The geographical distribution of animals, with a study of the relations ...

232 ZOOLOGICAL GEOGRAPHY. [part hi.of the Palaearctic region, it is evident that it owes many of itsdeficiencies and some of its peculiarities to the influence of theGlacial epoch, combined with those important changes of physicalgeography which accompanied or preceded it. The elevation ofthe old Sarahan sea and the complete formation of the Mediterranean,are the most important of these changes in the westernportion of the region. In the centre, a wide arm of the ArcticOcean extended southward from the Gulf of Obi to the Aral andthe Caspian, dividing northern Europe and Asia. At this timeour European and Siberian sub-regions were probably moredistinct than they are now, their complete fusion having beeneffected since the Glacial epoch. As we know that the Himalayashave greatly increased in altitude during the Tertiary period, it isnot impossible that during the Miocene and Pliocene epochs thevast plateau of Central Asia was much less elevated and lesscompletely cut off from the influence of rain- bearing winds. Itmight then have been far more fertile, and have supported a richand varied animal population, a few relics of which we see inthe Thibetan antelopes, yaks, and wild horses. The influenceof yet earlier changes of physical geography, and the relations ofthe Palsearctic to the tropical regions immediately south of it,will be better understood when we have examined and discussedthe faunas of the Ethiopian and Oriental regions.