_Mountains help to cause movement and change in the atmosphere._ Let us see how this takes place. Mountains expose on one side their masses of rock to the full heat of the sun. Rocks are capable of becoming highly heated under a blazing sun: we have known stone walls, even in England, to be almost too hot to touch; and perhaps the reader may have often noticed the quivering of the hot air as it rises from the ground on a summer day, especially over a road or any piece of bare rocky ground. This quivering tells us that the air is highly heated by the ground beneath, and is consequently rising. You know how the pebbles look beneath a clear running stream; and the things which we see through air in this state all seem to be similarly moving or quivering. It is easy then to imagine how masses of heated air would rise up from the side of a mountain-range which faces the sun,--that is, the southern side,--while on the other, or northern side they cast a soft shadow for leagues over the plains at their feet. In this way mountains divide a district into two different climates, with a light warm air on their southern slopes, and colder air on the northern, and the rising of the warm air will cause a certain amount of circulation and movement. Hence mountains help to make currents in the atmosphere, and these currents produce important consequences. When mountain-ranges trend more or less directly across the direction of prevailing winds, they always have a moist side and a dry one. In the torrid zone, where easterly winds prevail, the eastern slope is usually the moist side; but in higher latitudes, as, for example, in Europe, the western side of mountain-ranges receives the greatest amount of rainfall, because westerly winds prevail there. _Mountains are barriers dividing not only one nation from another, but separating also various tribes of plants and animals._ It will be readily understood that with the exception of birds, whose powers of flight render them independent of physical barriers, most animals find mountains more impassable than men do. We can make roads and railways, but they cannot thus aid their powers of locomotion; hence mountains put limits to their migrations. Still, climate and food supplies have a greater influence in determining the boundaries of zoölogical provinces (see chapter iv.).