Excluding Africa south of the Sahara from consideration, we may indeed say that the Old World has two regions of abundant rainfall and dense population, the one to the west and the other to the south-east, separated from each other by warm and cold deserts. Each of these two regions has given rise to its own civilisation, each has produced its own types of cultivated plants and domestic animals, and the root differences between the two must be regarded as largely the result of climatic conditions.
The monsoon countries are so named because of the regular seasonal reversal of the winds, which blow from land to sea in winter and from sea to land in summer, affording an example of a land and sea breeze on the gigantic scale. The result is that, subject to local modifications, the summer winds are moisture-bringing, and the winter winds are dry. Whereas, then, in the Mediterranean the heat of summer is largely wasted, from the agriculturist's point of view, on account of the scarcity of the water necessary for growth, in monsoon regions, unless the rain fail, as it sometimes does, the hot season is the moist season, and, therefore, other things being equal, growth must be faster here than in the Mediterranean area. The monsoon countries extend over a great stretch of latitude, and therefore temperature conditions vary greatly, while the great variety of surface-relief produces here abnormally heavy rainfall, and there desert conditions. The essential contrast with the Mediterranean type is, however, the summer rainfall.
Taking the globe as a whole we find that summer rainfall is more common than winter, and in addition to occurring in monsoon regions, it tends to occur in tropical regions generally. As we approach the equator from the tropics we find that the total fall increases, and tends to show two maxima, which occur when the sun is overhead, _i. e._ at the equinoxes. For our particular purpose, however, the climatic conditions in tropical and equatorial regions generally, though of great importance to the climatologist, are not of great interest, for except in monsoon countries the hot parts of the earth do not show the most highly developed human societies.
Let us turn next to that part of Europe which is outside the reach of Mediterranean influences. Here we find that the rain is distributed throughout the year, and is usually abundant, though it decreases in passing eastwards from the seaboard. Temperatures are naturally lower than in the Mediterranean basin, and winter frost plays an important part in determining the choice of cultivated plants. As the figures which we have already quoted for London and Paris suggest, the winter cold increases on passing eastward. Paris is colder in winter than London, though it lies south of it. Vienna is again colder than Paris. But the increase in winter cold is compensated for by an increase in the summer heat. In other words, as the distance from the sea increases in Europe the climate becomes drier and more extreme.