THE ALLIES OF THE SQUIRREL: MAMMALIA 431 



areas. The increased complexity of the teeth makes them 

 more efficient grinding-organs. In the latter part of the Age 

 of Mammals North America was broadly connected with Asia, 

 and the horse is known to have inhabited plains of all the 

 continents excepting Australia. After the horse had reached 

 practically its present state of development (in the early part 

 of the next succeeding period, the Age of Man) it seems to 

 have disappeared entirely from America, owing to causes not 

 thoroughly understood, though generally ascribed to the 

 oncoming cold of the Glacial Epoch. The horse persisted, 

 however, in Europe, and was one of the animals which prim- 

 itive man domesticated. The various uses to which the horse 

 could be put were gradually learned by man, for Professor 

 Osborn, of the American Museum of Natural History, says 

 there is " abundant proof that man first hunted and ate, then 

 drove, and finally rode the animal." It was reintroduced into 

 America by the Spaniards at the time of their conquest, and 

 soon ran wild. 



The carnivores were early represented by generalized 

 types, and later by dogs and saber-toothed cats. The latter 

 get their name from their lengthened canine teeth. Insec- 

 tivorous mammals, rodents, bats, ungulates of many kinds, 

 and even the primates also occurred, and in the waters of the 

 oceans were found cetaceans of different species. 



The Age of Mammals began in North America with a 

 warm climate, as in the case of previous periods, but toward 

 its close frigid conditions began to prevail, probably due to 

 the gradual elevation of the continental land-mass. The on- 

 coming cold produced in the northern part of both America 

 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 



