Popular Science Monthly/Volume 76/March 1910/Climate in Some of its Relations to Man

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By Professor ROBERT DeC. WARD


Climatology and Meteorology.—In a course of lectures dealing with the present status of meteorology the subject of climate, upon which I have the honor to address you this afternoon, finds an appropriate place. For meteorology and climatology are interdependent, and it is impossible to distinguish very sharply between them. In a strict sense, meteorology deals with the physics of the atmosphere, and those of you who have attended the preceding lectures in this course have listened to able discussions of the physical problems with which meteorologists are to-day concerned. The view taken by meteorology is largely theoretical, but the main object in the solution of most of these problems is to make this science of immediate practical service to man, in improving and extending our weather-forecasts.

When the term meteorology is employed in its broadest sense, climatology is a subdivision of meteorology. Climatology is largely descriptive. It rests upon physics and geography, the latter being a very prominent factor. In fact, climatology may almost be defined as geographical meteorology. The main object of climatology is also to be of practical service to man. Its method of treatment lays the most emphasis upon the elements which are of the most importance to life. Climate and health, climate and industries, climate and crops, climate and transportation—these are subjects of vital human interest. It is my privilege this afternoon to suggest a few of the points of contact between man and his climate. If my discussion seems disjointed and haphazard, I beg of you to remember that the subject is one of the widest possible range; it concerns all men, in all parts of the world. To select from this immense body of facts the few which it is possible to touch upon in an hour is like trying to decide which of a thousand snowflakes is the most symmetrical; or to determine, in a wonderful view across the snow-covered mountains on a brilliant winter's day, whether it is the sun, or the crisp air, or the snow, or the contour of the hills, or the grouping of the trees, or the picturesque farmhouses, which really contribute most to make the picture what it is.

The Climatic Zones.—So great is the variety of climates to be found in the world that it has long been customary to classify these climates into certain broad belts, which we call the zones. These were first suggested, on purely astronomical grounds, in the times of the early Greek philosophers and geographers. It is to be noted that these zones are really zones of sunshine, or of solar climate, a subject which Professor Libbey has already considered in his lecture on "Astronomical Climate" (January 19, 1909). The so-called "torrid" zone has the greatest annual amount of sunshine. It is the summer zone. The polar zones have the smallest amount of sunshine. They may well be called the winter zones. The temperate zones are intermediate between the tropical and the polar, in the matter of the annual amount and of the annual variation of sunshine.

The Temperature Zones.—The usual classification of the climatic zones on the basis of the distribution of sunshine serves well enough for purposes of simple description, but a glance at any temperature-chart shows at once that the lines of equal temperature (isotherms) do not coincide with the lines of latitude. In fact, in the higher latitudes, the lines of equal temperature often follow the meridians more closely than they do the parallels of latitude. The astronomical zones—i. e., the zones of light—therefore differ a good deal from the zones of heat. Hence, in recent years, it has become quite customary, at least in climatology, to limit the zones by lines of equal temperature, thus making a closer approach to the actual conditions of climate.

Characteristics of the Tropics.—The dominant characteristic of the great equatorial zone is the remarkable simplicity and uniformity of its climatic features. The tropics lack the proverbial uncertainty and changeableness which characterize the weather of the higher latitudes. Within the tropics, weather and climate are essentially synonymous terms. Eegular conditions, which depend upon the daily and annual march of the sun, are dominant. Irregular weather changes are wholly subordinate. In special regions only, and at special seasons, is the regular sequence of weather temporarily interrupted by an occasional tropical cyclone. These cyclones—the hurricanes of the West Indies and the typhoons of the China seas belong to the group—although infrequent, are notable features of the climate of the areas in which they occur. The devastation produced by one such storm often affects the economic condition of the people in the district of its occurrence for years.

Over nearly all the equatorial zone the difference between the average temperatures of the warmest and coldest months is less than 10°, and over much of it it is less than 5°. At Equatorville, in the interior of Africa, on the Congo, the difference between the average temperature of the warmest and coolest months is only a little over 2°. The variation in temperature during the day is usually larger than this seasonal difference. Thus, at Equatorville it is seven times as large. It has been well said that "night is the winter of the tropics." Over much of the equatorial zone the lowest temperatures usually do not fall below 60°. Maximum temperatures of 115°-120° occur over the deserts of northern Africa. In a true tropical climate seasons, in our sense, do not exist. The variations in temperature throughout the year are so slight that the seasons are not classified according to temperature, but depend on rainfall and the prevailing winds. The life of animals and of plants in the tropics, and of man himself, is regulated very largely, in some cases almost entirely, by rainfall. Although the tropical rainy season is characteristically associated with a vertical sun (i. e., "summer"), that season is not necessarily the hottest time in the year. In fact, the temperature is usually somewhat lower under the clouds, and hence the rainy season often goes by the name of "winter."

Within the tropics the equatorial belt of calms and variable winds—the "doldrums" of sailors—offers exceptionally favorable conditions for abundant rainfall. The rainfall is so heavy that the surface waters of the ocean are actually fresher than in the latitudes to the north and south. The sky is prevailingly cloudy; the air, hot and oppressive; heavy showers and thunderstorms are frequent. In the latitudes of this belt are the dense tropical forests of the Amazon and of equatorial Africa. Here drought and frost need not be feared. The belt of calms and rains shifts north and south of the equator after the sun. It is dreaded by seamen because sailing vessels are apt to be delayed in crossing it, but delays of great length are infrequent nowadays, since a careful study of the prevailing winds over the ocean areas has shown navigators at what points the belt of calms is narrowest, and where the crossing may be the most successfully made.

In striking contrast are the easterly trade winds, which blow toward the equator from about latitude 30° north and south. Of great regularity, embracing about one half of the earth's surface, the trades have long been favorite sailing routes because of the steadiness of their winds, the infrequency of their storms, the brightness of their skies and the freshness of the air, all of which are in pleasing contrast with the muggy, oppressive calms of the doldrums. All sailing routes which pass through the trade wind belts in any ocean are controlled by these winds. Steady winds like the trades certainly tempted the early navigators to put to sea. The famous voyage of Columbus was facilitated, if not made possible, by the northeast trade. The easy outward voyages of the early Spanish adventurers and colonists took them naturally to that portion of the Americas where they found even-tempered climates in which they and their descendants could live comfortably. The monsoons of India have from the earliest days of trade with the east been important agents in aiding commerce.

