AND ITS INHABITANTS 183 



great objection, however, has hitherto seemed to controvert 

 any such conclusion. Tropical countries may indeed have 

 made no appreciable contribution to civilization at any time, 

 but no later than two or three thousand years ago many 

 countries where climatic energy is now comparatively low were 

 the seats of the highest civilization. How was this possible? 

 The answer is that a great mass of evidence seems explicable 

 only on the hypothesis of pulsatory changes of climate during 

 historic times. This evidence has been discussed so fully in 

 other publications that it seems unnecessary to repeat it here. 6 

 It will be enough to state the main conclusions with almost no 

 details of proof. With these in mind we can apply them to 

 concrete instances and see how climate appears to have been 

 related to historic crises. 



HISTORICAL CHANGES OF CLIMATE 



During the past few decades the opinion of geographers as 

 to historical changes of climate has followed a course almost 

 identical with that of geologists as to earlier and greater 

 changes. Formerly climatic uniformity was supposed to be 

 the "normal" condition and variations were supposed to be 

 rare and exceptional. Today geologists universally believe 

 that glacial periods have occurred irregularly from the earliest 

 times to the most recent, and that these have been broken into 

 alternate glacial and interglacial epochs, while other less ex- 

 treme changes have been frequent. Coming to historical times 

 a belief in similar but smaller climatic pulsations is now almost 

 universal among American geographers, as is indicated by 



6 See The pulse of Asia, 1907; Palestine and its transformation, 1911; and 

 The climatic factor, 1914. Briefer treatments embodying further modifications 

 of the original hypothesis, together with replies to certain criticisms, are found 

 in The solar hypothesis of climatic changes, Bull. Geol. Soc. America, vol. 25, 

 1914, pp. 477-590; in Civilization and climate, 1915, ch. XI; and in Climatic 

 change and agricultural decline as elements in the fall of Rome, Quart. Jour. 

 Econ., vol. 31, 1917, pp. 173-208. 



