WCP2037

[1] [p. 359]

Edinburgh, 24th July 1880.

Alfred R. Wallace, Esq.

Dear Mr. Wallace, — Many, many thanks for your interesting letter, giving me a brief outline of the general drift of your views on the cause of geological climate. I shall be looking with interest to the appearance of your book. Of course I cannot make out from your brief statement what those causes really are on which you rely: only I do trust you don't put much stress on the effects of changes in the physical geography of the globe. There is one serious objection to all theories of geological climate based on great changes in the physical geography. These changes must be assumed to take place with irregularity, and also with extreme slowness, and it will be difficult to get them to harmonise with warm Interglacial periods. The amount of evidence uncollected, which lies scattered over the various journals and papers of America and the Continent, is perfectly astonishing. This will be brought out very strongly by Dr. James Geikie¹ in his forthcoming work on Prehistoric Man in Europe. It is, I think, now beyond question that the Glacial epoch consisted of a succession of cold and warm periods, which must be accounted for in any theory of geological climate. In fact, these periods are the general test of theories of climate. It seems to me difficult to conceive how it is possible that they could have depended on changes in physical geography. They have all along convinced me that the only possible way of explaining them is by the hypothesis that these alternations were [2] [p. 360] due to the precession of the equinoxes at the time of high eccentricity. After fifteen years' study of the subject, I have been unable to find any serious objection to this theory. Nearly all the objections which I have seen have been based upon misapprehension of the subject.

There is another point on which I think considerable misapprehension prevails, that is, in supposing that the great accumulation of snow and ice on Arctic and Antarctic regions results from a high elevation of the land. True, on elevated mountains all over the globe we have perpetual snow; but this is not owing simply to elevation, but to certain physical conditions which come into operation at great altitudes, but these conditions are obtained on Polar continents without any elevation. This I have endeavoured to show in a letter in Nature, July 1. We have almost positive evidence that the Antarctic continent is a vast, flat plain, very little elevated above sea-level. It contains, no doubt, some elevated mountains, but these, of course, cannot determine the general character of the climate. Greenland does not appear to contain in the interior great mountain chains. Its mountains are simply a fringe along its western edge. True, the surface of the inland ice is found everywhere to slope gradually upwards towards the interior, but there is no evidence to conclude that the ground under the ice does so. The thickening of the ice, as we proceed inwards, is a necessary condition of an ice-sheet. Without this there could be no general dispersion of the ice. I hope you will excuse me for going into these details. I thought I might draw your attention to them, so that you might give them your consideration when you are treating this vexed subject, but it is very unlikely you have fallen into any of these traps.

I am your ever truly | James Croll