324 THE POPULAR SCIENCE MONTHLY, 



vital actions, not only of man as at present constituted, but of the 

 mammalia generally, such heat hinders the evolution of bodily energy 

 only during part of the day, and, by the abundance of materials lor 

 living which it fosters, aids social development in most ways more 

 than it impedes it in some ways. 



I do not ignore the fact that in recent times societies have evolved 

 most, both in size and complexity, in temperate regions. I simply 

 join with this the fact that the first considerable societies arose, and 

 the primary stages of social development were reached, in hot cli- 

 mates. Joining these two facts, the entire truth would seem to be 

 that the earlier phases of progress had to be passed through w^here 

 the resistances ofiered b}' inorganic conditions were least ; that, these 

 phases having been passed through, and the arts of life having been 

 advanced, it became possible for societies to develop in regions where 

 the resistances were greater ; and that further developments in the 

 arts of life, and further discipline in cooperation going along with 

 them, enabled societies inheriting the resulting advantages to take 

 root and grow in regions which, by climatic and other conditions, 

 offered relatively great resistances. 



Taking the most general view of the facts, we must therefore 

 say that, solar radiation being the source of those forces by which 

 life, vegetal and animal, is carried on, and being, by implication, the 

 source of the forces displayed in human life, and consequently, in social 

 life, it results that there can be no considerable social evolution on 

 tracts of the earth's surface where solar radiation is very feeble. We 

 see that, though, contrariwise, there is on some tracts a solar radiation 

 in excess of the degree most favorable to vital actions, yet the con- 

 sequent hindrance to social evolution is relatively small. Further, 

 we conclude that an abundant supply of light and heat is requisite 

 during those first stages of progress in which social vitality is small. 



Passing over such traits of climate as variability and equability, 

 whether diurnal, annual, or irregular, all of which have their effects 

 on human activities, and therefore on social phenomena, I will name 

 here one other climatic characteristic that appears to be an impor- 

 tant factor. I refer to the quality of the air in respect of dryness or 

 moisture. 



Either extreme brings indirect impediments to civilization, which 

 we may here note before observing the more important direct effects. 

 That great dryness of the air, causing a parched surface and a scanty 

 vegetation, negatives the multiplication needed for advanced social 

 life, is a familiar fact. And it is a fact, though not a familiar one, 

 that extreme humidity, especially when joined with great heat, may 

 raise unexpected obstacles to progress ; as, for example, in some parts 

 of East Africa (Zungomero), where, according to Captain Burton, 

 " the springs of powder-fiasks exposed to the damp snap like toasted 



