Another burning question which fell to be considered was the evidence for or against interglacial warm periods; a question which even at the present time is almost as keenly debated as it was in 1874 when _The Great Ice Age_ was published. As years went past and successive editions of the book appeared, the battle of land ice _versus_ floating ice may be said to have been definitely closed; and James Geikie’s name as an expounder of glacial geology came to be more and more closely associated with his views on the glacial succession (the question, that is, whether there was only one epoch of glaciation, or several epochs separated by periods in which the climate was much milder). On this subject, as on glacial erosion, James Geikie took up his position very strongly from the first and maintained it to the end. In considering this branch of James Geikie’s scientific work we may trace a considerable amount of influence exerted on his thought by James Croll, one of the most remarkable geologists that Scotland has produced. Croll was a man who started life in a very humble position; he educated himself, and by sheer power of intellect and dogged perseverance he attained eminence in scientific work. He was at once a metaphysician and a physicist, but it is only with the latter aspect of his teaching that we are here concerned. Though quite unschooled in mathematics he was daunted by no difficulties, and by laborious calculation he worked out intricate problems of astronomy; and the accuracy of his solutions was afterwards confirmed by professional mathematicians and astronomers. Croll saw at a very early stage that to land ice must be ascribed practically the whole of the glacial phenomena of Scotland. He inferred from a number of facts that the North Sea had been filled with Scandinavian ice during the maximum stages of glaciation, and he suggested lines of research to James Geikie, John Horne, and Benjamin Peach that led to important contributions to the literature of Scottish glacial geology. Croll pondered long and deeply over the causes of the Ice Age and the origin of changes of climate in general, and the result was a series of papers in which he maintained that changes in the eccentricity of the earth’s orbit and the obliquity of the ecliptic, accompanied by other astronomical processes, might produce epochs of cold climate in one hemisphere with warmer conditions in the opposite hemisphere. These changes might under certain conditions be sufficient to account for the existence of ice-sheets. During the later years of his life Croll was a member of the staff of the Geological Survey of Scotland, and this silent, reserved, studious man was widely recognised as a most original thinker.