The most desirable house sites in the tropics are very commonly on the top of some elevation, exposed to the trades. The sea breeze, also, is an important climatic feature on many tropical coasts. "With its regular occurrence, and its cool, clean air, it serves to make many districts habitable for white settlers, and has deservedly won the name of "the doctor." The location of dwellings is often determined by the exposure of a site to this wind. For this reason, many native villages are placed as near the sea as possible. The houses of well-to-do foreigners often occupy the healthiest and most desirable locations, where the sea breeze has a free entrance, while the poorer native classes live in lower, less exposed, and less desirable places. A social stratification is thus determined by the sea breeze. In our own latitudes, exposure to sunshine is very important, as is well known, in determining house sites. Lugeon's study of one of the principal valleys between Martigny and the Rhone Glacier, has brought out some interesting facts in this connection. In this district the villages, with one or two exceptions, are on the sunny side. In fact, a distinction of classes results from this difference. There is developed what may be called an "aristocracy of the sun." The people on the sunny side are more prosperous and better educated, and look with some contempt upon the people on the shady side.

The trades, except where they blow onto windward coasts, or over mountains, are dry winds. On the lowlands swept over by the trades, beyond the polar limits of the equatorial rain belt (roughly between latitudes 20° and 30°) are most of the great deserts of the world. The interior of Africa has been out of contact with the civilized world largely because of the deserts to the north and south of it. Goods and passengers go around rather than across these deserts. In the desert, population gathers in oases, as on islands. Here the trails followed by the caravans converge like sailing routes at sea. There are small Arabian towns, built at oases, where the houses are almost crowded on top of one another, producing something not unlike the modern "sky scraper" of an American city, where land is scarce and expensive. The overflow of the Nile results from the rainfall on the mountains of Abyssinia during the northward migration of the belt of equatorial rains; and one of the most difficult problems in the construction of the Panama Canal, viz., the control of the floods of the Chagres River, is due to a similar cause.

The monsoons, reference to which was made a moment ago, are a special development of the general trade wind system. Monsoon regions have summer rainfall, and these rains are particularly heavy where the winds have to climb over high land. Thus, in India, the precipitation is heaviest at the head of the Bay of Bengal, where in the Khasi Hills, at a height of a little less than a mile above sea level, the rainfall averages between 35 and 40 feet a year. This is about ten to twelve times as much as the rainfall of New York City, and all this water falls in less than six months. Truly, at that place, "it never rains but it pours." In certain parts of India stores of provisions are laid in before the rains begin, the preparations being similar to those made on board a vessel bound on a long voyage. Special mention may be made here of a peculiar relation between climate and man in the case of certain tropical mountains which are high enough to receive snow instead of rain on their upper slopes. These mountains furnish a supply of snow and ice for refrigerating purposes in the towns below them. Thus, in Ecuador, snow is carried to Quito from the upper slopes of Pichincha; to Riobamba and Ambato from the slopes of Chimborazo. Guayaquil was formerly supplied with ice in the same way. In Colombia, Popayan, in the department of Cauca, is also supplied with ice and snow from the neighboring mountains. In Mexico, snow is carried from the summit of Colima to the towns on the hot plains below. The occupation of the Indians who bring down this ice from their tropical mountain tops is a curious example of climatic control.

Characteristics of the Temperate Zones.—The so-called "temperate zones" occupy about one half of the earth's surface. The north temperate zone includes the greatest known extremes of temperature, and if the word "temperate" were not so firmly established it would be well to change the name to "intermediate." A marked changeableness of the weather is a striking characteristic of these zones. For most of the year, and most of the temperate zones, settled weather is unknown. Climate and weather are here by no means synonymous. The changeableness of the weather suggests our never-failing subject of conversation. In the tropics no one talks about the weather, for it is monotonously the same, day after day.

In the north temperate zone the differences in temperature between the warmest and coldest months reach 120° at their maximum, in northeastern Siberia. An average January temperature of—60° and an average July temperature of 95°, with maxima of over 120° and minima lower than—90°, occur in this same zone.

The prevailing winds of the "temperate zones" are the westerlies, which occupy about as much of the earth's surface as do the easterly trades. The westerlies are, however, less regular than the trades, being much confused and interrupted by storms. So common are such interruptions that the prevailing westerly wind direction is often difficult to discern without careful observation. The south temperate zone is chiefly water. Hence the westerly winds are there but little interfered with by land. "Roaring forties" is a well-known designation for the southern middle latitudes, and between latitudes 40° and 60° south the well-named "brave west winds" blow with a constancy and a velocity hardly known in the northern hemisphere. Storms, frequent and severe, characterize these southern hemisphere westerlies. Voyages to the west around Cape Horn against head gales, and in cold, wet weather, are much dreaded, and are apt to be long and dangerous.

Between the trades of the tropics and the westerlies of the temperate zones lies a debatable belt, shifting seasonally. Within it, stormy westerlies and drying trades alternately hold sway. It is known as the "subtropical belt." With prevailingly fair skies, even temperature and moderate rainfall, the subtropical belt is a favored climatic region, where invalids seek health, and an escape from the rigors of a cold winter is found by many who have the time, and the means, to leave their northern homes. The long list of well-known health resorts on the Mediterranean, and the shorter list for southern California—"the American Riviera"—bear witness to the popularity of this subtropical belt.

Seasons in most of the temperate zones are classified according to temperature—not, as in the tropics, by rainfall. The four seasons are important characteristics, especially of the middle latitudes of the north temperate zone. These seasonal changes are of the greatest importance in the life of man. They control his occupations, his crops, his place of residence, to a considerable extent his health.

The north temperate zone embraces so great a variety of climates that no single district can be taken as typical of the whole. Its climate has been called "a crazy quilt of patches." The south temperate zone, on the other hand, may be described as a piece of fairly uniform texture and appearance throughout. This is the effect of the large ocean surface. The whole climatic regime is more uniform than that of the northern zone. The south temperate zone may truly be called "temperate," but our own zone is certainly in the highest degree "intemperate."

