thousand years to
bring about, it cannot be doubted that considerable modifications in the
fauna and flora of both countries would be the result, although it is
impossible to predict {120} what the precise changes would be. We can
safely say, however, that some species would stand the change better than
others, while it is highly probable that some would be actually benefited
by it, and that others would be injured. But the benefited would certainly
increase, and the injured decrease, in consequence, and thus a series of
changes would be initiated that might lead to most important results.
Again, we are sure that some species would become modified in adaptation to
the change of climate more readily than others, and these modified species
would therefore increase at the expense of others not so readily modified;
and hence would arise on the one hand extinction of species, and on the
other the production of new forms.

But this is the very least amount of change of climate that would certainly
occur every 10,500 years when there was a high excentricity, for it is
impossible to doubt that a varying distance of the sun in summer from 86 to
99 millions of miles (which is what occurred during--as supposed--the
Miocene period, 850,000 years ago) would produce an important difference in
the summer temperature and in the actinic influence of sunshine on
vegetation. For the intensity of the sun's rays would vary as the square of
the distance, or nearly as 74 to 98, so that the earth would be actually
receiving one-fourth less sun-heat during summer at one time than at the
other. An equally high excentricity occurred 2,500,000 years back, and no
doubt was often reached during still earlier epochs, while a lower but
still very high excentricity has frequently prevailed, and is probably near
its average value. Changes of climate, therefore, every 10,500 years, of
the character above indicated and of varying intensity, have been the rule
rather than the exception in past time; and these changes must have been
variously modified by changing geographical conditions so as to produce
climatic alterations in different directions, giving to the ancient lands
either dry or wet seasons, storms or calms, equable or excessive
temperatures, in a variety of combinations of which the earth perhaps
affords no example under the present low phase of {121} excentricity and
consequent slight inequality of sun-heat.

_Present Condition of the Earth One