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(Cervidæ), which now commence to be very much more prominent. In the true Sivalik fauna of India there are a good many types which have never yet been found in Europe; such, for instance, as the Camels, which are specially characteristic of the American Tertiary strata. Furthermore, there are found, in the American formations of this age, a large number of forms, such as Bos, Equus, Hippopotamus, and Ursus, which do not appear at all in Europe until the later Pliocene times.
When the Pliocene times arrive, we begin to find a preponderating number of still existing genera present in the fossil beds, although the greater number of them have, at the present epoch, retreated southwards into the Oriental and Ethiopian Regions. This southward migration seems to have gone on throughout the Pliocene period, and was probably occasioned by the increasing cold caused by the gradual advent of the great Ice-age, which now began to make itself felt over the whole of the northern part of the globe. .
Finally, during the Glacial period the fauna assumed nearly its present form, containing large numbers of species that still survive. At this epoch, too, a connection appears to have been formed between the Old and New Worlds in the neighbourhood of Behring Strait, by means of which an interchange of animals took place, and resulted in occasioning the similarity which forms so marked a feature on a comparison of the Nearctic and Palæarctic faunas.
It is this similarity that has caused certain writers on geographical distribution to unite the Palæarctic and Nearctic Regions into one, whereas, as a matter of fact, paleontological evidence seems to show that, out of all the
four Regions embraced under the term “Arctogæa," the North American or Nearctic Region was the first to be separated from the main mass, and that the similarity is a comparatively modern element in the character of the two faunas.
(PLATE VIII., p. 216)
Most of the recent writers on geographical distribution have confined their attention to terrestrial Mammals, or at any rate have but casually alluded to the marine groups of that Class. The seven previous chapters having been devoted to the terrestrial Mammals, it is proposed now to examine the principal facts connected with the distribution over the world's surface of the Marine or aquatic members of the Class.
Aquatic Mammals which pass their lives entirely, or for the greater part, in the water are, of course, subject to very different laws of distribution from the terrestrial forms. As regards aquatic Mammals, land is of course an impassable barrier to their extension, and, subject to restrictions in certain cases, water offers them a free passage. Just the opposite is the case with the terrestrial Mammals, to which in most cases land offers a free passage, while seas and rivers restrain the extension of their ranges.
The groups of aquatic Mammals that are represented on the earth's surface at the present time are three in number, viz. : (1) the Sub-order of the Carnivora, containing the Seals and their allies, generally called the Pinni