Page:Popular Science Monthly Volume 54.djvu/255

Rh seasons and the climate in each particular country depend in part upon many minor contributory causes. It is not merely nearness to or distance from the equator that counts; we have to consider also relative distribution of land and water, elevation, prevalent winds, exposure, condensation, and many other elements of a complex problem. In Ecuador, for example, whose very name means the equator, the plain is always in scorching summer, the mountains are always in perpetual spring. The monsoons, again, produce in other countries some curious results: they depend themselves on the change of relative temperature in sea and land at different seasons; and they break upon the Himalayas with this odd and unexpected effect, that the snow line on the southern side of that vast range goes very far down, owing to the immense rainfall (or rather snowfall) and the consequent spread of snow fields and glaciers; while on the northern side it descends but a very little way, owing to the extreme desert drought and the great summer heat of the central Asiatic table-land. We have thus the apparent paradox that millions of Tibetans occupy towns and cultivate farms to the north at a height from three to four thousand feet above the snow line on the southern slope of the same mountains.

Looking at the matter broadly, then, and taking for granted the now generally accepted modern view that the great oceans and great continents have been relatively fixed (though liable to minor fluctuations and variations of outline) throughout all geological time, and that the earth's crust has not shifted from pole to equator or vice versa, we arrive at last at the following probable conclusions: There have always been seasons more or less marked, and these have been more or less organically answered by corresponding changes or cycles of change in plants and animals. Rain and drought have in many cases more to do with such changes than variations of temperature. The seasons, again, are less marked in the tropics than in temperate and circumpolar climates. Nevertheless, even near the equator, they exert and have always exerted certain organic influences—have resulted in annual cycles in the life of species. Even before the coming on of the Glacial epoch, the seasons were probably somewhat more marked in the temperate and polar regions than in the tropics, the longer day in summer and the greater directness of impact of the rays making the summer months always warmer. But for various reasons, among which we may presumably rank the absence in early ages of high land at the poles and of an accumulated polar ice cap, together with the existence of warm sea currents from the tropics to the poles, the winters of preglacial ages seem to have been relatively mild, perhaps (if we may judge by the types of plant life) milder than those of South Carolina and Georgia in our own period.