These considerations apply more particularly to the earlier records, while Mr. Brooks has confined his work chiefly to the later records, beginning with those of the Great Ice Age, in which climatic zones are clearly indicated by the limits of the ice; but in this problem one cannot confine one’s attention to a portion of the record, for the test of any explanation must be its sufficiency to explain all the past changes of climate. One will not be satisfied with an explanation of the Great Ice Age which does not explain at the same time the records of earlier ice ages, of which there is indubitable evidence in the Permo-Carboniferous and Pre-Cambrian periods, and the records of widespread tropical or sub-tropical conditions in the Carboniferous and Eocene Ages. Whether Mr. Brooks’ theory for the cause of the recent changes of climate satisfies this criterion must be left to each reader to decide. As Mr. Brooks says, the literature on this subject is now immense, and it is most unsatisfactory literature to digest and summarize. In the first place, many of the original observations which can be used in the study of past climates are hidden away in masses of purely geological descriptions, and a great deal of mining has to be done to extract the climatic ore. Then, again, most of the writers who have made a special study of climatic changes have had their own theoretical ideas and most of their evidence has been _ex parte_. To take a single example, for one paper discussing dispassionately the evidence for changes in climate during the historical period, there have been ten to prove either that the climate has steadily improved, steadily deteriorated, changed in cycles or remained unchanged. It is extremely difficult to arrive at the truth from such material, and still more difficult to summarize the present state of opinion on the subject. It may be complained that Mr. Brooks has himself adopted this same method and has written his book around his own theory. But was there any alternative? There are so many theories and radically different points of view that no writer could confine himself to the observations and say what these indicate, for the indications are so very different according to each theory in turn. And new theories are always being propounded; since Mr. Brooks commenced to write this book, Wegener has put forward his revolutionary theory according to which the polar axis has no stability, and the continents are travelling over the face of the globe like debris on a flood. Where is there solid ground from which to discuss climatic changes if the continents themselves can travel from the equator to the pole and back again in the short period of one or two geological epochs?