THE LIFE OF MAMMALS 
spotted; and the preglacial horses which followed them were 
probably a uniform reddish brown — an effect of the cold, 
rainy climate of the Pleistocene period, to which, perhaps, 
are also due the addition of a heavy mane and tail, useful as 
blanket and fly brush, and the broadening of the hoofs, adapted 
to walking on softer ground. 
Along with the disappearance of the side toes in the evolution of the 
horse goes a considerable increase in the proportionate length of the limbs. 
Evolution Greater length in the lower leg and foot enlarges the length 
of Horse. of the stride without decreasing its quickness and, therefore, 
gives the animal greater speed; but it puts an increased strain on the ankles 
and toe joints, and these must be strengthened correspondingly by con- 
verting them from ball-and-socket joints to pulley joints. Additional 
strength is obtained by the consolidation of the two bones of the forearm 
and of the leg into one. The concentration of the step on a single firmly 
shod toe serves likewise to increase the speed over smooth ground, although 
a hindrance to travel in rough or boggy places. The increase in length of 
limb renders it necessary for the grazing animal that the head and neck 
should lengthen, in order to enable the mouth to reach the ground; and com- 
parison of modern with early types shows that all these changes have gone 
on as fast as required. With them has proceeded a constant lengthening 
of teeth and improvement of them as grinders, until now they are capable 
of the thorough mastication of the flinty grasses of the dry uplands upon 
which few other animals can subsist. The general enhancement in size 
shows the effect of abundant food and increasingly favorable conditions 
as time went on. All these changes are adaptations to a life in a region of 
level, open, grassy plains. At first the race was better fitted for a forest life, 
but it has become more and more completely adapted to compete with ene- 
mies or rivals under the conditions which prevail in the high dry plains of 
the interior of the great continents. This evolution went on as gradually 
as the evolution of the plains themselves. Says Matthew: — 
“At the commencement of the Age of Mammals the western part of 
the North American continent was by no means as high above sea level as 
now. Great parts of it had but recently emerged, and the Gulf of Mexico 
Tertiary still stretched far up the valley of the Mississippi. The cli- 
America. mate at that time was probably very moist, warm, and tropical, 
as is shown by the tropical forest trees, found fossil even as far as Green- 
land. Such a climate, with the low elevation of the land, would favor the 
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