THE GLACIAL ErOCH. 47 



changes as they spread further and further from their starting-point, hut in hoih those left at tlieir 

 starting-point should be nearest the original miocene type from which they have descended without 

 undergoing the chances of change of form incident to change of place. 



As regards modified species which may have found their way from Asia to America, or, vice verfta, 

 the difference in their proportion in East and West ^Vmerica is to be sought for in the com- 

 paratively insular position of the latter, fenced off, as I have shown, by seas to the west, to the 

 greater part of the east, and also probably to the north. Hence East America, although furthest 

 from Asia, received the Asiatic species first, and West America only received them by regurgita- 

 tion from the East. 



Before leaving the consideration of the effect of the glacial epoch, I should wish to notice an 

 ingenious, cosmical speculation, in relation to it, which has recently been propounded. 



That epoch plays too important a part in questions relating to the geographical distribution 

 of plants and animals, to allow us to disregard as extraneous any views relating to it. I there- 

 fore make no apology to the reader for detaining him for a few minutes while we look into this 

 hypothesis and endeavour to estimate it at its real value. 



The speculation to which I refer is that lately made by Mr. Croll* in which he ascribes 

 some of the phenomena of the glacial epoch to the disturbance of the centre of gravity of the 

 earth. That gentleman has suggested that the submergence of the land in the north which 

 is believed to have followed the termination of the glacial epoch may have been due to a 

 distuibance or alteration of the centre of gravity of the earth, consequent on the enormous 

 weight of ice accumidated at the North Pole during that epoch. Mr. Jamiesonf had already 

 suggested that the earth's crust may have yielded imdor the weight of the ice, and so caused 

 the submergence ; but Mr. Croll, whde he also refers the residt to the weight of the ice, argues, 

 that instead of the earth sinking, the water, in adjusting itself to a different centre of gravity, 

 overflowed part of the land. Mr. Croll says, " The surface of the ocean always adjusts itself in 

 relation to the earth's centre of gravity, no matter what the form of the solid mass of earth may 

 happen to be. Now if a portion of the water of the ocean be converted into solid ice, and placed, 

 for example, around the northern polar regions, it will necessarily change the position of the 

 earth's centre of gravity. The centre of gravity will be removed a little to the north of its former 

 position. The water of the ocean will then forsake the old centre, and adjust itself in relation 

 to the new. The surface of the ocean will, therefore, rise towards the North Pole, and fall towards 

 the South ; in other words, there will be in relation to the sea-level a depression of the land on 

 the northern hcmisj)here, and an elevation on the southern. The extent of the rise of the ocean, 

 level, or, what is equally the same, the extent of the submergence, will be in proportion to the 

 weight of the ice-sheet. The weight, or the size, of the ice-sheet being known, we can determine 

 with the utmost certainty the extent of the submergences ; or controversely, the extent of the 

 submergence being known we can determine both the weight and the size of the ice-sheet. It 

 is singular why physicists should not have perceived the physical impossibility of an ice-sheet, 

 several thousands of feet in thickness being placed upon the northern hemisphere, stUl retaining 

 its former level in relation to this land, unless the ice-sheets be counterbalanced by one of equal 

 weight placed upon the southern hemisphere. But this leads to another residt. The submergence 

 of the land during the glacial epoch leads to the conclusion that the glaciation wa.s not contem- 



* Sec Letter in " Reader," Aug. 1865. t Jamieson, in " Quart. Journ. of Geology," xviii. 170, and .\i.\. 235. 