Characteristics of the Polar Zones.—The climate of the polar zones gains a peculiar character by reason of the longer or shorter absence of the sun. At the poles themselves, the day and the year are alike. In the Arctic climate, plants must make rapid growth in the short, cool summer. They grow and blossom with great rapidity and luxuriance where the exposure is favorable, and where the water from the melting snow can run off. Over great stretches of the northern plains the surface only is thawed out in the warmer months, and swamps, mosses and lichens are found above eternally frozen ground. In high latitudes, where the exposure is good, snow melts in the sun even when the temperature of the air in the shade is far below freezing. It has been reported that at Assistance Bay, latitude 74.5° north, in March, when the air temperature was about—25°, snow near stones and other dark objects melted in the sun. The temperature in the immediate vicinity of the North Pole is probably a little below—40° in January; below 32° in July and a few degrees below 0° for the average of the year. It may be noted, however, that northeastern Siberia has a January mean temperature which is 20° lower than that at the North Pole in the same month.

For the Antarctic our knowledge is still very fragmentary. The low temperatures of the south polar summer, which are probably due to the great continental mass of ice around the south pole, are responsible for much of the difficulty of Antarctic exploration. The average annual temperatures have been in the vicinity of 10°-15°; and the minima of an ordinary Antarctic winter go down to—40°, and below, but so far no minima of the severest Siberian intensity have been noted. The British expedition on the Discovery recorded a minimum temperature of—67.7°, and also noted—40° in midsummer. The highest temperatures have varied between about 35° and 50°. It is likely that near the south pole will prove to be the coldest point on the earth's surface in the average for the year, and also that the lowest winter and summer temperatures in the southern hemisphere will be found in the immediate vicinity of the pole.

The polar zones have a permanent deficiency of precipitation (1510 inches, or less). The polar deserts of snow and ice are therefore deserts in more senses than one, although it is natural that these extended snow and ice fields should tend to give an exaggerated idea of the actual amount of snowfall. So far as exploration has yet gone into the highest northern latitudes, rain has been found to fall in summer, and it is doubtful whether there are any places in the world, near sealevel where all the precipitation comes in the form of snow. Perhaps the interior of the south polar continent may never have rain. The snow of the polar regions is characteristically fine and dry, and it has been pointed out that the snow huts of the Eskimos could not be built with our kind of snow. At low polar temperatures flakes of snow are not found, but precipitation is in the form of ice spicules.

The inner polar areas seem to be beyond the reach of the most frequent and most violent storms, and as most of the observations thus far obtained from the Antarctic come from the marginal zone of great storm activity, violent winds and wet, disagreeable weather, they do not show us the features of the actual south polar climate. Extraordinary records of storm and gale have been brought back from the far south and the far north. During the long, dreary winter night the temperature falls to very low readings. Snowstorms and gales alternate at irregular intervals with calmer spells of more extreme cold and clear skies. There is no really warm season. The summer is essentially only a modified winter, especially in the Antarctic. Yet the Arctic summer, with its long days, crisp, clean air and sunshine, has many attractive qualities, and we may fairly safely predict a considerable development of summer resorts within the Arctic circle for the pleasure-loving, wealthy and unoccupied persons of the north temperate zone.

Climate and Health.—We have now seen something of the climatic zones and of their characteristics. Let us turn for a few minutes to the question of climate and health—a subject which is surely of the greatest concern to man. From the earliest times people have sought in atmospheric conditions an explanation of the occurrence of disease. Many fairly obvious facts naturally point to some relation of cause and effect in this matter. Some diseases are found principally in warmer climates; others seem to prefer the colder. Some are usually more active in the warmer, or the drier months; others have shown the contrary relation. High altitudes are free from some of the diseases which prevail near sea-level, and have certain favorable climatic characteristics long recognized in the treatment of disease. In the case of other diseases, again, altitude has no effect. Dry climates, especially deserts, whose air is usually exceptionally pure and aseptic, are generally healthful, and are beneficial in many cases where mountain climates are too stimulating. The climates within forested areas have proved especially favorable in cases of phthisis. Ocean air, pure and dust-free, with its saline constituents and equability of temperature, is beneficial to most persons as a moderate tonic and as a restorative in many illnesses. Winds are active ventilating and purifying agents where population is congested. Fogs and clouds, by cutting off sunlight, weaken one of the best agents in promoting health, for sunlight, in the words of Dr. Sternberg, is "one of the most potent and one of the cheapest agents for the destruction of pathogenic bacteria." In London, a higher death-rate follows a long fog, but this may result from the lower temperature during the fog, and not from any direct effect of the fog itself.

A Complex Subject.—Facts like these naturally prejudice one in favor of a causal connection between atmospheric conditions and disease. Nevertheless, such studies have often led to very contradictory conclusions. Some of the difficulty arises from untrustworthy statistics, but most of the disagreement comes from the fact that not only may each of the different weather elements have some effect in the production of the disease, but so many other factors are concerned in the matter that confusion and contradiction in the conclusions reached are inevitable. Sanitation, food, water, habits, altitude, character and moisture of the soil, race, traffic and other controls, serve to complicate the problem. In most studies of climate and health some, or even many, of these factors have not received attention. Overcrowding under unhygienic conditions, especially indoors during cold weather, and traffic by rail, steamship, caravan or on foot, are often more important than climate. The frequent escape of mountain, of desert and of polar peoples from epidemics is to be attributed in most cases to the smaller chance of importing disease because of little intercourse with the outside world, and of spreading it, when imported, because of the scattered population. It may be noted, however, that crowding indoors in winter, and the sparseness of population just referred to, are themselves climatically controlled.

Climate, Microorganisms and Disease.—The cause of disease is no longer sought directly in meteorological conditions, but in the effects, more or less direct, of these conditions upon the microorganisms which are the specific cause of the disease. Atmospheric conditions may help or may retard the development of the microorganism, and may strengthen or weaken the individuals' power of resistance. Winds used to be regarded as the chief agents in spreading epidemics: now it is known that disease can not be carried far by winds, for the microorganisms do not long maintain their power in the free air and under the sun. Eain has been supposed directly to control the distribution of disease: now we believe that precipitation acts only indirectly, through drinking-water, or through its control over the dust in the air. Dust from dry soil, and from city streets, may contain the germs of infectious diseases, and aggravate affections of the respiratory organs.

Geographical Distribution of Disease.—The scheme of classifying disease geographically, on a broad climatic basis, is attractive, but not very satisfactory. For, on the one hand, many diseases are practically universal in extent, showing great independence of climate, and, on the other, the history of many diseases is still in the making. In spite of this complexity, however, certain broad statements may be made, useful in enabling the layman properly to coordinate his ideas on the subject, and fairly accurate within reasonable limits.

