318 The History of our Earth. [July, 
190 feet. If one-half of this mass were further added to 
that existing at the North Pole, an additional displacement 
of 95 feet would ensue, making a total of 285 feet. The 
immediate result of this would be a rise of the sea level in 
the northern hemisphere—which in the latitude of Edin- 
burgh would amount to 234 feet—and a corresponding fall 
of the sea level and laying bare of lands now submerged in 
the southern hemisphere. The removal of 12 miles of ice 
from the South Pole would effect in the latitude of Edin- 
burgh arise of the sea to the height of 800 to rIooo feet 
above its present level. This is on the supposition that the 
interior of the earth is solid to the centre. If it be fluid, 
the displacement of the centre of gravity and the consequent 
modification of the sea level would be much greater. 
Both Mr. Croll and Mr. Belt, then, agree that the occur- 
rence of a Glactal epoch must greatly affect the distribution 
of land and water, and might easily produce cataclysms,— 
in case, for instance, that ice-masses melted away very 
rapidly, and especially if they disappeared more quickly in 
one locality than they formed in another. Mr. Croll— 
though he does not enter at length into this matter—admits 
that the withdrawal of the water from shallow seas would 
connect lands now isolated, and allow of the migration of 
plants and animals from one continent to another, or from 
continents to what are now islands. The depth of Behring’s 
Straits does ‘not exceed 30 fathoms. A lowering of the 
ocean level in the northern hemisphere to the extent of 
200 feet would therefore connect the Old and New Worlds, 
and allow of an interchange both between their plants and 
their animals. Such a lowering could only occur when the 
southern hemisphere was glaciated, and the northern was 
enjoying its warm interglacial period. Consequently organic 
forms, now only found in the temperate regions of Asia, 
would press on into Siberia, cross the bridge, and establish 
themselves in America, where, when the temperature be- 
came less genial, they would gradually migrate southwards. 
Similarly in Europe, during the interglacial period, England 
would be part and parcel of the Continent, and the beds of 
the North Sea and the Channel would be dry land. Hence 
animals of southern regions were enabled to enter Britain, 
and have left their fossilised bones in proof of their former 
presence. The existence of closely allied or identical forms 
in countries now separated by wide seas, has led some 
naturalists to the assumption of a double origin, whether by 
special creation or by development—a view for which there 
is now no necessity. It is a remarkable fact, easily 
