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. ↑| 8 blocks not present in 1869 1872; present in 1859 1860 1861 1866 | | We do not know that the Glacial epoch was strictly simultaneous at these several far distant
points on opposite
sides of the world.
But we have good evidence in almost every case, that the epoch was included within
the latest geological period.
We have, also, excellent evidence, that it endured for an enormous time, as measured by years, at each point.
The cold may have come on, or have ceased, earlier at one point of the globe than at another, but
seeing that it endured for long
at each, and that it was contemporaneous in a geological sense, it seems to me
probable that it was, during a part at least of the period, actually simultaneous throughout the world.
Without some distinct evidence to the contrary, we may at least admit as probable that the glacial action was simultaneous on the eastern and western sides of North America, in the Cordillera under the equator and
under the
warmer temperate zones, and on both sides of the southern extremity
of the continent.
If this be admitted, it is difficult to avoid believing that the temperature of the whole world was at this period simultaneously cooler.
But it would suffice for my purpose, if the temperature was
at the same time lower along certain broad belts of longitude.
On this view of the whole world, or at least of broad longitudinal belts, having been simultaneously colder from pole to pole, much light can be thrown on the present distribution of identical and allied species.
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