such as to
produce any considerable amount of perpetual snow, this would be increased
whenever a high degree of excentricity concurred with winter in _aphelion_,
and diminished during the opposite phase. On all mountain ranges,
therefore, which reached above the snow-line, there would be a periodical
increase and decrease of snow, and when there were extensive areas of
plateau at about the same level, the lowering of the snow-line might cause
such an increased accumulation of snow as to produce great glaciers and
ice-fields, such as we have seen occurred in South Africa during the last
period of high excentricity. But along with such depression of the line of
perpetual snow there would be a corresponding depression of the alpine and
sub-alpine zones suitable for the growth of an arctic and temperate
vegetation, and, what is perhaps more important, the depression would
necessarily produce a great _extension_ of the area of these zones on all
high mountains, because as we descend the average slopes become less
abrupt,--thus affording a number of new stations suitable for such
temperate plants as might first reach them. But just above and below the
snow-line is the area of most powerful disintegration and denudation, from
the alternate action of frost and sun, of ice and water; and thus the more
extended area would be subject to the constant occurrence of land-slips,
berg-falls, and floods, with their {517} accompanying accumulations of
_debris_ and of alluvial soil, affording innumerable stations in which
solitary wind-borne seeds might germinate and temporarily establish
themselves.

This lowering and rising of the snow-line each 10,500 years during periods
of high excentricity, would occur in the northern and southern hemispheres
alternately; and where there were high mountains within the tropics the two
would probably overlap each other, so that the northern depression would
make itself felt in a slight degree even across the equator some way into
the southern hemisphere, and _vice versa_; and even if the difference of
the height of perpetual snow at the two extremes did not average more than
a few hundred feet, this would be amply sufficient to supply the new and
unoccupied stations needful to facilitate the migration of plants. It is
well known that all great mountain ranges have undergone such fluctuations,
as proved by ice-marks below the present level of snow and ice.

But the differences of temperature in the two