40 LAND MAMMALS IN THE WESTERN HEMISPHERE 



of Pleistocene Man in Europe, where fMammoth bones have 

 been abundantly found, and also that there were Siberian 

 rhinoceroses similarly protected against the cold. fM^-i^'inioth 

 remains with hide and flesh, but much less complete, have like- 

 wise been found in Alaska. 



In a cavern in southern Patagonia an expedition from the 

 La Plata Museum discovered, with the remains of a gigantic, 

 extinct fground-sloth, large pieces of the skin still covered 

 with hair and affording most welcome information as to the 

 colouration of these most curious animals. The skin had been 

 preserved from decay by deep burial in dry dust. Mummies 

 of Pleistocene rodents have been found in the dry caves of 

 Portugal, whereas in the ordinary caves which are damp or 

 wet, only bones are preserved. Unfortunately, as has been 

 said, such instances of complete preservation are very rare, and 

 none are known of mammals more ancient than those of the 

 Pleistocene epoch. 



In general, it may be said that the higher the geological 

 antiquity of a skeleton is, the greater is the chemical alteration 

 which it has undergone. Bones of Pleistocene or later date 

 have, as a rule, suffered little change beyond the loss of more 

 or less of their animal matter, the amount of such loss depending 

 chiefly upon exposure to the air. Bones which, for thousands 

 or tens of thousands of years, have been buried in dense cave- 

 earth, in an antiseptic peat-bog, or in asphalt, are often per- 

 fectly sound and fresh when taken up. Skeletons of the ante- 

 cedent (Tertiary) period are, on the other hand, very frequently 

 petrified; that is to say, the original substance of the bones 

 has been completely removed and replaced by some stony 

 material, most commonly lime or flint. This substitution took 

 place very gradually, molecule by molecule, so that not only 

 is the form of the bone or tooth most accurately reproduced, 

 but the internal, microscopic structure is perfectly retained 

 and may be studied to as great advantage as in the case of 

 modern animals. 



t Extinct. 



