Page 235 - Lokmanya Tilak Samagra (khand 2)

22 SAMAGRA TIL~K- 2 • THE ARCTIC HOME
causes the equatorial regions to bulge out, a change in the axis
would give rise to a second equatorial protuberance, which,
however, is not observable and that 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 plea-
sant 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 thousand of years, and
those who support Lyell's theory are of opinion that the dura-
tion of the Glacial epoch must be taken to be about 200,000
years in order to account for all the geographical 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 is enormous; but in the present
state of geological evidence it is difficult to decide in favour of
any one of these views. All that we can safely say is that the
duration of the Pleistocene period, which included at least two
Glacial and one Inter-Glacial epoch, must have been very much
longer than the period of time which has elapsed since the com-
mencement of the Post-Glacial period.
According to Sir Robert Ball the whole difficulty of find-
ing out the cause of the Glacial period vanishes when the solu-