THE EFFECT OF CLIMATE AND WEATHER 325 



progress; the centuries of diminishing storminess when the 

 optimum swung northwestward appear to have been times 

 of distress and decline. 



This conclusion is by no means universally accepted. In 

 fact many able people assail it vigorously and even ridicule it. 

 They say that such an hypothesis is unnecessary because 

 other historic and cultural conditions afford a full explanation 

 of the rise and fall of civilization. They also say that the facts 

 on which the hypothesis is based are scanty and are subject 

 to various interpretations. Therefore it is wise to suspend 

 judgment, but it is highly significant to see the way in 

 which independent lines of investigation dovetail. One line, 

 the earliest, suggests that the great centers of ancient 

 civilization rose to their highest levels when their climates 

 were more moist and variable, and hence nearer to the 

 physiological optimum than at present. Another shows 

 that the optimum varies according to the degree of civilization, 

 and man's consequent ability to protect himself from low 

 temperature and excessive dryness. A third indicates 

 that at present the distribution of civilization and progress is 

 almost identical with that of climatic energy. All three 

 together suggest, although they do not prove that in the past 

 as at present, the distribution of civilization has been closely 

 determined by the physiological effect of climate. 



CLIMATE AND RACIAL CHARACTERISTICS 



This brings us to the vexed question of the relation of 

 climate to racial characteristics. So far as external character- 

 istics are concerned, the case is fairly clear. In general the 

 pigmentation of the races of the world varies according to the 

 intensity of the sunlight, for pigment appears to be primarily 

 a protection against ultraviolet light. The center of the fair 

 Nordics today, and we know not how long in the past, is 

 Scandinavia where the sunlight, especially in its shorter ultra- 

 violet wave lengths, is never very strong. The blackest races 

 are all found in low latitudes. Here and there to be sure, 

 we find relatively fair people in low latitudes and moderately 

 dark tribes in high latitudes, but in most cases this is the 

 obvious result of migration. When once a race has acquired 

 a given pigment, it presumably requires a long time for a new 



