EVOLUTION OF ELEPHANTS 361 



same time, from over an area of some millions of square 

 miles." 



Many and various causes may occasion the death of 

 great numbers of animals in a short space of time. For 

 example, climatic changes, absence of water, great depth of 

 snow, or certain extreme changes of temperature. Dr. 

 Bailey Willis mentions that, in parts of the Argentine, 

 animals are dependent for water upon ponds in water-holes, 

 around which frequently hundreds of cattle will congregate, 

 and in dry years these water-holes contain less and less oi 

 water until they finally dry out completely, thus causing hun- 

 dreds of animals to die of thirst. 



We know that in our Western country for many suc- 

 cessive years horses, cattle, and sheep can safely depend 

 upon the forage on the plains for their sustenance, but there 

 come years when the snow is too deep and when a large 

 percentage perish. It is quite possible that some such cli- 

 matic change as an unusually deep snow or a very intense 

 cold caused the death and extermination of the mammoth. 

 In Alaska and in Siberia, when an animal dies in a great 

 mass of snow and ice, the remains will probably be preserved, 

 as were those of the two great mammoths that have been 

 found in Siberia. Though it is quite possible that an animal 

 may die and be thus preserved, as were the mammoths that 

 have been found ; still the extermination of the great probo- 

 scidians that existed from the most northerly clime to the 

 end of Patagonia remains a mystery. 



The structure of mastodon teeth suggests that the animal 

 consumed soft herbage and could not exist in an era of 

 great cold, and it is quite possible that as mastodon skele- 

 tons are frequently found in swamps, the animals had met 

 their death there while in search of food. The fact that 

 mammoth teeth are made up of cellular rings, Siberian (E. 

 primigenius) mammoth teeth being more highly specialized 



