As a first step therefore towards a solution of the problem of the glacial period we may start with the axiom that it requires abundant evaporation, combined with a temperature low enough to precipitate an excess of that evaporation in the solid form. This does not necessarily imply any great and permanent refrigeration of the whole earth, for although this would give the cold it would not give the evaporation, and would tend rather to extend the conditions of Siberia than those of Greenland. Longer and colder winters with shorter and hotter summers would seem more adapted to the growth of glaciers. But for a more exact investigation our next step must be to inquire what are the causes which may have produced these postulates of a glacial period, lower temperature with larger evaporation. They may be classed under two heads. 1st. Geographical causes, arising from latitude, aërial and oceanic currents, and a different distribution of sea and land. 2nd. Cosmic causes, such as variations of solar and terrestrial heat, passage through colder regions of space, the position of the poles, precession, and the eccentricity of the earth's orbit. All these have had supporters in their time, but the result of the latest science has been to leave only two seriously in the field--Lyell's theory of a different distribution and elevation of sea and land, carrying with it changes in aërial and oceanic currents; and Croll's theory of the effects of precession combined with high eccentricity of the earth's orbit. Thus, of the geographical causes, latitude is no doubt an important factor in determining temperature, but it cannot of itself be the cause of the glacial periods, for it has remained unchanged through all the vicissitudes of heat and cold in geological times. The latitude of Greenland and Spitzbergen is presumably the same now as it was in the Miocene period, when they were the seat of a luxuriant temperate vegetation; and at the present day we have only to follow the isothermal lines to see to what a great extent climate in the same latitudes is modified by other influences, such as the Gulf Stream.