वेदों की प्राचीनता एवं काल-निर्णय / ओरायन (लोकमान्य बाल गंगाधर तिलक - खगोलीय वैदिक शोध)

26 THE ARCTIC HOME IN THE VEDAS.
axis means a change in the equator, and as the earth owing to its
diurnal rotation causes the equatorial regions to bulge out, a
change in the axis would give rise to a second equatorial protu-
berance, which, however, is not observable, and the theory cannot,
therefore, be accepted. A gradual cooling of the earth would
make the Polar regions habitable before the other parts of the
globe; but a succession of Glacial epochs cannot be accounted for
on this theory.
Thus out of the various theories advanced to account for the
vicissitudes of climate in the Pleistocene period only two have
now remained in the field, the first that of Lyell which explains
the changes by assuming different distribution of land and water
combined with sudden elevation and submergence of large landed
areas, and the second that of Croll which traces the glaciation to
the precession of the equinoxes combined with the high value of
the eccentricity of the earth's orbit. Lyell's theory has been
worked out by Wallace who shows that such geographical changes
are by themselves sufficient to produce heat and cold required to
bring on the Glacial and Inter-Glacial periods. We have seen
that in earlier geological ages a pleasant and equable climate
prevailed over the whole surface of the globe owing mainly to
different distribution of land and water, and the theory advanced
by Lyell to account for the Glacial epoch is practically the same.
Great elevation and depression of extensive areas can be effected
only in thousands of years, and those who support Lyell's theory
are of opinion that the duration of the Glacial epoch must be
taken to be about 200,000 years in order to account for all the geo-
graphical and geological changes, which, according to them, were
the principal causes of the Glacial period. But there are other
geologists, of the same school, who hold that the Glacial
period may not have lasted longer than about 20 to 25
thousand years. The difference between the two estimates