324 HUMAN BIOLOGY 



One feature of these climatic pulsations has undoubtedly 

 been a certain change in mean temperature, but changes of 

 this sort have evidently been slight. Even at the height of 

 the .Glacial Period the average temperature of the earth was 

 probably not more than 15° or 20°f. lower than now and 

 since the dawn of civilization the temperature has pre- 

 sumably not fluctuated more than perhaps a tenth or at 

 most a fifth as much as this, far too httle to be readily evident 

 either from the ordinary historic records or from the known 

 facts as to plants and animals. On the other hand the amount 

 of storminess has apparently varied considerably from 

 century to century. Part of the evidence is found in ruins, 

 irrigation ditches, and traces of old fields in areas where the 

 water supply is now utterly inadequate; another part appears 

 in the level of salt lakes, the location of ancient roads, the 

 rate of growth of ancient trees, and many other features 

 which indicate a greater water supply during some centuries 

 than during others. Such evidence, be it noted, applies 

 mainly to the drier parts of the world, where even a slight 

 change in rainfall may produce serious results. 



This is not the place to discuss the matter in detail, but 

 greater rainfall appears to indicate greater humidity and 

 greater storminess; and greater storminess means more 

 frequent changes of temperature. Thus although the average 

 temperature of any given part of the earth has probably 

 changed very little during historic times, the degree of 

 humidity and still more the degree of variability from one 

 day to the next appear to have varied considerably. All this 

 means that during the Glacial Period the optimum climate 

 was located much nearer to the equator and to the great 

 deserts than at present. Since then it has moved poleward 

 and at the same time toward the margins of the continents, but 

 the movement has been irregular. During the historic period 

 for centuries at a time, especially in the era ending with 

 the time of Christ, the optimum /or the stage of human culture 

 then existing appears to have been located in the dry lands 

 around the Mediterranean and in western Asia where 

 ancient civilization made its greatest progress. The cen- 

 turies of greatest storminess when the climate most nearly 

 approached the optimum appear to have been periods of 



