CAUSE OF THE EVOLUTION 



THK evolution of tlic liorsc, a(laj)tinji; it to live on the dry plains, 

 |)r()l)al)ly went hand in hand with the evolution of the plains 

 themselves. At the coniinenceiiienl of the A^e of Mannnals the 

 westein part of tiie North Ameiicnn continent was Ia' no means as high 

 above sea level as it now is. ( licat parts of it had hut iceently emerged, 

 and the (Julf of Mexico still stretched far up the valley of the Mississippi. 

 The climate at that time was ):)r()bal)l>' veiy moist, warm and tropical, 

 as is shown hy the tropical foi-est trees, found fossil even as far as Green- 

 land. Such a climate, with the low elevation of the land, would favor the 

 growth of dense foiests all over the country, and to such conditions of life 

 the animals of the beginning of the mammalian period must have been 

 adapted. Diu'ing the Teitiary the continent was steadily rising above the 

 ocean le\-el, and at the same time othei' infhiences were at woi'k to 

 make the climate continually colde'- and drier. The coming on of a cold, 

 dry climate restiicted and thinned the foi'ests and caused the aj)pearance 

 and extension of ()i)en, glassy plains. The an(ient forest inhabitants 

 weie forced either to reticat and disai)pear with the forests, or to adapt 

 themselves to the new conditions of life. The ancestors of the horse, 

 following the latter course, changed with the changing conditions, and 

 the race became finally as we see it to-day, one of the most highly special- 

 ized of animals in its adaptation to its pecuUar environment. At the end 

 of the Age of Mammals the continents stood at a higher elevation than at 

 present, and there was a broad land connection between Asia and North 

 America, as well as those now existing. At this time the horse became 

 cosmopolitan, and inhabited the plains of all the great continents, except- 

 ing Australia. 



It is a (juestion whether the direct ancestry of the modern horse is to 

 be searched for in western America or in the little known interior plains 

 of eastern Asia. It is also unknown why the various species which in- 

 habited North and South America and Europe during the early part of 

 the Age of ]\Ian should have become extinct, while those of Asia (horse 

 and wild ass) and of Africa (wild ass and zebra) still survive. ]Man, since 

 his ai)pearance, has played an important part in the extermination of the 

 larger animals: but thei'e is nothing to show how far he is responsible 

 foi- the (lisai)))eaiance of the native^ Amei'ican s};)ecies of horse. 



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