1 72 EVOLUTION OF THE EARTH 



steps in his final physical evolution, and acquired his present 

 invariable inner temperature. 



In this connection it is important to note that among white 

 men the best conditions for mind and for body do not appear 

 to be quite the same. Variability seems, if anything, to be 

 even more stimulating to mental than to physical activity. 

 Tests of school children made by Lehman and Pedersen in 

 Copenhagen and my own study of the marks of about 1,600 

 students at West Point and Annapolis indicate that mental 

 activity is greatest at a temperature decidedly lower than the 

 optimum for physical energy. For negroes the difference, as 

 indicated by daily tests of twenty-two students at Hampton 

 Institute for sixteen months, is much less. Perhaps this indi- 

 cates that after the separation of the white and negro races 

 there came a period of low temperature and of climatic stress 

 which modified the white man's mental response, but did not 

 affect the negro because he had gone too far south. 



If the conclusions here presented are sound, a knowledge of 

 the temperature, humidity, and variability of any region enables 

 us to determine what effect its climate would have upon the 

 physical and mental energy of all mankind. It is thus possible 

 to prepare a map showing the distribution of either kind of 

 energy or of the two combined. Such a map, in which physical 

 and mental energy are regarded as of equal importance, is 

 shown in Figure 36. 5 The map is subject to correction when 

 fuller data are available, but that will not change its main 

 outlines. The heavily shaded areas are those of great energy. 

 The agreement of these areas with the places where civiliza- 

 tion is highest is too marked to escape notice. In order to 

 bring the matter more clearly before us, Figure 37 has been 

 prepared. This is a map showing the distribution of civiliza- 

 tion according to the opinion of fifty competent judges in 

 fifteen different countries. The judges have been grouped in 



6 The map is fully discussed in "Civilization and Climate." 



