engendered. Those who become discontented with the old religion. are more than usually ready to accept any new idea which some religious enthusiast may propose. This seems to have been the case when Mohammed came upon the scene of action after the prolonged period of increasing aridity which culminated with a sudden. access of dryness in the first half of the seventh century. Without the genius of Mohammed that long period of adversity might have come to an end without any serious upsetting of the old conditions; but on the other hand, without the discontent and unrest fostered by years of distress Mohammed might have appealed in vain, for he would have had to speak to men who did not desire change instead of to those who ardently longed for it.

Thus far we have spoken of internal conditions which would make for the downfall of nations under growingly adverse physical conditions. External conditions would be equally unfavorable. When discord arises between nations it is far more likely to lead to war if the people of one and still more of both countries are dis-' contented. And more than this, foreign invasion may often arise simply because the rulers feel that the best way to avoid trouble at home is to lead their discontented subjects against an enemy. the case of nomadic tribes such as those of the vast regions of central Asia a period of prolonged aridity brings many of them face to face with the alternative of absolute starvation or migration. There is no question as to which will be chosen by a people who are constantly in motion. When they wander beyond their own territories into those of their neighbors, where also distress probably prevails even if not to so great a degree, fighting inevitably ensues. There is not grass and water enough for all, and someone must move on. Each onward movement brings the migrating bands into conflict with new tribes, and a movement once started may persist for a generation or two, and may be felt across a continent, thousands of miles from the home of the tribe which first moved. Such seems to have been the genesis of many of the great migrations which finally overwhelmed both Greece and Rome. Possibly and indeed probably a certain number of migrations of this sort might have occurred had there been no changes of climate, for the mere pressure of increasing population would sometimes start them, but that they would have been so severe or prolonged as they were seems hardly probable. A steady decline in the areas available for pasturage and in the amount of grass even in the areas where flocks could still be supported must have been a terrible incentive to migration, especially when it lasted five or six centuries, from the time of Christ to that

of Mohammed. In the diagram, to be sure, the decrease in rainfall does not appear to have been so great as in the period from 400 to 200 B. C., but this is largely due to the relatively small number of trees upon which the curve of the earlier period is based, and to the consequent exaggeration of that portion.

At a later time two other events similar to the great barbarian invasions took place, although their duration was by no means so prolonged. In these, according to available evidence, the elements of human ambition and human greatness appear to have figured more prominently than in the earlier barbarian migrations. From about 1000 A. D. to 1200 A. D. the climate of central Asia and of the rest of the world in the same latitude seems to have grown steadily drier. Once again distress and discontent must have reigned among the tents of Central Asia. Here, as in the days of Mohammed, no great concerted movement might have arisen, had it not been for the ambitions of one man. Genghis Khan may have been no more ambitious and no abler than other gifted men of his race, but he happened to live at a time when his people had been brought by nature to a condition of discontent favorable to his aspirations. Therefore, it would seem, he was able in a few years to arouse all the tribes of the steppes and deserts, and sweep over Asia with an almost unparalleled devastation. A century and a half later, in the last quarter of the fourteenth century, another ambitious Asiatic, Timour the Lame, arose, and emulated Genghis Khan. In Timour's case, also, physical conditions seem to have favored his projects, for after half a century of greatly improved conditions, a rather rapid decrease in rainfall took place just at the time when he began his conquests. How much this had to do with the matter I do not attempt to determine, but it should at least be carefully considered before any conclusions are drawn as to Timour and his conquests. Not much later, and in this same period of increasing aridity, the Turks advanced from their dry place of sojourn in the arid centre of Asia Minor and overwhelmed the last shattered remnants of the Byzantine Empire.

The portion of the history of the Roman Empire which centres around the Augustan Age stands in marked contrast to the periods which we have just been discussing. The Californian curve indicates a period of favorable climatic conditions from about 100 B. C. to 75 A. D. Even the low point at the birth of Christ is high compared with the centuries which precede and follow this period of prosperity. During these two hundred years the wars of Rome were very different in character from those which prevailed both before

and after. No great rivals like Carthage threatened the very existence of Rome; nor did rude barbarians like the Goths of later days pour in across her frontiers. She fought to extend her boundaries, her ambitious citizens engaged in battle with one another for the sake of personal ambition, and she quarrelled somewhat with Parthia, a state which met her on terms almost of equality so far as the relative positions of the two were concerned in Asia. In a word the wars of this period were of the kind that are characteristic of prosperity, and were not at all of the devastating kind which arise when the inhabitants of semi-arid regions migrate or plunder because of the impossibility of living at home. Similar conditions prevailed six or seven hundred years earlier when Assyria was at the height of her power and fought to expand her boundaries. In her case, however, the era of prosperity and freedom from harassing invasions was by no means so long as in that of Rome.

It is not possible to go through the course of history and pick out all the cases where prosperity due to favorable climatic conditions may have influenced the political fortunes of a nation, but it would be a most profitable exercise. Often, unquestionably, the influence of favorable climatic environment may have been completely nullified by political causes, or by personal ambitions, or other purely historical considerations, such as the discovery of a new art like the manufacture of iron, or of a new country such as America. Therefore, even if the theory here set forth contains large elements of truth, it is not to be expected that climatic pulsations should invariably be accompanied by the political and social results which would be expected if these physical matters were the only ones concerned in history. Nevertheless it is probable that their influence can be traced in scores of places where hitherto it has been unsuspected.

