Lyell has shown, is the relative
position of the land and water. Mr. Croll believes that the last great
glacial period occurred about 240,000 years ago, and endured, with slight
alterations of climate, for about 160,000 years. With respect to more
ancient glacial periods, several geologists are convinced, from direct
evidence, that such occurred during the miocene and eocene formations, not
to mention still more ancient formations. But the most important result
for us, arrived at by Mr. Croll, is that whenever the northern hemisphere
passes through a cold period the temperature of the southern hemisphere is
actually raised, with the winters rendered much milder, chiefly through
changes in the direction of the ocean currents. So conversely it will be
with the northern hemisphere, while the southern passes through a glacial
period. This conclusion throws so much light on geographical distribution
that I am strongly inclined to trust in it; but I will first give the facts
which demand an explanation.
In South America, Dr. Hooker has shown that besides many closely allied
species, between forty and fifty of the flowering plants of Tierra del
Fuego, forming no inconsiderable part of its scanty flora, are common to
North America and Europe, enormously remote as these areas in opposite
hemispheres are from each other. On the lofty mountains of equatorial
America a host of peculiar species belonging to European genera occur.
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