present. No great changes in
climate were liable to take place at that time. During the next fifty
thousand years the eccentricity steadily increased. Towards the end
of that time all that was necessary to produce a glacial epoch in the
Northern Hemisphere was favorable geographical causes, and that our
earth should reach its point nearest the sun in Summer. This it must
have done when about half that time had elapsed.

We can in imagination see what a slow deterioration of climate took
place. Thousands of years would come and go before the change would be
decisive. But a time must have at length arrived when the vegetation
covering the ground was such as was suited only for high northern
latitudes. The animals suited for warm and temperate regions must have
wandered farther south; others from the north had arrived to take their
place. We can see how well this agrees with the changes of climate at
the close of the Pliocene Age. The snows of the commencing Glacial Age
would soon begin to fall, finally the sun would not melt them off of the
high lands, and mountain peaks, and so a Glacial Age would be ushered
in.

We have referred to the fact that the earth reaches its perihelion point
a little earlier each year, and, as a consequence, we would have periods
of mild climate alternating the cold. This extended period of time,
equal to twenty-one thousand of our ordinary years, has been named the
Great Year of our globe. Mr. Wallace has pointed out some very good
reasons for thinking Mr. Croll's theory must be modified on this
point. He thinks that when once a Glacial Age was fairly fastened on
a hemisphere, it would retain its grasp as long as the eccentricity
remained high, but whenever the Summer of the Great Year came to that
hemisphere, it would melt back the glacial ice for some distance, but
this area would be recovered by the ice when the Winter of the Great
Year supervened. These effects would be different when the eccentricity
itself became low. Then we would expect the glacial conditions to vanish
entirely when the Summer of a Great Year comes on.<30>

As we have made the theoretical part of this chapter already too long,
we must hurry on. We can only say that this view is founded on the fact
that when a country was covered with snow and ice, it had so to speak,
a great amount of cold stored up in it, so much, in fact, that it would
not be removed by the sun of a new geological Summer. This ought to be
acceptab