of peasants and shepherds, wringing from upland pasture and scant field a miserable subsistence." 24

Thus far we have discussed climatic differences as though they occurred in certain fixed bands circling the earth. But climate in any given locality is, as we all know from our own experience, a relatively variable quantity. Aside from seasonal change there is much latitude of variation. This climatic cycle when considered in its wider aspect, that is, leaving out the temporary fluctuations of the year, and concentrating our attention upon changes that occur or recur over the period of many years or even centuries, is an important cause of movements of population. Recognition of the part played in history by climatic changes has led to the formulation of the theory of pulsatory climatic changes by Ellsworth Huntington.25 "It seems to be true, as a principle, that, in regions occupied by the ancient empires of Eurasia and northern Africa, unfavorable changes of climate have been the cause of depopulation, war, migration, the overthrow of dynasties, and the decay of civilization; while favorable changes have made it possible for nations to expand, grow strong, and develop the arts and sciences.'' 26 However, this by no means implies that all invasions and all prosperity are supposed to be due to climatic causes, but merely that climate has been one of the important factors in producing such results.27

"In relatively dry regions increasing aridity is a dire calamity, giving rise to famine and distress. These, in turn, are fruitful causes of wars and migrations, which 24 Ibid., p. 558.

25 The Pulse of Asia, 1907.

26 Huntington, F.--Palestine and Its Transformation, 1910, p. 251. 27 Huntington, "Changes of Climate and History," p. 215.

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engender the fall of dynasties and empires, the rise of new nations, and the growth of new civilizations. If, on the contrary, a country becomes steadily less arid, and the conditions of life improve, prosperity and contentment are the rule. There is less temptation to war, and men's attention is left more free for the gentler arts and sciences which make for higher civilization." 28 Among primitive men the nature of the province which a tribe happens to inhabit determines its mode of life, industries, and habits; and these in turn give. rise to various moral and mental traits, both good and bad. Thus definite characteristics are acquired, and are passed on by inheritance or training to future generations. If it be proved that the climate of any region has changed during historic times, it follows that the nature of the geographic provinces concerned must have been altered more or less. For example, among the human inhabitants of Central Asia, widespread poverty, want, and depression have been substituted for comparative competence, prosperity and contentment. Disorders, wars, migrations have arisen. Race has been caused to mix with race under new physical conditions, which have given rise to new habits and character. The impulse toward change and migration received in the vast arid regions of Central Asia has spread outward, and involved all Europe in the confusion of the Dark Ages." 29

The pulsations of climate which have been important factors in the movements of populations both in prehistoric and historic times are of several types. The first type of climatic change is that of the Glacial period, during which great fluctuations took place, probably simul28 The Pulse of Asia, p. 14. 29 Ibid., pp. 15-16.

taneously, throughout the northern hemisphere. It is held, with some evidence, that once in thirty-six years, on the average, we pass through what may be called a climatic cycle. There are two extremes during a cycle, at one of which the climate of continental regions for a series of years is unusually cool and rainy, with a low barometric pressure and relatively frequent storms; while at the other it is comparatively warm and dry, with high pressure and few storms. These changes are most extreme in mid-continental regions, decreasing toward the sea-coast. Thus the Glacial period as a whole represents the largest type of pulsation. But upon it are superposed the great pulsations known as Glacial epochs, each with a length measured probably in tens of thousands of years. The steady progress of these cycles is interrupted by smaller changes of climate, such as those of which there is evidence during historic times in Central Asia. Finally the climate of the world pulsates in cycles of thirty-six years, and even these are interrupted by seasonal changes and by storms.

We have often heard it said that civilization has advanced from east to west. Mr. Huntington considers it more accurate to say that civilization has advanced from south to north. The civilizations which started in Egypt and Babylonia, we know, spread to Persia, slightly farther north. Then Syria, Greece, and Carthage became dominant. Next, Rome until its decline, and then an obscure period of transition until France, Austria, and the states of southern Germany grew in prominence. Finally, during modern times, the northern nations of Europe have risen to power. The common explanation has been that as man became more civilized he also became better adapted to colder and moister climatic conditions. Mod

ern man has presumably a higher nervous organization. But Huntington believes that since first the race gained the rudiments of civilization, it has always made most rapid progress under essentially the same climatic conditions. "The conditions are that the summers shall have a sufficient degree of warmth and of rainfall to make agriculture easy and profitable, but not enough to be enervating; that the winters shall be cool enough to be bracing, but not deadening; and that the relation of summer to winter shall be such that with forethought every man can support himself and his family in comfort the year round, while without forethought he and his will suffer seriously." 30 These conditions appear to have been present in each of the great nations of history at the time when it has risen to the highest degree of civilization and power. During the early part of the Christian Era there was a relatively sudden desiccation in Central Asia. During the previous centuries the region was moist and fertile. It supported a vast population of men and animals. When the rainfall decreased fifty per cent., flocks of sheep diminished and the inhabitants were obliged to migrate in search of food. As these nomadic tribes pressed outward from Central Asia, they came in contact with others. Peoples pressed upon peoples, confusion spread in every direction, the wave of migration was felt in Europe two thousand miles away. In Cæsar's time, Europe was cold and swampy, but as it became warmer the throngs of primitive peoples, driven from behind by the hordes of restless nomads who had forsaken the arid Caspian basin, swarmed into this fertile country. Climatic changes in Rome sapped the strength of the original population, so that in time the 30 Ibid., pp. 381-382.

governing power fell into the hands of vigorous northern barbarians. The civilization of Rome declined.31

We have now examined two kinds of climatic change, latitudinal and pulsatory. It remains to consider the influence of more local changes, what we commonly call the weather. Careful investigation has confirmed the popular belief that clear, cool weather is invigorating. Dexter, in his book, "Weather Influences," has made a study of the influence of various meteorological conditions upon the conduct of school children, upon the occurrence of crime, and upon the number of errors made by bank clerks. In damp, muggy weather people feel disagreeable and suppose themselves ready to do all sorts of evil things. As a matter of fact they do not do them, for the vital functions are so far depressed that there is no surplus energy to spend in doing anything very active, either good or bad. Dry, windy days stimulate the vital processes, unless it be exceptionally warm, and create a surplus of energy which finds expression in work

31 The evidence employed to substantiate the theory of pulsatory climatic changes is roughly of four kinds. The first kind consists of physiographic phenomena such as river terraces, lake strands, denuded mountain slopes, desiccated springs, and rivers whose salinity has increased. A second kind consists of archeological phenomena, ruins of great cities in places whose supply of water is not now one-tenth large enough to support such a population as once existed. A third kind of evidence consists of historic accounts of famines, of old roads across the desert which to-day are impassable. Finally, evidence is based upon plant life. The thickness of the rings of annual growth in old trees has been found to be proportional to the amount of rainfall. Huntington measured the rings of annual growth of 450 of the Big Trees of California, Sequoia gigantea, and plotted the curve of climatic pulsation indicated by variation in these rings. The trees which were measured were from 230 to 3200 years old. Eighty were over 2000 years old and three more than 3000 years old. The data was, therefore, quite comprehensive. The curve showed a remarkable verification of the theory of pulsatory climatic change. The dry periods corresponding with the periods of desiccation shown by other phenomena. See “Changes of Climate and History."