THE ALLIES OF THE SQUIRREL 437 



and the Eurasian land-mass conditions so severe that to the 

 period the name Glacial Epoch is given. During its contin- 

 uance all of North America north of a line drawn from 

 New York through Pennsylvania, Indiana, Missouri, South 

 Dakota, Montana, and Oregon was covered at different 

 times with a layer of ice, which in certain regions grew to be 

 a mile thick over the land, destroying all life or forcing it to 

 migrate southward to escape the rigors of the climate. As 

 there were several invasions and retreats of the ice, there 

 may be said to have been several glacial epochs, separated 

 by long periods of warmer weather, when the animal and 

 plant life could slowly work its way back on the edge of the 

 retreating glaciers. There is a peculiar interest to this period, 

 inasmuch as it introduces the last of the great geological eras, 

 the Quaternary Period, or the Age of Man. 



A conspicuous feature of the mammalian life of the Age 

 of Man was the great size of many of the species. After the 

 opening glacial epoch the climate became mild again, and 

 this seems to have forced the development of abundant 

 vegetation and great mammalian forms. One of the largest 

 and most widely distributed species was the mammoth 

 (El'ephas yrimiqe'nius, Fig. 229), a proboscidean larger than 

 the elephant of today and covered with a thick coat of hair, 

 an adaptation to cold temperate regions. Its remains have 

 been found frozen in the ice of Siberia, the hair and flesh 

 perfectly preserved. Early man knew of this great mammal, 

 for a drawing of the creature is in existence, made on a piece 

 of its own tusk (Fig. 230). The mastodons were somewhat 

 similar to the mammoths, but fitted on the whole for a 

 warmer climate. There are over thirty species of mastodons 

 known, of nearly world-wide distribution. They have be- 

 come extinct within so short a time, geologically speaking, 

 that traditions of their existence as living animals occur 

 among men. 



