350 James Croll — On the Glacial Epoch. 



the following considerations. If we suppose that the climate of the 

 Glacial epoch was brought about mainly by changes in the physical 

 geography of the globe, we must assume that these great changes 

 took place, geologically speaking, at a very recent date. Then when 

 we ask what ground is there for assuming that any such change in 

 the relations of sea and land as is required actually took place, 

 the submergence of those areas is adduced as the proof. Did it 

 follow as a physical necessity that all submergence must be the 

 result of subsidence of the land, and not of elevations of the sea, there 

 would be some force in the reasons adduced. But such a conclusion 

 by no means follows, and, a priori, it is just as likely that the 

 appearance of the ice was the cause of the submergence as that the 

 submergence was the cause of the appearance of the ice. Again, a 

 subsidence of the land to the extent required would to a great 

 extent have altered the configuration of the country, and the main 

 river-systems of Europe ; but there is no evidence that any such 

 change has taken place. All the main valleys are well known to 

 have existed prior to the Glacial epoch, and our rivers to have 

 occupie4 the same channels then as they do now. In the case of 

 some of the smaller streams, it is true, a slight deviation has resulted 

 at some points from the filling up of their channels with drift 

 during the Glacial epoch ; but as a general rule all the principal 

 valleys and river-systems are older than the Glacial epoch. This, of 

 course, could not be the case if a subsidence of the land sufficiently 

 great to account for the submergence of the areas in question, or 

 changes in the physical geography of Europe necessary to produce a 

 Glacial epoch, had actually taken place. The total absence of any 

 geological evidence for the existence of any change which could 

 explain either the submergence of the areas in question or the 

 climate of the Glacial epoch, is strong evidence that the submergence 

 of the Glacial epoch, as well as of the areas in question, was the result 

 of a simple oscillation of sea-level resulting from the displacement of 

 the earth's centre of gravity by the transference of the ice-cap from 

 the southern to the northern hemisphere. 



Oscillations of sea-level in relation to Distribution. — The oscillations 

 of sea-level resulting from the displacement of the earth's centre of 

 gravity help to throw new light on some obscure points connected 

 with the subject of the geographical distribution of plants and 

 animals. At the time when the ice would be on the southern 

 hemisphere during the Glacial epoch, and the northern hemisphere 

 enjoying a warm and equable climate, the sea-level would be 

 several hundred feet lower than at present, the North Sea 

 would probably be dry land, and Great Britain and Ireland 

 joined to the continent, thus opening up a pathway from the con- 

 tinent to our island. As has been shown in former papers (Phil. 

 Mag. November, 1868; August, 1864; June, 1867, Supplement), 

 during the inter-glacial periods the climate would be much warmer 

 and more equable than now, so that animals from the south, such as the 

 hippopotamus, hyasna, lion, Elephas antiquus and Bhinoceros mega- 

 rhinus, would migrate into this country, where at present they could 