Tropics: General Physiological Effects.—Tropical monotony of heat is associated with high relative humidity, except over deserts and in dry seasons. The air is therefore muggy and oppressive. This "hot-house air" has an enervating effect. Energetic physical and mental action are often difficult, or even impossible. The tonic effect of a cold winter is lacking. These conditions have certain fairly well-established physiological effects, which, combined with less power to do work, greater fatigue from work, and lowered vitality, render the body less able to resist disease.

Hygiene in the Tropics.—Under the peculiar conditions of tropical climates, the resident who comes from a cooler latitude should take special precautions regarding his mode of life and personal hygiene. A rational, temperate life, especially the avoidance of alcoholic excess; regular exercise; non-fat-producing food; clothing suited to the climate; all possible sanitary precautions; protection against mosquitoes; frequent change of climate by returning to cooler latitudes—all these are important. It seems like a contradiction, but it is a fact, that the danger of becoming chilled in the tropics is very great and must be carefully guarded against. General Wolseley is reported to have said of the tropics, "not to get cold is to avoid almost certainly all the causes of disease," and a recent writer has well said that these words should be inscribed on the walls of all barracks in the tropics. The situation may be summed up in this rule: "Respect the sun, and rain and wind; clothe with a view to avoiding chill; live temperately." On the Calcutta docks are painted the words: "Beware of the sun."

Tropical Diseases.—Certain diseases are so much at home in the tropics that they have come to be known as tropical diseases. This designation, however, does not mean diseases confined to the tropics, but is employed in a meteorological sense for diseases associated with, but not solely, or even necessarily directly due to, high temperatures. Sir Patrick Manson has made it clear that the difference between the diseases of tropics and extra-tropics lies in the specific cause of these diseases. For the development of certain disease germs, tropical temperatures are required; or a third organism, other than the disease germ itself and man, may be necessary. If this organism is a tropical species, as in the case of the tsetse fly, the disease is a tropical disease. "The more we learn," Dr. Manson says, "about these [tropical] diseases, the less important in its bearing on their geographic distribution, and as a direct pathogenic agency, becomes the rôle of temperature per se, and the more the influence of the tropical fauna." The fact that plague, and leprosy, and to some extent cholera as well, are practically limited to the tropics, is the result of modern sanitary precautions in the extra-tropics. The unsanitary conditions among tropical peoples favor the spread of these, and similar, diseases, and not the climate per se. Nevertheless, it is clear that these very unsanitary conditions are "more or less an indirect outcome of tropical climate."

General Conclusions: the Tropics.—All parts of the equatorial zone are not equally disagreeable or hostile to the white race. Many elderly persons, and those who are overworked, may find rest from nervous tension in the enervating climate of the tropics. Much-needed relief from the heat at sea-level may be obtained at tropical mountain stations, and many of these have become well-known health resorts. In India, the hill stations are crowded during the hot months by civilian and military officials, and it has been well said that India is governed from 7,000 feet above sea-level.

Acclimatization of the White Race in the Tropics.—The acclimatization of the white race in the tropics is a question of vast importance. Upon it depend the control, government and utilization of the tropics. It is a very complex problem, and it has been much discussed. It is complicated by race, diet, occupations, habits of life and the like. To discuss it fully is impossible at this time. The gist of the matter is this: white residents from cooler latitudes, on coming into the tropics, must adjust themselves physiologically to the new climatic conditions. During this adjustment there is more or less strain on various organs of the body. The strain may be too severe, then the individual suffers. The adjustment is usually much retarded and hindered by a persistence in habits of food, drink and general manner of living which, however well suited to the home climate, do not fit tropical conditions. During the adjustment, especially if complicated by irrational habits, the body is naturally sensitive to the new diseases to which it is exposed. Even should no specific disease be contracted, there are anasmic tendencies and other degenerative changes. Experience teaches that white men can not, with impunity, do hard manual labor under a tropical sun, but that they may enjoy fairly good health as overseers, or at indoor work, if they take reasonable precautions.

Acclimatization, in the full sense of having white men and women living for successive generations in the tropics, and reproducing their kind without physical, mental and moral degeneration—i. e., colonization in the true sense—is impossible. Tropical disease and death rates, as has been abundantly shown, can, however, be greatly reduced by strict attention to sanitary laws. And with increasing medical knowledge of the nature and prevention of tropical diseases, as well as by means of modern sanitary methods, a white resident in the tropics will constantly become better able to withstand disease. For greater comfort, for better health and for greater success, properly selected hill stations will, however, always be essential to northerners who have to live in the tropics, especially to white women and children.

It has been well said that the white soldier in the tropics is "always in campaign; if not against the enemy, at least against the climate." This sentence may be made to fit the case of the white civilian in the tropics by making it read: the white race in the tropics is always in campaign against its enemy, the climate.

Health in the Temperate Zones: General.—In the temperate zones the organs of the body act more equally than in the warmer and cooler latitudes. The winter cold is met by means of warm clothing, heated houses and other means of protection. Unless too severe, or too prolonged, the cold winter acts as a healthful stimulant upon body and mind. In the tropics, the body is unused to adjusting itself to temperature changes, because such changes are there slight, and is readily affected by them. But the frequent, sudden and severe changes of many parts of the temperate zone are usually borne without serious discomfort or injury, if the body is in good health, and is accustomed to adjusting itself readily to these changes. The habit of keeping houses very warm in winter, and of having the air indoors very dry, weakens the body's power to resist the cold outdoors, especially if the air be damp, and aggravates affections of throat, lungs and nose. The summers, although hot in the lower latitudes of these zones, and marked by spells of warm weather even to their polar limits, are not characterized by such steady, uniform moist heat as is typical of much of the tropics. When the heat is extreme, and the relative humidity is high, night and day, sunstroke is occasionally noted, but the invigorating cool of autumn and winter are never far off, and may always be trusted to bring relief.

Winter and Summer Diseases of the Temperate Zones.—It is natural that such marked seasonal and such sudden weather changes as ours should be reflected in the character, distribution and frequency of the diseases which are found in these zones. Diseases of the respiratory system, bronchial and rheumatic affections, diseases that result from colds and chills, pneumonia, bronchitis, influenza, diphtheria, whooping cough, are all common in climates with sudden marked temperature changes, especially if these changes are accompanied by cold, damp winds. These diseases are also most frequent in the winter months, when the weather changes are more common and more severe, and when, in consequence, the vitality of the body is lowered and its power of resistance against the attack of the disease germs is weakened. A greater prevalence of diseases of the respiratory system, catarrhs and rheumatic affections in cool, moist weather, with sudden changes, has been shown by Weber, and several investigators have found a higher mortality after a greater variability of temperature. Many contagious or infectious diseases, such as diphtheria, influenza, measles and scarlet fever, for example, are also more common in the colder season, not because the lower temperatures are the direct controlling factor, but largely because the colder weather drives people indoors; houses and buildings generally are less well ventilated; more clothing is worn, less attention is paid to personal cleanliness and there is increased opportunity for contagion, especially among the poorer classes. Obviously, these are indirect effects of meteorological conditions.

