf, arctic willow, cassiope, bryanthus, primulas and fringed gentians, flourish, while in the subtropical sections, the lime, the olive and the pomegranate are grown, and even the more sensitive though less poetic banana and alligator pear. Everywhere the African pelargoniums, the "geranium" cherished by the eastern housewife and tenderly nurtured within her furnace-heated house, runs riot, growing into good-sized shrubs and frequently used for porch coverings or hedges. The castor bean, described in all botanical text-books as an annual, here becomes a tree living for years, and grown for ornament and shade. Between these two extremes boreal and subtropical, are all "the intermediate zones; the cool temperate, where rye, red currants and apples flourish, and the warm temperate with the almond, apricot and fig.
But great as is the range of temperature and its effect on vegetation, rainfall and atmospheric humidity are fully as varied and play even a more important role over a large part of the state in determining the character of the vegetation. The normal annual rainfall in certain localities of the northwest coast region runs nearly to one hundred inches. At San Diego, also on the coast, it is only a little over nine inches. On the deserts, lying east of the mountains which have robbed
