_Migration across the Equator._—Mr. Belt[103] and others have felt some difficulty in understanding how, according to theory, the plants and animals of temperate regions could manage to migrate from one hemisphere to the other, seeing that in their passage they would have to cross the thermal equator. The oscillation to and fro of the thermal equator across the geographical, removes every difficulty in regard to how the migration takes place. When, for example, a cold period on the northern hemisphere and the corresponding warm one on the southern were at their maximum, the thermal equator would by this time have probably passed beyond the Tropic of Capricorn. The geographical equator would then be enjoying a subtropical, if not a temperate condition of climate, and the plants and animals of the northern hemisphere would manage then to reach the equator. When the cold began to abate on the northern and to increase on the southern hemisphere, the thermal equator would commence its retreat towards the geographical. The plants and animals from the north, in order to escape the increasing heat as the thermal equator approached them, would begin to ascend the mountain heights; and when that equator had passed to its northern limit, and the geographical equator was again enjoying a subtropical condition of climate, the plants and animals would begin to descend and pursue their journey southwards as the cold abated on the southern hemisphere. CHAPTER XV. WARM INTER-GLACIAL PERIODS. Alternate Cold and Warm Periods.—Warm Inter-glacial Periods a Test of Theories.—Reason why their Occurrence has not been hitherto recognised.—Instances of Warm Inter-glacial Periods.—Dranse, Dürnten, Hoxne, Chapelhall, Craiglockhart, Leith Walk, Redhall Quarry, Beith, Crofthead, Kilmaurs, Sweden, Ohio, Cromer, Mundesley, &c., &c.—Cave and River Deposits.—Occurrence of Arctic and Warm Animals in some Beds accounted for.—Mr. Boyd Dawkins’s Objections.—Occurrence of Southern Shells in Glacial Deposits.—Evidence of Warm Inter-glacial Periods from Mineral Borings.—Striated Pavements.—Reason why Inter-glacial Land-surfaces are so rare.