His glacial work and its significance are alike discussed by Dr Flett in the second part of this volume, and need not be treated here, but a word or two is necessary to explain the intense interest which glacial problems aroused among all the Survey men during the period we are considering, and in James Geikie in particular. A quotation from a letter of thanks written by Charles Darwin, after receiving a copy of the second edition of _The Great Ice Age_, will throw some light upon this. The letter is dated 26th October 1876, and Darwin says:--“The subject [_i.e._, the Ice Age] is one which fascinates me, chiefly owing to a little incident which I will mention as showing the grand progress of geology. When I was a boy an acute old gentleman who had attended to geology and natural history showed me a boulder in Shropshire, and assured me solemnly that the world would pass away before any one could explain how this great stone came from Cumberland or Scotland. This made a deep impression on me, and you may believe how delighted I was some forty years ago when floating ice action was first broached, to be followed some years afterwards by glacier action.” We see from this letter that the thought of the mystery of the great boulder haunted Darwin for years, but the young geologists of the Survey were confronted not with one boulder, but, day after day, week after week, with an accumulation of only half-explained mysteries. When they started work the view that a large part of the British Islands had been covered by land ice, and that the boulder-clay was the record of its passage, had, after a period of neglect, again come into prominence; but it was very far from being universally accepted (see the historical discussion in Part II.). Then, and for many years to come, the view that the boulder-clay, erratics, and so forth had been dropped by floating icebergs still commanded many followers. The suggestion that there was not one Glacial Period only but a series of advances and retreats, with well-marked interglacial periods between, had yet to be born. Thus the subject was a burning one at the moment, and in their meetings, whether in the field or, when the field-work was done for the season, at headquarters, the members of the Survey had much to discuss and to tell, many fragments of evidence to piece together. They seem all to have been greatly interested in the subject; but that James Geikie made it so peculiarly his own was partly due to the constructive imagination which enabled him to visualise, in a series of brilliant flashes, not the country as he saw it, but the former conditions to which it bore testimony. This constructive imagination was aided also, as has been indicated, by constant toil and by ceaseless comparison, by means both of personal visits and through the writings of others, of local conditions with those of other regions and of other lands.