From great wars and movements of the nations let us turn back to internal affairs, and see how a change of climate in the direction of aridity would affect the composition of a race in its own home. The chief effects would come through disease. Probably insidious diseases such as malaria, consumption, neurasthenia, and the like are the most important sifters of the wheat from the chaff in the physical make-up of a nation, but great epidemics are much more startling and more easily studied. In the case of the plague there is possibly some connection between the times of its occurrence and the times of increasing aridity. As yet the question has never been worked out, and I mention the matter here not as something in regard to which we have any certain knowledge, but merely as an illustration of the interesting type of problems which confront the stu

dent who chooses to investigate the relation of human history to changes in man's physical surroundings.

It is sufficient here to call attention to the two worst instances of plague that have ever been recorded in history. The first is defined by the Encyclopædia Britannica as "the great cycle of pestilence, accompanied by extraordinary natural phenomena, which lasted fifty years [542-592 A. D.], and is described with a singular misunderstanding of medical terms by Gibbon in his forty-third chapter". A reference to the Californian curve shows that this occurred near the end of the long and terrible period of increasing desiccation which began, mildly no doubt, in the first century after Christ, and which during its long centuries may possibly have played so large a part in driving the barbarians into Europe, and in preparing the way for the Prophet of Islam. The seventh century, as well as the latter half of the sixth, was also a time of severe plagues, and this, to judge from our curve, appears to have been the driest and hence most famine-stricken period during three thousand years. After this, when the climate ceased to deteriorate and began to improve, the plague seems to have been somewhat assuaged.

The next of the really terrible plagues was that known as the Black Death. This reached southern Europe in 1346 or 1347 A. D., after having scourged Asia for an unknown period. Even in these modern days of rapid travel an appreciable number of years elapse before the plague can travel across a continent, and in earlier days when communication was far slower, the movement must have been much less rapid. For instance, in 1798 plague prevailed in Georgia and the Caucasus, where it continued to be more or less prevalent until 1819 or later. Meanwhile it spread to Baghdad in 1801, to Armenia and Constantinople in 1802, to Astrakhan in 1805 or thereabout, to Smyrna and Constantinople once more in 1808 and 1809, to Bucharest by land and Malta by sea in 1813, and finally to Dalmatia and the northeastern coast of Italy in 1815. If the spread of this plague from the eastern end of the Black Sea to the northern end of the Adriatic required seventeen years, during a period of relatively active communication, the spread of an earlier plague across the unfrequented deserts of Asia and across two or three times as great a distance would presumably require half a century. Therefore we seem to be justified in framing the working hypothesis that the Black Death may have originated during the famines which in some of the drier parts of Asia must have accompanied the period of aridity lasting from 1100 A. D. to the end of the thirteenth century. In the curve derived from the trees of California it will be

seen that the dry period does not end until 1300. From that time until the appearance of the plague in southern Europe is only forty

six years.

The plague is not the only disease which may have been influenced by changes of climate. Malaria, although far less fatal than the plague, is far more dangerous in its ultimate effects. The plague passes over the land and is gone; the dead are dead, and the living have suffered no serious injury. Malaria, on the contrary, hangs on year after year, not killing its victims, but sapping their energy and vitality. The presence and the abundance of malaria are closely associated with climate and topography. Without entering into any discussion of the origin of malaria, let me point out how a change toward aridity in a country like Greece and, to a less extent, Italy, would probably foster the disease.

Malaria is pre-eminently a disease of tropical and subtropical countries whose climate is characterized by alternate wet and dry seasons. Except in the perennially moist portions of the tropics, the streams of such regions are subject to seasonal floods which spread over wide areas for a short period and then disappear, leaving innumerable stagnant pools and swamps, ideal breeding places for the anopheles mosquito. Permanent bodies of water usually contain fish which eat the mosquito larvae and reduce their numbers, or else the water moves sufficiently to carry away most of the eggs that are laid in it. When the climate of a subtropical country becomes drier, the conditions which favor the mosquito are intensified. This comes primarily from the death of vegetation upon the mountains. The scarcity of vegetation allows the soil which had formerly been held in place by roots and by the cover of dead leaves to be washed rapidly away. The streams are thereby overloaded and begin to fill their valleys with sand and gravel, while the flowing water is forced to wander hither and thither over broad flood plains in innumerable channels, which form pools when the floods are assuaged, or else the water loses itself in marginal swamps. The streams also become intermittent and no longer contain large quantities of fish. Thus everything co-operates to reduce the number of streams which flow steadily throughout the year and to increase the number of bodies of stagnant water in which the mosquitoes may live. This in itself may produce most widespread effects. How great they are may be judged from the success of the United States government in eradicating malaria at Panama by the opposite process of reducing the number of places where mosquitoes can breed.

AM. HIST. REV., VOL. XVIII.—16.