In the warmer months, fevers and diseases of the digestive system, diarrhœa, malaria, typhoid fever, are prevalent. Thus there are usually two maxima of mortality: one in the colder season, when the changeableness of temperature is greatest, chiefly due to respiratory diseases, and another in the warmer months, largely due to infant mortality from disorders of the bowels.

Climate and Man: General.—Let us turn now to some larger, more general, relations of climate and man. Man's climatic environment affects him in many ways. His clothing, dwellings, food, occupations and customs; his physical and mental characteristics; his systems of government; his migrations; his history—all are affected to a greater or less degree.

Civilized man protects himself more or less successfully against unfavorable climatic features. Thus, there is a gradual transition from the primitive shelter made of branches of trees, of skins or leaves, to the permanent and highly elaborate modern building, which is both heated and cooled artificially. There is also a transition from the primitive and scanty clothing made of leaves or bark, where trees grow, or the skin of an animal, where trees are lacking, or where warmer clothing is needed, to the manufactured or perhaps imported garment of wool, cotton or silk. Again, there is the increasing variety of food, from that of primitive man, supplied directly where he lives, to the highly varied diet found in a civilized community to-day, to which distant latitudes are made to contribute their local delicacies. Nowhere has man given a more striking exhibition of his ingenuity in meeting and overcoming, at least partially, the obstacles put in his way by climate than in his construction and operation of railroads. Transportation by rail is necessarily closely affected by climatic conditions, for trains have no protection against snow, wind or heat. The trans-Siberian railway was constructed with great difficulty because of frozen soil, spring thaws and upheaved tracks. Across the rivers and across Lake Baikal, rails were laid on the ice during construction times. Later, the trains were carried across the lake in winter on ice-breaking ferry-boats. The snow-blockades on the northern railroads of America led to the invention and use of the ingenious and effective rotary snowplough, and to the construction of snow fences and of the highly interesting modern snow sheds, made in sections, which may be "telescoped" into one another in summer, in order to prevent the destruction of many miles of these sheds by fire. The campaign of a modern street railway system against the winter's snow is carefully planned in the previous summer, and a mild, open winter means a saving of money, time and labor, which results in increased earnings and larger dividends. The freezing of harbors at the termini of the northern railroads is a serious handicap in many countries. Russia's desire for an ice-free port at the terminus of the trans-Siberian railway on the Pacific led to her acquisition of Port Arthur, and ultimately to the war with Japan. The construction of railroads across deserts presents many difficulties. Ties dry up and twist; the danger from fire is greatly increased; fire patrols are often necessary; fuel is expensive and must be imported; water, for men and for locomotives, must be brought in by water-trains, tank cars or pipe-line; drifting sands cover the track and must constantly be shoveled off; the blowing sand hinders seeing, and increases friction and wear on the rolling-stock; watchmen are employed to guard against accidents from blowing sand on the track. A curious effect of sand-blasting is noted in the California desert, where the telegraph poles along the railroad are so worn near their bases by the blowing sand that they have to be protected by piles of stones. In the dense vegetation of the tropics, the roadway is constantly being overgrown, and men must be kept at work cutting down the weeds and underbrush. This involves great expense, and seriously reduces the earnings of the roads. Recently, tank-cars, which frequently spray the right of way with a strong poison, have come into use, as on the Guayaquil-Quito line in Ecuador, and elsewhere.

All this man has brought about in his combat with climatic conditions. But he can not change his climate. Slight local modifications may be secured here and there, as by planting trees to serve as wind-breaks, or in the case of protection against frost by the use of "smudges," or screens, or fires, or by erecting lightning rods to guard buildings against the danger of being struck. Man can not make it rain; nor can he prevent hail from falling, nor can he change his climate by planting forests. No such modification is possible in man's climatic environment as has been accomplished on the surface of the land under human agency. The atmosphere is as essentially unalterable as it is all-pervading.

Some Old Views Regarding the Effects of Climate on Man.—It is, however, easy to go too far in calling upon climate to explain certain phenomena which we may otherwise find it difficult to account for. This was the mistake formerly made by many writers on this subject. The broad generalizations of Montesquieu, Voltaire, Hume, Buckle and others, furnish interesting reading, and contain much that is suggestive and instructive, but they usually carry us well beyond the range of reasonable probability. Even Hippocrates's observations on climatic controls are not without value to-day.

Factors in the Problem other than Climate.—To most of these older writers, climate meant more than it does to-day. It included much of what is now termed our whole physical environment. We must remember that we are dealing here with large, highly complex phenomena. Man moves readily from place to place, from climate to climate. His food, drink, habits, occupations; to some extent his physical and mental characteristics, change in consequence. Inheritance, intermarriage, environment, opportunities, soil and many other factors enter in to determine what changes individual man and the race as a whole shall undergo. Time is a very important element in the final result, for in time a gradual adaptation to new conditions takes place. Climate is but one of many controls, albeit a most important one, for it largely determines what many of the other factors, such as diet, customs and occupations, for example, shall be. The task of giving climate its proper place as a factor controlling the life of man as a whole is a difficult one, which can not be definitely and satisfactorily solved to-day—or to-morrow.

Climate and Habitability.—Climate determines where, as well as how, man shall live. It classifies the earth's surface for us into the socalled habitable and uninhabitable regions. The deserts of sand and the deserts of snow and ice, whether the latter be near sea-level or high up on mountain tops, are alike climatic, the former because of aridity, the latter because of cold. The only non-climatic deserts are recent lava flows. Where a soil is present which is not frozen much over half the year, and where there is reasonable temperature and sufficient rainfall, plants and animals are found, ranging from few and lowly forms where conditions are hardest and where all life has to be especially adapted to these conditions, to the greatest abundance where conditions are most favorable.

