THE ALLIES OF THE SQUIRREL 435 



horse of today has been accomplished by a gradual increase 

 of size, a reduction in the number of toes, and a reduction in 

 number and an increase of complexity in the teeth. The 

 main steps in the evolution of the horse are shown in 

 Fig. 228. The changes in the limbs fit the animal for rapid 

 locomotion over level, grassy ground. The teeth become 

 more complex and 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 primitive 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 ani- 

 mal. " 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 



