What is the explanation of these relationships? Assuming that there has been a land connexion, it can have been either by way of Antarctica or Polynesia. The earthworms cannot endure a very severe climate, but on the other hand there is a total absence of any tropical forms common to Australia and South America, and the general dissimilarity of the faunas shows that the connexion cannot have been available for a very long period. A study of the oceanic depths suggests that the Antarctic connexion is the more probable. A comparatively slight elevation would connect Patagonia and the Falkland Islands with the South Shetlands and Graham Land, and an elevation of 12,000 feet would give a large land connexion between Australia and the opposite coast of Antarctica via Kerguelen. Forbes even postulates an immense Tertiary Antarctica in which several forms of animals and plants were able to evolve, but except possibly in the case of the edentates this supposition is not necessary.
The course of events may provisionally be taken as follows: In late Tertiary times an elevation of at least 12,000 feet in the South Polar regions caused a great increase in the area of Antarctica, which was united to South America on the one hand and Australia on the other. The northern shores of this continent were far to the north of their present position, and though the interior was very cold the coast lands had at first a moderate temperature, and for a short time allowed animals to migrate from Australia to South America or vice versa. But the high mountains of the interior were already glaciated, and ice-sheets gradually crept down their slopes. Owing to the small precipitation the advance of the ice-sheets was slow, but ultimately, probably in late Tertiary times, they approached the coast, and the track along which migrations had taken place was closed. The distribution of animals and plants shows quite clearly that the land connexion was maintained into the period of refrigeration. The shores of the continent being further north, the pressure gradient between the pole and the present coast was less, and consequently the winds were lighter. This and the diminished loss by calving into glaciers allowed the ice to become thicker than it is now.
Hedley apparently considers that the migrations referred to above took place in an interglacial period, but the Patagonian beds in which the fossil marsupials are found are Tertiary and not Quaternary. No direct evidence of an interglacial period has been found in Antarctica, nor, considering the intensity of the glaciation which the country is even now undergoing, is any such to be expected, and we can only infer from the bipartition of the Glacial period in Australia, New Zealand and South America—which, in New Zealand at least, was associated with submergence—that there was probably a similar bipartition in Antarctica. Nordenskjold states that the submarine relief showing river erosion which, in Tierra del Fuego, was developed partly at least in the interglacial period, is also developed in West Antarctica. It is improbable that the ice ever entirely vanished from the continent. We shall see in [Chapter XIV] that even the comparatively brief warm period known as the post-glacial climatic optimum extended to the Antarctic coast, and this is additional argument for extending the much greater interglacial oscillation southward beyond its known limits in Tierra del Fuego, Australia and New Zealand, but here the matter must be left.
BIBLIOGRAPHY
Hedley, C. “The palæographical relations of Antarctica.” London, Proc. Linnæan Soc., 124, 1911-2, p. 80.
Lydekker, R. “A geographical history of mammals.” Cambridge University Press, 1896, pp. 125 ff.
David, T. W. E. “Antarctica and some of its problems.” London, Geogr. J. 43, 1914, pp. 605-30.
Nordenskjold, O. “Antarktis.” Handbuch Regional Geologie, Heft 15, 1913.