At
this period the equatorial climate at the level of the sea was probably
about the same with that now experienced at the height of from five to six
thousand feet under the same latitude, or perhaps even rather cooler.
During this, the coldest period, the lowlands under the equator must have
been clothed with a mingled tropical and temperate vegetation, like that
described by Hooker as growing luxuriantly at the height of from four to
five thousand feet on the lower slopes of the Himalaya, but with perhaps a
still greater preponderance of temperate forms. So again in the
mountainous island of Fernando Po, in the Gulf of Guinea, Mr. Mann found
temperate European forms beginning to appear at the height of about five
thousand feet. On the mountains of Panama, at the height of only two
thousand feet, Dr. Seemann found the vegetation like that of Mexico, "with
forms of the torrid zone harmoniously blended with those of the temperate."
Now let us see whether Mr. Croll's conclusion that when the northern
hemisphere suffered from the extreme cold of the great Glacial period, the
southern hemisphere was actually warmer, throws any clear light on the
present apparently inexplicable distribution of various organisms in the
temperate parts of both hemispheres, and on the mountains of the tropics.
The Glacial period, as measured by years, must have been very long; and
when we remember over what vast spaces some naturalised plants and animals
have spread within a few centuries, this period will have been ample for
any amount of migration.
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