| From these several facts, namely from the glacial action having extended all round the northern and southern hemispheres— from the period having been in a geological sense recent in both hemispheres— 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 formerly
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 excentricity
of the earths
orbit.
All these causes tend towards the same end; but the most powerful appears to be the influence
of the excentricity
of the orbit upon oceanic currents.
It follows from
Mr. Crolls
researches, that
cold periods regularly recur every ten or fifteen thousand years; but that at much longer intervals the cold,
owing to certain contingencies, is extremely severe, and lasts for a great length of time.
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 in relation to our present subject,
the most important result 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 is
with the northern hemisphere, when
the southern passes through a glacial period.
These
conclusions
have, as we shall immediately see, a most important bearing
on geographical distribution;
but
I will
first give the facts, which demand an explanation.
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