rain or snow. The presence of a high mountain range on the west coast stops the clouds which gather about its peaks in summer, while the Sacramento Valley, a few miles to the west, swelters in a daily temperature of 100°. In the winter snow piles up to an incredible depth to be available as water supply to the coast in summer, but supplying only a few little streams for the wide arid stretches of Nevada to the east. Barring a little from the north all the water for the rest of our country must work its way north and west from the Gulf of Mexico and the Atlantic. The result is that west of the one-hundredth meridian, which passes through central Kansas, the supply is inadequate for ordinary crops. Another result is that the west coasts of America, as well as of Europe and Asia, taken as a whole, are much warmer and subject to fewer extremes than the east coasts. This is shown by the following comparison: The corresponding distribution of fog and cloud are not to be ignored. The visitor to San Francisco who neglects to carry his overcoat when he leaves the hotel in the morning of a sunny, summer day is likely to freeze when the fog rolls in about the middle of the afternoon. The man in New York at the same season is likely to look like a drowned rat if caught far from shelter without an umbrella, but in San Francisco the Sunday-school picnic may be announced a week in advance without the eastern reservation of "weather permitting." London is covered with a fog from fifty to seventy-five days a year. On the north Pacific coast 65 per cent of the days are cloudy; in the region of the Great Lakes, 50 per cent; the Mississippi and Missouri valleys, 45 to 50 per cent; while Arizona and New Mexico have one of the sunniest climates on earth, only 30 per cent of the days being cloudy. Our rainfall corresponds to the same conditions. On the north Pacific coast it varies from 60 to 100 inches annually; lower Mississippi Valley, 30 to 40 inches; upper Mississippi and Missouri valleys, 15 to 40 inches; Arizona and New Mexico, 10 to 20 inches (including areas, however, in which practically not a drop of rain falls for two or three years at a stretch), and the eastern coastal plain, 40 to 60 inches. The effect of mountains on rainfall is most strikingly shown in some tropical countries like Santo Domingo where there is an annual average of 12 inches per month in the mountains while a near-by coast has only 20 inches per year. DISTRIBUTION OF ORGANISMS It is becoming a custom among naturalists to classify organisms under three heads: bacteria, plants, and animals, for the first differ in many ways from the others. Bacteria Bacteria and the one-celled animals called protozoa are the smallest of living beings. In as much as water is an essential part of all cells, bacteria can live and multiply only in moisture. Some of the bacteria form spores, which we may think of as seeds, and exist indefinitely under dry conditions, but reproduction will occur only where it is moist. The bulk of them are to be found, then, in water or moist earth but since they are so small and light large numbers exist at any given time in the air. Where the air is dryest, therefore, the fewest bacteria are found. In the Alps one observer detected only 345 per cubic meter, whereas 4,750 were found in the air in Paris. They seem to be most abundant in humid regions of warm countries. Plant Distribution So obvious, indeed, is the paramount importance of water in its influence on their form, structure, and distribution that the classification of plants into hydrophytes, mesophytes, and xerophytes, based on this criterion, is universally recognized as the most fundamental of all ecological classifications. Plants which require a maximum water supply-typically, plants that actually grow in the water-are termed hydrophytes. Plants which require a minimum water supply, notably desert perennials, are termed xerophytes. Plants which are intermediate in their water requirements are termed mesophytes: these include the plants of ordinary moist soils. The soil moisture conditions influence the amount of water which the plant is in position to absorb from the outside world, while the atmospheric moisture conditions influence the amount it is liable to lose through the phenomenon of transpiration, the water actually available to the plant for carrying on its various activities being represented, of course, by the balance between what is absorbed and what is lost. When the amount of available water falls below a certain point, the plant wilts, and two of the greatest sources of danger the plant has to face are insufficient absorption and excessive transpiration.11 So varied are the demands of plants that there is hardly a place on earth where some plant does not grow. Even the face of the rocks may be covered with lichen. A trip across our country from New York to San Francisco is a revelation to the observant traveler of the variety of natural conditions and of plant life. He leaves a region brown or white in winter and green in summer, where the plants of China will flourish as well as native varieties, and arrives at the west coast green in winter and brown in summer to find the "gums" of Australia side by side with the olives of Italy. From a region heavily watered he passes to areas increasingly arid until farming is possible only through irrigation. He breaks through the heavily wooded eastern hills to the hillless prairies and understands the Colorado preacher's comment on Iowa as a land where "there is no more scenery than in a pan of skimmed milk." He crosses mountain ranges and sees little patches of scraggly juniper and wonders if some wag is responsible for calling them "forest reserves." The open spaces impress him as large enough to provide for untold added millions of men-until he asks himself how these men are to make a living. Just such contrasts exist in all parts of the earth. Influence of Rainfall In as much as the water is seldom available in equal amounts throughout the year, all plants must be adjusted to the peculiar condition of the area they occupy. In Haiti the telegraph lines are festooned with air plants which secure needed moisture from the air, while the trees in Louisiana may be draped in Spanish moss. In the desert, moisture is conserved in the fleshy leaves of cactus or, as in other cases, the roots of the shrubs extend a considerable distance about the plant and are near the surface to take advantage of whatever rain may fall. Growth and formation of seed must occur when the rainy season is on or reproduction may be prevented. When man appears and tries to cultivate this dry country he comes to appreciate the old saying: Little drops of water An annual average of 6 to 10 inches of water makes general agriculture possible, provided the rain comes at the right time. A much heavier rainfall coming in a brief period and followed by a long drought may force man to abandon the territory or change his crops; if the rains are light, the water may evaporate ere the crops are benefited; if very heavy they may beat down and destroy the crops and wash the land. Fog may bring little precipitation but may be very helpful by preventing transpiration, while dew often prevents freezing. As we go from the equator to the poles we find a certain regularity in the yearly distribution of rain. The equatorial regions have two wet and two dry seasons. Subequatorial and subtropical regions have one wet and one dry season. Deserts lying 25 to 30 degrees from the equator have two brief periods of rain and two hot and dry seasons. In the temperate zone the distribution is very irregular but scattered through the year while in the polar regions the distribution is quite even but small in amount. These phenomena are produced by the rotation of the earth and the other factors already mentioned. In another chapter we shall discuss agriculture under these different conditions. It may be noted in passing that an increase in rain may be a great benefit to man while a drought is always serious. Hann has noted the following increase in wheat in Australia: 12 6 bad years.......13.5 inches rain.... 6.6 bushels per acre. 5 next best years....15.4 inches rain....10.0 bushels per acre. 7 best years........18.5 inches rain.... 12.4 bushels per acre. By contrast we may note a reduction in the corn crop of the United States, due to the drought of 1913, which was estimated at 750,000,000 bushels. Animal Distribution The influence of moisture on the distribution of animal life is no less important. The abundance of the lower forms in water and wet regions has been noted already. If these chance to be of the disease-producing types the welfare of the higher forms may be affected. "It is a matter of universal observation that in tick or tick-infested countries dry seasons result in the reduction; moist seasons in the increase of |