I turn from the effects to the causes of that great glacial period which has been described in the last chapter. This line of investigation is peculiarly interesting in the search after human origins, for it affords the only chance of reducing the vague periods of immense duration shown by geology, to something like a definite chronology of years and centuries. If astronomical causes, the dates of which admit of mathematical calculation, can be shown to have been, if not the sole or principal, yet one of the causes which must have influenced the phenomena of the glacial epoch, we may assume these dates for the occurrence of the human remains which accompany these phenomena. Otherwise we must fall back on immense antiquity, which may mean anything from 50,000 or 100,000, to 500,000 or 1,000,000 years, since the first authentic evidence for palæolithic man. The first step towards an investigation of the cause of glacial periods, is to consider what are the conditions of the actual ones which are now prevailing. We have one such period in Greenland, another in the Antarctic region, a third in high mountain chains like those of Alaska, and of the Swiss and New Zealand Alps. In all these cases we find certain common conditions. High land in high latitudes, rising in great masses above the snow-line or temperature which condenses water in the solid form; and winds which are charged with great quantities of watery vapour raised by evaporation, to be so condensed. Cold alone is insufficient to produce glaciers and ice-caps, as may be seen by the example of the coldest regions in the world, Siberia and the tundras of Northern Asia and of North America, where the earth is permanently frozen to a depth of many feet; but there are no glaciers, The reason obviously is, that there is no sufficient supply of moist air from warm oceans to furnish more snow in winter than is melted in summer. Heat is in a certain sense as necessary as cold to account for glacial periods, for snow and ice can no more than other things be made out of nothing, and every snowflake implies an equal amount of aqueous vapour raised somewhere else by evaporation. But if an abundant supply of liquid or gaseous water is combined with cold sufficient to condense it into the solid form, it becomes fixed, and if the summer heat is insufficient to melt the excess of snow, it necessarily accumulates. The growth of glaciers, follows as an inevitable consequence. The snow is converted into ice by pressure and by alternate freezing and melting, and this grows year by year, until an equilibrium is established by the ice pushing down glaciers into lower levels, where the melting is more rapid, or into the sea, where the front is floated off in icebergs, and drifts into lower latitudes. The process is the same as that by which the rainfall on high levels is drained off by rivers into the sea, so that an equilibrium is established between waste and supply.