Man is influenced by much the same controls as those which affect plants and the lower animals. From the highest latitudes he is excluded by cold. The highest altitudes are hostile both because of cold and of diminished pressure. The deserts of sand are uninhabited, or thinly populated, by reason of aridity. Forests, where rainfall is abundant, are unfavorable to a dense population. The trees must be cleared away before settlement is easy. The waves of civilization, as one writer has expressed it, beat up against the forest, but only with difficulty do they break through it. The equatorial forests of Africa; the densely wooded Amazonian provinces of Peru; the forests of northern Sumatra; the eastern forested slopes of Central America, left longest to the native tribes, while the western, more open, and drier slopes were first settled by white men, and are best developed—these are examples of the repelling effect of dense tree-growth where the advance of civilized man is concerned. Even the earlier American civilizations, the Aztec and the Inca, halted before forested areas. The Incas were almost as much hemmed in by the forests on the east as by the Pacific on the west. Travel through dense forests is difficult. Narrow paths, along which travelers move in Indian file, are the natural, and in fact the only, ways of communication, unless travel can be by boat. It requires no wide stretch of the imagination to see a connection between the method of carrying goods in the African forests, on the backs or heads of negro porters, and the slave trade, which sells the man who carried the goods as well as the goods. Many of the natives who secure the rubber from the Amazonian forests, or from those of the Congo, are to-day subjected to hardships which equal those of slavery.

Man is widely distributed over the earth's surface. The coldest place in the world in January is a large Siberian city, Verkhoyansk, while one of the hottest places in the world is Massowa, on the Red Sea, the capital of the Italian colony of Eritrea. But the life of man is harder here and easier there, according to climatic conditions, and the scarcity or abundance of plant and animal life.

Man is distributed in great belts around the world, corresponding roughly to the broad zones of vegetation, desert, steppe and forest, the limits of which are set by temperature and rainfall, but man is far more dependent on rainfall than on temperature. There are certain common conditions of life which affect the people who live in the same zone in the same broad, general way. This, as Ratzel first pointed out, means that there is a climatic factor at work to maintain differences between the people of different zones, in spite of the great movements which are constantly tending to produce uniformity. All the regions of sparse population are gradually being encroached upon by an invasion from their borders. Forests are being cleared, and replaced by agricultural lands. Wheat and corn are replacing grass on the steppes, especially where irrigation can be practised. Deserts are being reclaimed here and there where water is available. The more civilized man becomes, the denser the population which the different parts of the earth can be made to support. From the wandering hunting and fishing tribes of the African forest or of the borders of the Arctic Sea, through the farming populations of the cleared forest and of the steppe, to the crowded industrial centers of the modern city, there is such a gradation. It is the story of a more complete to a less complete mastery of man by his environment.

But in spite of all that man can do, the larger climatic limitations persist. The Greenland desert of snow and ice, the Saharan desert of sand: these remain, deserts.

Primitive Civilization and the Tropics.—There are reasons for thinking that primitive, prehistoric man, in his earliest stages, when most helpless, was an inhabitant of the tropics; that he lived under the mild, uniform, genial climate of that zone, where food was easily obtained and protection against the inclemencies of the weather least necessary. There has been a belief that southern Asia, with its numerous bays and archipelagoes, was probably the cradle of humanity. Civilized man is believed by many to have appeared first on the delta formed at the head of the Persian Gulf by the Tigris and Euphrates rivers. Ancient civilizations seem to have developed in the drier portions of the tropics, where irrigation was necessary in order to insure abundant and regular crops, and where lived races more energetic and more hardy than those of the damper and rainier portions of the tropics, with more luxuriant vegetation. Within the tropics, the greatest progress later came, not on the damp lowlands, but on the less fertile plateaus of Mexico and Peru, where the Aztecs and the Incas made their marvelous progress in the drier, cooler and somewhat more rigorous climates over 7,000 or 8,000 feet above sea-level.

The Development of the Tropics.—Within the tropics, under the equatorial sun, and where there is abundant moisture, animal and plant life reach a very full development. Here are the lands which are most valuable to the white man because of the wealth of their tropical products. Here are the tropical "spheres of influence" or "colonies" which are among his most coveted possessions. It is in this belt that food is provided for man throughout the year without labor on his part; where shelter and clothing are so easily provided, and often so unnecessary, that life becomes too easy. Nature does too much; there is little left for man to do. The simplicity of life, so far as providing food is concerned, has been emphasized by writers almost without number. Captain Cook put the case very emphatically when he said that a South Sea Islander who plants ten bread-fruit trees does as much towards providing for his family as does a man in northern Europe who works throughout the year.

In a debilitating and enervating climate, without the necessity of work, the man who inhabits the tropics not unnaturally lacks the will to develop himself, and also the will to develop the resources of the tropics. Voluntary progress towards a higher civilization is not reasonably to be expected. The tropics must be developed under other auspices than their own. As Professor John E. Commons has well put it: "Where nature lavishes food and winks at the neglect of clothing and shelter, there ignorance, superstition, physical prowess and sexual passion have an equal chance with intelligence, foresight, thought and self-control." The energetic and enterprising nations of the world have not developed under the easiest conditions of life in the tropics. As Edward Whymper's Swiss guide said of the natives of Ecuador: "It would be good for tropical peoples to have a winter."

The Labor Problem in the Tropics.—" What possible means are there of inducing the inhabitants of the tropics to undertake steady and continuous work, if local conditions are such that from the mere bounty of nature all the ambitions of the people can be gratified without any considerable amount of labor?" In these words, Alleyne Ireland well sums up the labor problem in the tropics. If the natives are, on the whole, disinclined to work of their own accord, then eitber forced native labor, which is contrary to the spirit of the times, or imported indentured labor, becomes inevitable if the tropics are to be developed. With few exceptions, and those where the pressure of a large population necessitates labor, effective development has been accomplished only where imported Chinese, Japanese or coolie labor has been employed, usually under some form of contract. Negro slavery began in the West Indies, under early Spanish rule, and its perpetuation was certainly in part aided by climatic controls. The best development of many tropical lands depends to-day upon Chinese or Japanese labor. It will be so in the Philippines.

With a large native class which is indolent, working intermittently for low wages, or which is bound under some form of contract, it follows that the native or imported laboring classes are separated by a broad gulf from the upper, employing class, which is usually essentially foreign and white. The latter class tends to become despotic, the former, servile. Marked social inequalities thus result, accentuated by the fact that the foreign-born white is usually debarred from all hard labor in a hot, tropical climate. White laborers are not likely to become dominant in the tropics for two reasons: first, because the climate is against them; and second, because the native is already there, and his labor is cheaper. White men are not doing the hard daily labor of India, of Java, of the Philippines, or even of Hawaii. They are directing it.

