Hemisphere, only to melt away again some hundred thousand years later.
Even after the idea of ice ages was generally accepted—it was first
proposed in the eighteen-thirties by Louis Agassiz, a protégé of Cuvier—
no one could explain how such an astonishing process could take place. In
1898, Wallace observed that “some of the most acute and powerful
intellects of our day have exerted their ingenuity” on the problem, but so
far “altogether in vain.” It would take another three-quarters of a century
for the question to be resolved. It is now generally believed that ice ages
are initiated by small changes in the earth’s orbit, caused by, among other
things, the gravitational tug of Jupiter and Saturn. These changes alter
the distribution of sunlight across different latitudes at different times of
year. When the amount of light hitting the far northern latitudes in
summer approaches a minimum, snow begins to build up there. This
initiates a feedback cycle that causes atmospheric carbon dioxide levels to
drop. Temperatures fall, which leads more ice to build up, and so on. After
a while, the orbital cycle enters a new phase, and the feedback loop begins
to run in reverse. The ice starts to melt, global CO 2 levels rise, and the ice
melts back farther.
During the Pleistocene, this freeze-thaw pattern was repeated some
twenty times, with world-altering effects. So great was the amount of
water tied up in ice during each glacial episode that sea levels dropped by
some three hundred feet, and the sheer weight of the sheets was enough
to depress the crust of the earth, pushing it down into the mantle. (In
places like northern Britain and Sweden, the process of rebound from the
last glaciation is still going on.)
How did the plants and animals of the Pleistocene cope with these
temperature swings? According to Darwin, they did so by moving. In On
the Origin of Species, he describes vast, continental-scale migrations.
As the cold came on, and as each more southern zone became fitted for arctic beings
and ill-fitted for their former more temperate inhabitants, the latter would be supplanted
and arctic productions would take their places.... As the warmth returned, the arctic forms
would retreat northward, closely followed up in their retreat by the productions of the