OVIBOS MOSCHATUS. 25 



arctic of known living mammalia, the Musk Sheep, since the evidence which it offers must 

 be weighed against that offered by the other mammalia. And among these, that of 

 the Mammoth and Woolly Rhinoceros must be put out of court, because the first possessed 

 a sufficiently elastic constitution to endure the severity of a Siberian climate, and to 

 flourish alike in Italy and in the southern states ; and it is very probable, from the wide 

 range of the latter, that it had similar capacities of enduring climatal extremes. Either 

 the Musk Sheep must have wandered into the temperate regions at the time, or the 

 associated animals must have been fitted to endure the severity of a climate in which the 

 Musk Sheep now lives. The former alternative seems to me to be far more likely to be 

 true than the latter. I should be inclined to consider that the skull in question belonged 

 to an animal that had strayed from its usual arctic haunts in the winter, southwards into 

 the country more usually occupied by the animals with which it was found, and this 

 view is considerably strengthened by an appeal to like cases of migration at the pre- 

 sent day. 



In North America, for example, the Bison ranged, in Hearne's time, over the open 

 rushy plains as far to the north and east as the southern shore of Athabasca Lake in 

 lat. 59°, while on the colder shores of Hudson's Bay, a little to the north of Fort 

 Churchill in the same latitude, that explorer found proofs of the presence of Musk Sheep. 

 In an unusually severe winter there would be nothing extraordinary in the latter animal 

 occasionally straying south of Athabasca Lake, and its bones being mingled with those of 

 the Elk, Waipiti, and Bison. I should therefore view this isolated case of the occurrence 

 of the most arctic of all the ruminants on the banks of the Thames, during the time of the 

 deposits of the Lower Brick-earths, as altogether exceptional, and not affecting the sum of 

 the evidence as to climate afforded by Rhinoceros megarhinus, B. hemitcechus, Cervus 

 elaphus, C. capreolus, Elephas antiquus, Hippopotamus major, and indeed all the other 

 mammalia of the group found at Ilford, Gray's Thurrock, or Erith. 



§ 6. The Age of the Deposit at Crayford. — The relation of the lower Brick-earths to 

 the Glacial period, under which name are comprehended the complex phenomena offered 

 by — 1, the development of an ice sheet like that of Greenland ; 2, the submergence of the 

 land beneath the sea ; 3, the glacier period, is one of those difficult and delicate questions 

 which cannot be solved definitely in the present state of our knowledge. There are, how- 

 ever, two considerations which are of considerable value in coming to any conclusion 

 whatever. In the first place we know, that the mammalia inhabiting the English side of 

 the great valley of the North Sea in Pleiocene times, lived under a temperate climate; and 

 it is only reasonable to suppose that, as the /temperature became lowered in the northern 

 regions, the northern animals would gradually pass southwards, and occupy the 

 feeding grounds, which had been before those of the animals inhabiting the temperate 

 zone. This must have taken place at the very beginning of the Glacial period in Great 

 Britain, for the lowering of the temperature which dispossessed them of their ancient 



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