The Government of Tropical Possessions.—The government of European possessions in the tropics has thus far been determined chiefly by three considerations: (1) The general incapacity of the natives, through ignorance, or lack of interest, or their undeveloped condition, to govern themselves properly. (2) The fact that the white residents are generally comparatively few in number and are only temporarily in the country, to make money and then go home again. This white population is often composed chiefly of men—soldiers, officials, merchants, adventurers. There is little inducement to found permanent homes. (3) The marked class distinctions already referred to. These generalizations must obviously not be carried too far, but what has been said is in the main true. The white residents constitute a caste, and naturally become the rulers, the home government retaining general control, often by force of arms. The native population, although largely in the majority, may have little or no voice in its own government. This is clearly not a democracy. It thus comes about that the tropics are governed largely from the temperate zone; the standards, ideals, motives, come from another land. And where governed under their own auspices, as independent republics, the success has not been great. Buckle first strongly emphasized the point that hot countries are conducive to despotism and cold countries to freedom and independence; and James Bryce has recently clearly set forth the climatic control of government in an essay on "British Experience in the Government of Colonies."[2] The very Europeans who exercise the controlling power in the tropics, themselves tend to become enervated if they live there long; they lose many of the standards and ideals with which they started; they not uncommonly tend to fall towards the level of the natives rather than to raise the standards of the latter. The peculiar situation which may arise from the government of a tropical possession in which the white race does not become acclimated has been emphasized by Dr. Goldwin Smith in a recent discussion of British rule in India. He says:

British empire in India is in no danger of being brought to an end by a Russian invasion. It does not seem to be in much danger of being brought to an end by internal rebellion. Yet it must end. Such is the decree of nature. In that climate British children can not be reared. No race can forever hold and rule a land in which it can not rear its children.

The future of tropical possessions and "spheres of influence" offers many problems of great complexity, the solution of which is largely controlled by the factor of climate.

Climate and Man in the Temperate Zones: General.—Intermediate in location, in mean temperature and in their physiological effects, the temperate zones, whatever was the condition in the past, are to-day clearly the center of the world's civilization, as they have also been the scenes of the most important historical developments for several centuries. From the temperate zones have come the great explorers and adventurers of the past, and are coming the exploiters and colonizers of to-day. In the occurrence of the temperate zone seasons lies much of the secret—who can say how much of it?—of the energy, ambition, self-reliance, industry, thrift, of the inhabitants of the temperate zones. The monotonous heat of the tropics and the continued cold of the polar zones are both depressing. Their tendency is to operate against man's highest development. The seasonal changes of the temperate zones stimulate man to activity. They develop him physically and mentally. They encourage higher civilization. A cold, stormy winter necessitates forethought in the preparation of clothing, food and shelter during the summer. Carefully planned, steady, hard labor is the price of living in these zones. Development must result from such conditions. In the warm, moist tropics, life is too easy. In the cold polar zones it is too hard. Temperate zone man can bring in what he desires of polar and tropical products, and himself raises what he needs in the great variety of climates of the intermediate latitudes. Near the poles the growing season is too short. In the moist tropics it is so long that there is little inducement to labor at any special time. The regularity and the need of outdoor work during a part of the year are important factors in the development of man in the temperate zones. Where work is a necessity for all, labor becomes dignified, well-paid, intelligent, independent. Behind our civilization there lies what has been well called a "climatic discipline"—the discipline of a cool season which shall refresh and stimulate, both physically and mentally, and prevent the deadening effect of continued heat. On the other hand, a very long winter is about as unfavorable as a very long summer. If outdoor work is seriously interrupted, progress is retarded. It is not surprising to learn that the difficulty of keeping farm-hands through the long winter has in the past been a handicap in western Canada, and that it was urged against the abolition of slavery in Russia that it would be impossible, without some form of compulsion, to keep farmhands through the winter.

Northward Movement of Civilization in the North Temperate Zone.—The gradual migration of the center of civilization away from the tropics, and the highest development of the human race, not where life is easiest, but in extra-tropical latitudes, are significant. "Slowly but surely," as Benjamin Kidd says,[3] "we see the seat of empire arid authority moving like the advancing tide northward. The evolution of character which the race has undergone has been northwards from the tropics." From the Mediterranean region, where the world's civilization, its commerce and its power were long centered, westward through Spain and Portugal, the migration continued farther and farther north in Europe, until Holland, and then England, became the dominant power. From lands of more genial climates to lands of colder and longer winters, but also of the most active and energetic races, the migration has taken place.

Present-day Migrations in the Temperate Zones.—Within the north temperate zone especially, and also across from the north to the south temperate, vast, peaceful migrations are taking place, determined to no small degree by climatic considerations. From Europe and Asia to the United States alone, a million people a year are now migrating. These aliens have shown marked tendencies to settle where climate, soil and occupations are most like those of their old homes, although the fact that most of them land at one port on the eastern seaboard, the concentration of industries in certain sections, and other artificial controls, have operated very effectively to counteract and interfere with this tendency. Scandinavians, for example, have gone largely into the northwest; and in the future, unless steps are at once taken to prevent it, the southern parts of the United States will doubtless have a population predominantly of Latin blood. I say this although I am well aware of the very homogeneous "native" character of the southern population to-day, and of the high birth-rate among that population. Canada has grown slowly, partly on account of the repelling effect of her long, cold winters and her generally severe climate.

This migration within the temperate zone is peopling Canada, South Africa and Australia with the same stock as that which occupies the home-land of the British Isles. Therefore, institutions and government essentially similar to those at home are possible in these colonies of England beyond the seas. The case is very different in tropical climates, as has been seen. Russia will later be found to gain great strength from the fact that she has expanded eastward within the same zone. I think it was Leroy-Beaulieu who first pointed out what a unifying influence in Russia is the severe winter cold and the snowfall. In spite of the many factors which make for diversity and lack of coherence, there comes a great factor of unification in the possibility of continuous sleighing over those immense stretches of country, from north to south and from east to west, when the frozen rivers can be crossed without bridges and when the traveler, on his sledge, can journey straight across country to the farthest limits of the empire.

