The movement of the seed or fruit of a spe cies into a new formation or country is often determined by natural barriers. Winds bear seeds for long distances, but they are powerless to carry them across oceans, or over high mountain ridges. Similarly, a desert region is a barrier to seeds brought from a moist climate, and a cold climate prevents the naturalization of species coming from a warm country. The chance that seeds will germinate and grow is greatest when they are carried into a habitat similar to the original one and it is least when they are left in habitats very different from it. It is unquestionable that seeds have often been carried into many places where they were un able to secure a foothold. This fact explains why many species are found only in certain countries, or localities, and why it is that each formation retains a more or less distinctive impress.
North American The vege tation of the North American continent owes its general features to the gradual decrease of to the northward, and the more or less con stant decrease in the rainfall in passing from the coasts to the interior. The greatest develop ment of forests is found in the warm coast regions of the southeast, and of the Pacific. The poorest vegetation is found in the north, and on high mountains, where the temperatures are low, and in the interior where the rainfall is slight. The character and distribution of vegetation are chiefly determined by heat and water. As a result, the vegetative covering falls into zones corresponding in a general way to zones of tem perature. If the distribution of moisture were uniform over the continent, the series of zones would be as follows: (1) the zone of ever green tropical and subtropical trees; (2) the zone of deciduous trees; (3) the zone of cone bearing trees; (4) the zone of grasses and other herbs; (5) the zone of mosses and lichens; (6) the zone of ice and snow. The rainfall de creases regularly from the coast inland, while a high mountain range makes an abrupt change in the amount. The Appalachian, Rocky Moun
tain and Sierra Nevada ranges act as barriers to the passage of moisture-laden winds, and turn into grass land or desert, regions that are sufficiently warm to be forested. The Appa lachian barrier is too low to be very effective, and the forests yield to prairies only slowly and far inland. The Sierra Nevada and the Rocky Mountains are almost complete barriers, and they enclose a parched desert. The height of these ranges causes an abundant condensation on their slopes, and in consequence they are more or less heavily wooded. On account of the altitude, the temperature is low, and the forests are merely southerly extensions of the great boreal zone of pines and spruces. A general survey of the North American conti nent would show it to be wooded on the eastern, western, and southern coasts. In the north, there is a zone of grass and moss-covered bar rens. In the interior there is a region of plain and prairie, stretching unbroken from Atha basca to Texas, and between the two great Cordilleran ranges from Washington to Central America lies a great desert region, broken re peatedly by intersecting lines of mountains. Riming southward from the great northern forest mass of the continent are the three mountain systems. In the low Appalachian system, the arctic vegetation of the north is found on a few alpine peaks alone, but in the higher Rocky Mountains and the Sierra Nevada this long southward extension of dwarf herbaceous vegetation is almost continuous. All carry the northern pines and spruces far south, but in the lower range, these disappear in Virginia, while on the higher ranges they persist almost to the Mexican boundary. North Amer ica is thus seen to be covered with belts of vegetation running east and west, which are completely interrupted in the interior by high mountain ranges, which, together with the Appa lachians, also serve to carry the northern forests southward in three long tongues.