448 Climate of the Glacial, Period. (October, 
and Mr. Darwin has explained their presence by supposing 
that during the glacial period they were driven to the high 
lands of the tropics by the advancing ice, and that on its 
retreat they followed it north and south. A glacial period 
in one hemisphere only would not afford this means of mi- 
gration; the plants and animals driven south by the 
northern ice would always have a hot zone to the south of 
them, which they could not pass. 
Another class of evidence that favours the theory of the 
glacial periods of the two hemispheres having existed at the 
same time, is that connected with the lowering of the sea- 
level. Mr. Alfred Tylor, some time ago, advanced the 
theory that the piling up of ice in the northern hemisphere 
would lower the level of the ocean 600 feet. Mr. Croll has 
lately discussed the question,* and comes to the conclusion 
that, if each hemisphere was glaciated alternately, the level 
of the ocean would be raised, and not lowered, in the one in 
which the ice accumulated ; by the melting of the ice of the 
opposite pole and the shifting of the centre of the earth’s 
gravity towards that covered by an ice-cap. Though I can- 
not agree with Mr. Croll’s estimate of the thickness of the 
ice, and think that it could not possibly have been highest 
at the pole, I have no doubt that a great lowering of the 
level of the ocean could not have arisen by the accumulation 
of ice at one pole, if at the same time that now existing at 
the other was melted off. But if the glacial periods of the 
two hemispheres were simultaneous, the water abstracted 
from the sea and frozen into ice at the two poles, and that 
impounded in the great lakes of Northern Europe, America, 
and Asia, by the blockage of the northern drainage of the 
continents by ice, must have lowered the level of the ocean 
to a great extent. 
In my “ Naturalist in Nicaragua” I stated that this de- 
crease in the volume of the ocean could not have been less 
than 1000 feet. I was thus guarded because we had at that 
time no proof of the ice having descended from the north 
upon Northern Asia, and there was no certainty that the 
Polar basin had been filled with it. Since then I have my- 
self found evidence in Siberia that the Arctic Sea was filled 
with ice, which was piled up so high that it overflowed the 
low lands as far as lat. 52° N. Calculating from this data, 
I find that the lowering of the sea-level—on the supposition 
that the ice was equal in the two hemispheres at the same 
time—could not have been less than 2000 feet, and may 
* Geological Magazine, July and August, 1874. 
