INFLUENCE OF CONDITIONS 
growth of dense forests all over the country, and to such conditions of life 
the animals of the beginning of the mammalian period must have been 
adapted. During the Tertiary the continent was steadily rising above the 
ocean level, and at the same time other influences were at work to make 
the climate continually colder and drier. The coming on of.a cold, dry 
climate restricted and thinned the forests and caused the appearance and 
extension of open, grassy plains. The ancient forest inhabitants were 
forced either to retreat and disappear with the forests, or to adapt themselves 
to the new conditions of life. The ancestors of the horse, following the latter 
A RAcE HORSE: EXTREME OF MODERN DEVELOPMENT OF THE EQUINE TYPE. 
course, changed with the changing conditions, and the race became finally 
as we see it to-day, — one of the most highly specialized of animals in its adap- 
tation to its peculiar 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, excepting Australia.” 
“About the early or mid-Pliocene period,’’ Osborn informs us, ‘‘there 
apparently occurred the long journey of the true American breed of horses 
into Asia and Europe, and over the newly made land bridge Migration 
of Panama or of the Antilles into South America. That the of Primi- 
true Old World horse actually came from Americais inferred tive Horses. 
because of the sudden appearance in the Upper Pliocene of the Siwalik Hills 
of northern India, in northern Italy, and in England, of five species of the 
true horses (Equus), of which no ancestors have been found in either Eu- 
rope or Asia. Another strong argument for their American origin is found 
357 
