the long-continued inhospitable condition thus produced, and it is quite possible that, but for the glacial period, the civilization of mankind would have been much further advanced than at present, and most of the awkward questions which are troubling us now would have been settled ages ago. They might, however, have been succeeded by other questions quite as awkward, for the solving of perplexing problems of social relations seems part of the destiny of man. It has been suggested that the glacial age may have aided human advancement, by forcing primitive man to adopt new methods of shelter, clothing, and food-getting, in self-defense against the cold. Thus, instead of hindering it may have helped to break the reign of savagery.
Here it may be well to advert to another probable refrigerating agency of snow to which no attention has hitherto been paid. Aërial snow—snow that forms in the upper strata and is melted at lower levels of the air—may have always been an important agent in the cooling of the earth, aiding essentially in the upward transport of heat during the ages when the surface was at a high temperature. In those ages the great quantity of water vapor in the air hindered the free radiation of heat, whose conveyance upward was mainly accomplished by warm ascending currents. This may have been greatly aided by the conversion of the vapor of these vertical winds into snow in the upper air, the descent of this snow, and the exhaustion of much of the lower heat in melting it.
Such a state of affairs may have extended much further back in time than would at first thought be deemed possible; perhaps to that period when the earth was still too hot to permit the existence of liquid water, and the substance of the present oceans was held in the air as water vapor. Even then the rarer regions of the atmosphere were probably chilled below the temperature of congelation, and a snow limit existed, though very much higher than at present. The range of vapor must also have extended much higher than at present, possibly far within the region of congelation. Therefore, at the period when the surface heat prevented the existence of liquid water, there may have been a continuous formation and fall of snow in the upper strata of the atmosphere. The melting of this snow at lower levels, and the vaporizing, at still lower levels, of the rain which it yielded, must have been highly important agents in the upward transit of the surface heat.
There is thus much reason to believe that the snow-fall, which within the recent period has played so prominent a part in terrestrial affairs, has been from a very early era an active agent in the cooling of the earth, the snow limit of the atmosphere gradually descending through the ages until, in the glacial era, it nearly