It is interesting to observe how immediately controlled by the special weather conditions or even one season these voluntary migrations may be. Years of sufficient rainfall and abundant crops in the United States are always followed by a large immigration. A failure of crops in Europe, whether it be of wheat in one country, or of fruit in another, or of potatoes in another, resulting from drought, or storms, or excessive rainfall, always promotes a larger exodus from the country concerned. There is, furthermore, a considerable seasonal migration across the Atlantic. Thousands of Italians come to the United States in the spring, to work during the warmer months, when farm and outdoor laborers are in demand, and return to the milder climate of Italy for the winter. Similarly, there is a seasonal migration, also chiefly of Italians, to Argentina at harvest time.

In connection with these larger migrations, there is an interesting tendency westward, observable not only in the westward "course of empire," but in the advantages enjoyed, in the belt of prevailing westerly winds, by those who live in the western quarters of cities. The "west ends" are usually the most fashionable and the newest sections of these cities, while the quarters to leeward, the "east sides" and "east ends," are inhabited by the poorer classes.

The Continents and the Temperate Zones.—So far as the continents are concerned, in their relation to the zones, Europe is well situated, being almost altogether in the temperate zone, and open to the ocean on the west, so that nearly all parts of it are well watered.

Asia is an overgrown continent. Much of it is in the temperate zone, it is true, but the interior is so far from the sea that the climate is severe, and the rainfall very deficient. This condition of hopeless aridity is depressing in the extreme, and this region is prevented from becoming thickly populated, or important, on that account.

Most of Africa is within the tropics. Its plateaus will furnish areas not wholly unfavorable for white settlement. The southern part of Africa is just within the marginal subtropical belt of the south temperate zone. The same is true of Australia. Most of the latter continent is a trade-wind desert, and therefore hopelessly arid.

South America is, unfortunately for white occupation, widest within the tropics, while its southern portion tapers off into the temperate zone. As a future home for the white race, it offers much less attractive possibilities than it would were the continent narrow within the tropics, and broad to the south. Its western portion is peculiar in having the tempering influence of high plateaus in the interior, and of a cool ocean current along the coast.

North America is widest in the temperate zone. This is one of its greatest assets. It suffers from the extreme cold of its winters in the north, and from the rain-shadow effect of its western mountains, which gives the interior basin and part of the western plains deficient precipitation. The interior of North America has more favorable rainfall conditions than Asia, because our continent is narrower. The eastern portion of North America is freely open to the Atlantic and the Gulf of Mexico, and this condition is much better than is the case in Asia. Most of the United States is wonderfully adapted, climatically, to serve as the home of a dense population.

The Life of Man in the Polar Zones: a Minimum of Life.—In the polar zones a "monotony of cold" replaces the "monotony of heat" of the tropics, and instead of the spur of the temperate zone seasons there is the depressing, long, polar night. There is a minimum of life. Plants are few and lowly. Land animals which depend upon plant food must therefore likewise be few in number. Farming and cattle raising cease. The reindeer, which manages to find sufficient food in the lowly Arctic vegetation, is the mainstay of the Arctic natives. But the reindeer must wander far and wide in search of their moss. And many reindeer are needed to provide sustenance for one man. Population is small, and scattered. There are no permanent settlements at all within the Antarctic Circle. In the Arctic, human settlements are fairly well scattered over a considerable range near the margins of the zone, but with increasing latitude man is more and more rarely seen, and finally he disappears altogether. There will never be permanent settlements at the poles.

Life is hard. Man seeks his food by the chase on land, but chiefly in the sea. Hardly one tenth of Greenland's population could live there without food from the sea. It has been well said that, with every degree of higher latitude, man is forced more and more to obtain his food from the sea. Gales, snow and cold, cause many deaths on land, and also at sea. It has been estimated that about one twenty-fifth of the population of Iceland perishes through being lost in snowstorms, by freezing or by drowning. The polar limit of permanent human settlements is believed by Bessels to be fixed, not by the decreasing temperature, but by the increase in the length of the night, which shortens the time during which man can lay up food, by hunting and fishing, to last him through the polar night.

Culture in the Polar Zones.—Under such adverse conditions it is not hard to see that progress towards a higher culture is not a reasonable expectation. There is little time in which man may seek to develop and satisfy his higher needs. Much truth is contained in Guyot's somewhat picturesque statement:

The man of the polar zones is the beggar overwhelmed with suffering, who, too happy if he but gain his daily bread, has no leisure to think of anything more exalted.

A sparse population, not far advanced in culture or in social relations, is inevitable under polar conditions of climate.

Deserts of Sand and Deserts of Snow.—There is a singular similarity, in their relation to man, of the deserts of sand, near the equator, and the frozen deserts of snow, near the pole, to which I have referred. The relations are interesting, for they illustrate very clearly how similar climatic controls, acting through plant and animal life, affect the life of man in the same large way. I can not select a better example for closing my discussion this afternoon.

Deserts of sand and deserts of snow: both alike repel man. Both are largely or wholly destitute of vegetation, of wood and of water. The yellow desolate waste of the sand desert is matched by the monotonous white surface of the snow desert. There are no opportunities for accumulating wealth in either. Travel is difficult in both. In one, the camel is the typical beast of burden; in the other, the reindeer and the clog are man's most useful possessions. The monotonous heat and glare and silence of the sand desert find their counterpart in the cold, and glare, and silence of the snow desert. The air is generally clear in both, except for the dust over the sand desert and the ice-needles in the air of the snow-desert. In both deserts, man is very limited in his food supply: in the Sahara the date, in Greenland the seal, are typical staple articles of diet. The aridity in one, and the cold in the other, are man's great enemies. The inhabitants of both deserts are nomadic. Settlements of some permanency are found in oases or along the edges of the sand desert, where there is water; similarly, the natives of the far north live along the edges of the ice desert, where they can best find their food. The sand deserts are deserts because they are arid. The snow deserts are deserts because they are cold. Denudation of exposed rocks in the desert of sand is largely due to the action of wind, carrying sand; and denudation of the surfaces of ice in the desert of snow is due to the action of wind carrying ice spicules. The polar deserts are perhaps on the whole better suited to life than the sand deserts, for the former supply water from the melted snow and ice. Man has, however, a harder struggle to protect himself against the cold than against the heat, for he needs more clothing, and better shelter, and fire. In both deserts life is isolated and primitive. The sand desert is crossed by caravans and trade routes between the more populous lands on either side, and the people of these deserts have more contact with civilization than do most of the natives of the far north. The polar desert of snow and ice: who travels across it except the occasional explorer, seeking the Pole?

  1. Lecture delivered at Columbia University, March 2, 1909.
  2. Century, March, 1899, 718-729.
  3. "Control of the Tropics," 51-52.