_The Explanation of the Difficulty._—As an explanation of these puzzling phenomena, I suggested, in the Philosophical Magazine for November, 1868, that these southern animals lived in our island during the warm periods of the glacial epoch, while the northern animals lived during the cold periods. This view I am happy to find has lately been supported by Sir John Lubbock; further, Mr. James Geikie, in his “Great Ice Age,” and also in the Geological Magazine, has entered so fully into the subject and brought forward such a body of evidence in support of it, that, in all probability, it will, ere long, be generally accepted. The only objection which has been advanced, so far as I am aware, deserving of serious consideration, is that by Mr. Boyd Dawkins, who holds that if these migrations had been _secular_ instead of seasonal, as is supposed by Sir Charles Lyell and himself, the arctic and southern animals would now be found in separate deposits. It is perfectly true that if there had been only one cold and one warm period, each of geologically immense duration, the remains might, of course, be expected to have been found in separate beds; but when we consider that the glacial epoch consisted of a long _succession of alternate cold and warm periods_, of not more than ten or twelve thousand years each, we can hardly expect that in the river deposits belonging to this long cycle we should be able to distinguish the deposits of the cold periods from those of the warm. _Shell Beds._—Evidence of warm inter-glacial periods may be justly inferred from the presence of shells of a southern type which have been found in glacial beds, of which some illustrations follow. In the southern parts of Norway, from the present sea-level up to 500 feet, are found glacial shell beds, similar to those of Scotland. In these beds _Trochus magus_, _Tapes decussata_, and _Pholas candida_ have been found, shells which are distributed between the Mediterranean and the shores of England, but no longer live round the coasts of Norway.