This is but a special case of the general problem of climatic changes in the history of the earth. We now know that the Pleistocene glaciation is not what it was once supposed to be, a unique phenomenon in .geological history. On the contrary, in at least three other periods, Algonkian, Cambrian, and Permian, we have found evidence of glaciation rivalling or equalling that of the Pleistocene. At other times mild and equable climates have extended far into the Arctic regions.

In attempting to explain these remarkable changes, three kinds of agencies have been called upon: (1) Astronomical, or change in the position of the earth's axis, in the eccentricity of its orbit, in the heat radiated from the sun, etc., but these are now very generally discarded. (2) Geographical, or changes in the arrangement of land and sea, in the height of the land, direction of ocean currents, etc., but none of these seems to afford any real help in solving the problem. (3) The most promising and widely favoured agency of climatic changes is now sought in variations in the composition of the atmosphere, especially in the quantity of carbon dioxide present. Whatever may eventually come of this, it has not yet advanced beyond the stage of an interesting speculation.