James Croll — On the Glacial Epoch. 351 



not live in consequence of the cold. We have therefore an explana- 

 tion, as was suggested on a former occasion {Pliil. Mag. Nov. 186B), 

 of the fact that the bones of these animals are found mingled in the 

 same grave with those of the musk ox, mammoth, reindeer, and 

 other animals which lived in this country during the cold periods of 

 the Glacial epoch ; the animals from the north would cross over 

 into this country upon the frozen sea during the cold periods, while 

 those from the south would find the English Channel dry land 

 during the warm periods. 



The same reasoning will hold equally true in reference to the old 

 and new world. The depth of Behring Straits is under thirty 

 fathoms ; consequently a lowering of the sea-level of less than 200 

 feet would connect Asia with America, and thus allow plants and 

 animals, as Mr. Darwin believes, to pass from the one continent to 

 the other.^ During this period, when Behring Straits would be dry 

 land, Greenland would be comparatively free from ice, and the Arctic 

 regions enjoying a comparatively mild climate. In this case plants 

 and animals belonging to temperate regions could avail themselves 

 of this passage, and thus we can explain how plants belonging to 

 temperate regions may have, during the Miocene period, passed from 

 the old to the new continent, and vice versa. 



As has already been noticed, during the time of the greatest 

 extension of the ice, the quantity of ice on the southern hemisphere 

 might be considerably greater than what exists on the entire globe at 

 present. In that case there might, in addition to the lowering of 

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

 of gravity, be a considerable lowering resulting from the draining of 

 the ocean to form the additional ice. This decrease and increase in 

 the total quantity of ice which we have considered would affect the 

 level of the ocean as much at the equator as at the poles ; conse- 

 quently during the Glacial epoch there might have been at the 

 equator elevations and depressions of sea-level to the extent of a few 

 hundred feet. 



Extent of submergence on the JiypotJiesis that the earth is fluid in the 

 interior. — But we have been proceeding upon the supposition that 

 the earth is solid to its centre. If we assume, however, what is the 

 general opinion among geologists, that it consists of a fluid interior 

 surrounded by a thick and rigid crust or shell, then the extent of 

 the submergence resulting from the displacement of the centre of 

 gravity for a given thickness of ice must be much greater than I 

 have estimated it to be. This is evident, because, if the interior 

 of the globe be in a fluid state, it, in all probability, consists of 

 materials differing in density. The densest materials will be at the 

 centre, and the least dense at the outside or surface. Now the 

 transference of an ice-cap from the one pole to the other will not 

 merely displace the ocean — the fluid mass on the outside of the 

 shell — but it will also displace the heavier fluid materials in the 

 interior of the shell. In other words, the heavier materials will be 

 attracted by the ice-cap more forcibly than the lighter, consequently 

 ... 1 Origin of Species, chap. xi. (iifth editioji). 



