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ia we have the Oriental region very fairly defined; so that the relation of the geographical with the zoological primary divisions of the earth is sufficiently clear. In order to make these relations visible to the eye and more easily remembered, we will put them into a tabular form: Regions. Geographical Equivalent. Palaearctic EUROPE, with north temperate Africa and Asia. Ethiopian AFRICA (south of the Sahara) with Madagascar. Oriental TROPICAL ASIA, to Philippines and Java. Australian AUSTRALIA, with Pacific Islands, Moluccas, &c. Nearctic NORTH AMERICA, to North Mexico. Neotropical SOUTH AMERICA, with tropical N. America and W. Indies. The following arrangement of the regions will indicate their geographical position, and to a considerable extent their relation to each other. N E A R C T I C--P A L A E A R C T I C | | | | | ORIENTAL | ETHIOPIAN | NEO- | TROPICAL AUSTRALIAN May 4th. Diameter of spot 31deg 24' June 4th. ,, ,, 28deg 0' ,, 17th. ,, ,, 22deg 54' July 4th. ,, ,, 18deg 24' ,, 12th. ,, ,, 15deg 20' ,, 20th. ,, ,, 18deg 0' We thus see that Mars has two permanent snow-caps, of nearly equal size in winter but diminishing very unequally {55} in summer, when the southern cap is reduced to nearly one third the size of the northern; and this fact is held by Mr. Carpenter, as it was by the late Mr. Belt, to be opposed to the view of the hemisphere which has winter in _aphelion_ (as the southern now has both in the Earth and Mars), having been alone glaciated during periods of high excentricity.[9] Before, however, we can draw any conclusion from the case of Mars, we must carefully scrutinise the facts, and the conditions they imply. In the first place, there is evidently this radical difference between the state of Mars now and of the Earth during a glacial period--that Mars has no great ice-sheets spreading over its temperate zone, as the Earth undoubtedly had. This we know from the fact of the _rapid_ disappearance of the white patches over a belt three degrees wide in a fortnight (equal to a width of about 100 miles of our measure), and in the northern hemisphere of eight degrees wide
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