Lowering of the Sea-Level 205 
easily leave. It is the answer to the question, What became 
of the many peculiar tropical American genera of animals 
and plants, when a great part of the tropics was covered with 
ice, and the climate of the lower lands much colder than now? 
For instance, the Helicon and Morphos are a group of 
butterflies peculiar to tropical America, containing many 
distinct genera which, on any theory of descent from a 
common progenitor, must have originated ages before the 
glacial period. How is it that such peculiarly tropical 
groups were not exterminated by the cold of the glacial 
period, or if able to stand the cold, that they did not spread 
into temperate regions on the retreat of the ice? I believe 
the answer is, that there was much extermination during the 
glacial period, that many species and some genera, as, for 
instance, the American horse, did not survive it, and that 
some of the great gaps that now exist in natural history 
were then made; but that a refuge was found for many 
species, on lands now below the ocean, that were uncovered 
by the lowering of the sea caused by the immense quantity 
of water that was locked up in frozen masses on the land. 
Mr. Alfred Tylor considers that the ice cap of the glacial 
period was the cause of a great reduction of the level of the 
sea, amounting to at least 600 feet.1 But if we admit that 
the ice existed in both hemispheres at the same time, we shall 
have to speculate on a lowering of the level of the sea to at 
least 1000 feet. We have many facts tending to prove that 
during the extreme extent of the glacial period the land stood 
much higher relatively to the sea than it now does. Pro- 
fessor Hartt believes that during the time of the drift, Brazil 
stood at a much higher level than at present,” and we can, 
on the supposition of a general lowering of the sea all over the 
world, account for the distribution of animal life over islands 
now separated by shallow seas. Thus Mr. Bland, in a paper 
read before the American Philosophical Society, on ‘‘ The 
Geology and Physical Geography of the West Indies, with 
reference to the distribution of Mollusca,” states his opinion 
that Porto Rico, the Virgins, the Anguilla group, Cuba, the 
Bahamas, and Hayti, once formed continuous dry land that 
obtained its land molluscs from Central America and 
1 Geological Magazine, vol. ix. p. 392. 
2 Geology and Physical Geography oy Brazil, by Ch. Fred. Hartt, p. 573. 
