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 for 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 climates. Joining these two facts, the entire truth would seem to be that the earlier phases of progress had to be passed through where the resistances offered by 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 consequent 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 important 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-flasks exposed to the damp snap like toasted
