Could the Megarhine or Southern species of Rhinoceros have dwelt
under the same climate as the Musk -sheep ? I think this altogether
impossible. Had the constitution of the former animal been sufficiently
elastic, it would have held its own ground against the invading
animals from the north and east after the manner of its
fellow the R. leptorhinus of the French and Italian Pliocenes.
There is, however, one view that has the merit of explaining the
difficulty, and which therefore is probably true. During the depression
of North Germany and the greater portion of Britain, those
parts of the Preglacial continent now represented by France and
the south of England were not submerged ; for in that case they
would present some traces of the deposit of icebergs, which were so
numerous in the North Sea of the period. It is hardly within
reason to suppose that all proof of submergence beneath the Glacial
sea could have been removed by subaerial denudation from so large
an area, while to the north of the Thames, and in North Germany,
it is so abundant and so ample. It is therefore probable that the
valley of the Lower Thames roughly marks the position of the
ancient Glacial coast-line in Britain, and that to the south the relics
of the Preglacial continent extended without a break through France
into Italy ; while to the north the look-out was over a dreary expanse
of sea burdened with icebergs, like that off the coast of Newfoundland.
The temperate or moderately warm climate that prevailed over the British portion of the Preglacial continent before the depression took place, must have been lowered by the presence of so much melting ice as is implied by the presence of the Boulder-clay, and especially in the neighbourhood of the coast-line, independently of any great change flowing from some other unknown and cosmical cause. This climatal change must have banished to a certain extent the Preglacial mammals from the area over which it was felt ; but nevertheless it is highly consistent with what we know of the migration of Herbivores to suppose that now and then some of them, such as R. megarhinus, may have ventured northwards as far as the valley of the Thames.
Again, M. Lartet has shown, in his memorable essay on the migration of mammals, that the Arctic division of the Postglacial or Quaternary mammalia (the Mammoth and Tichorhine Rhinoceros and the rest) invaded Europe from their ancient home in the north of Asia (where they probably dwelt during the Pliocene epoch, Tertiary, in Western and Central Europe) at the commencement of the European Quaternary or Postglacial period, the change in the Pliocene temperature, coupled very possibly with the depression of land in North Siberia, causing the animals inhabiting that area to advance westwards and southwards and to occupy the feeding- grounds till then belonging to the Pliocene fauna. This immigration very probably began while Northern Europe was being depressed beneath the waves in the Boulder-clay epoch. If this be admitted, there is nothing improbable in the hypothesis that the North Asiatic immigrants would gradually creep round the shores of the Glacial
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