heres--from its having lasted in both during a great length of
time, as may be inferred from the amount of work effected--and lastly,
from glaciers having recently descended to a low level along the whole
line of the Cordillera, it at one time appeared to me that we could not
avoid the conclusion that the temperature of the whole world had been
simultaneously lowered during the Glacial period. But now, Mr. Croll,
in a series of admirable memoirs, has attempted to show that a glacial
condition of climate is the result of various physical causes, brought
into operation by an increase in the eccentricity of the earth's orbit.
All these causes tend towards the same end; but the most powerful
appears to be the indirect influence of the eccentricity of the orbit
upon oceanic currents. According to Mr. Croll, cold periods regularly
recur every ten or fifteen thousand years; and these at long intervals
are extremely severe, owing to certain contingencies, of which the most
important, as Sir C. 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.
On the Organ Mountain