Reasons for the Contrast between Mountains and Plains.— (1) The Effect of Altitude.—There are three chief reasons why the life in mountains differs from that in plains, namely, (1) altitude, (2) climate, and (3) relief. Altitude alone is relatively unimportant. People with heart trouble, to be sure, cannot live even at an altitude of 5000 feet, and most people find difficulty in breathing at altitudes of 10,000 feet or more. Nevertheless when people go to high alti tudes the body soon adapts itself to the new• conditions. An increase in the number of red corpuscles in the blood enables it to absorb oxygen more rapidly, and thus the rarity of the air, which is the great difficulty at high altitudes, is robbed of much of its effect. When people come down from the mountains this excess/if red corpuscles makes them feel very strong, but it quickly passes away. This prompt change in the blood enables people to adapt themselves to any altitude where the climate and relief make it possible to get a living. Denver, for example, has become a great city a mile above sea level, Mexico City is half a mile higher, Quito prospers at an altitude of nearly two miles, and certain villages in the Andes and Tibet raise barley and sheep nearly three miles above the sea. In each case a high plain makes it possible for the city or village to grow up in spite of the altitude.
(2) How the Climate in Mountains and Plains.—In the loftiest villages of the Andes and Tibet the villagers think little about the altitude, but much about the climate. This is because altitude influences climate in three main ways: (a) Temperature decreases with altitude. In the free air the fall is about F. for 400 feet of altitude in summer and for 500 feet in winter. The fall is more marked where mountains rise steeply above a lowland as in the Alps, than in regions like the Great Plains where one can rise from sea level to Denver almost without noticing any grade. Nevertheless the average yearly temperature at Denver is about 3° lower than at Indianapolis, which lies in the same latitude but 4500 feet lower. (b) The greater the altitude the more variable is the temperature. The rarity of the air allows the sun's heat to pass through it readily and thus the earth's surface is quickly warmed, but the same con dition also allows the earth's heat to pass away rapidly at night, so that there are great extremes. The relief also causes variability, for cool air may flow down a valley at night while warm air rises by day. (c) Mountains are also more cloudy and rainy than plains, for the currents of air that approach them must rise. Hence the air is cooled and its water vapor condenses into clouds and rain. On a perfectly clear day in the plains of California one can often see great banks of clouds enshrouding the crests of the Sierras only 40 miles to the east. While the dry brown grass of the plains shows that no
rain has fallen for months, the dense pine forests of the mountains, and the little brooks flowing amid rich green grass or thick brakes of flourishing bushes betoken rain in plenty.
(3) The Great Importance of Relief.—In the rest of this chapter we shall confine ourselves largely to relief, the third of the great reasons for the contrast in the life of mountains and plains, but it must be remembered that climate and relief work together so closely that they often cannot be separated.
The Uneven Distribution of Population in Mountain Regions and the Even Distribution in Plains: Switzerland versus Iowa.— One of the conditions where the effect of relief alone can be most clearly seen is in the different distribution of the population among mountains and in plains. Fig. 30 is a map of Switzerland showing the density of population. Notice how irregularly the people are scat tered. There is a great concentration in the northern lowland, where most of the people live, while among the mountains the inhab itants are distributed here and there without apparent order, but really along the main valleys. Contrast this with Fig. 31, a similar map of the plain of Iowa. How evenly the people are distributed! Practically every township of 36 square miles contains about a thousand. A few cities have grown up, but are quite evenly spaced throughout the State and not concentrated in one section, as in Swit zerland. The reason is obvious: In Switzerland people cannot live in any large numbers in the rugged portions and must concentrate in the valleys; in Iowa the plain is so uniform that people can live anywhere.
The Sparsity of Population in Mountains Compared with the Density in Plains.—There is a strong contrast between mountains and plains in the density of population as well as in the distribution. This is due to the combined effects of relief and climate. In Califor nia there is a beautiful district called Alpine County. In 1890 its population numbered 667, or a little less than one for every square mile. In 1900, this had fallen to 509, in 1910 to 309, and in 1920 to 243. Some 80 miles west of Alpine County, Sacramento County, though only a little larger, contained 40,000 people in 1890, or more than 40 to the square mile, while in 1900 the population numbered 46,000, in 1910, 68,000, and in 1920, 91,000. Why should one county contain only one person in 3 square miles and show little or no prospect of containing more, while the other contains 92 people to the square mile? The answer is simply that Alpine County is one of the most mountainous parts of the United States, while Sacramento County is a smooth plain.