Page:Popular Science Monthly Volume 54.djvu/248

234 are reaped from four to seven months after. But in the Delta, rice, cotton, and indigo are sown in the spring (March or April) and harvested in October, [November, and December. Here, irrigation and temperature come in as disturbing elements, for the Delta feels something of the cold of winter.

I could give many other instances, but these will suffice. As a general rule, we may say that in the temperate and frigid zones the seasons for plants and animals are ruled by heat and cold, but that in tropical and even in subtropical climates, rainfall and drought, themselves largely due to the same circumstances, are the ruling factors.

Again, everybody knows that winter and summer, and the other phenomena which simulate or accompany them, such as wet and dry seasons, depend upon the fact that the earth's axis is not perpendicular to the plane in which the earth moves round the sun, but slightly inclined to it. Now, a year in itself, viewed as a measure of time, is merely the period which it takes the earth to perform one such complete revolution. During one half of each such revolution the north pole is turned at a considerable angle toward the sun, and during the other half, the south pole. When the north pole is so turned we call it summer in the northern hemisphere; when the south pole is being favored, and the north is receiving less light and heat, we call it winter. Let us suppose for a moment that the earth had not got this twist or kink in its axis; that the equator was always presented exactly toward the sun; what then would happen? Obviously, there would be no change of seasons. The day and night would have fixed lengths which never varied; climate would in each place be uniform and, barring accidents of elevation or distribution of land and water, the climate of each place would also depend entirely the whole year round on its distance from the equator. Roughly speaking, the temperature of a district would be the temperature it now possesses in March and September, only not quite so cold as March nor so warm as September, owing to the absence of accumulated heat from summer or of reserves of ice and snow from winter. In one word, under such conditions there would have been climates—marked belts of climate; but there would not have been seasons.

Seasons, however, depend in great part, as Mr. Alfred Russel Wallace has ingeniously shown, on a great many things besides this mere inclination of one end or the other of the earth toward the sun in June and January. Much must be laid to the count of accumulated stores of heat or cold; and though accumulated cold is physically a misnomer, still for all practical purposes we may apply the words fairly enough to the ice caps of the pole and the