Gradually these useful animals
were adapted to different uses; and at length different breeds were
evolved. Climate and food supply had much to do with the size and the
character of the breeds. In the Shetland Islands the animals are
naturally dwarfed by the cold, bleak winters, and the scant vegetation
on which they subsist. In middle Europe, where the summers are long and
the winters mild, vegetation is luxurious, and the early horses
developed large frames and heavy muscles. The Shetland pony and the
Percheron draught-horse are the two extremes of size.

What man has done in changing the types of horses is to emphasize
natural differences. The offspring of the early heavy horses became
heavier than their parents. The present draught-horse was produced,
after many generations, all of which gradually approached the type
desired. The slender racehorses, bred for speed and endurance rather
than strength, are the offspring of generations of parents that had
these qualities strongly marked. Hence came the English thoroughbred and
the American trotter.

We can read in books the history of breeds of horses. Our knowledge of
what horses were like in prehistoric times is scant. It is written in
layers of rock that are not very deep, but are uncovered only here and
there, and only now and then seen by eyes that can read the story told
by fossil skeletons of horses of the ages long past.

Geologists have unearthed from time to time skeletons of horses. It was
Professor Marsh who spent so much time in studying the wonderful beds of
fossil mammals in the western part of this country, and found among them
the skeletons of many species of horses that lived here with camels and
elephants and rhinoceroses and tigers, long before the time of man's
coming.

How can any one know that these bones belonged to a horse's skeleton?
Because some of them are like the bones of a modern horse. It is an easy
matter for a student of animal anatomy to distinguish a horse from a cow
by its bones. The teeth and the foot are enough. These are important and
distinguishing characters. It is by peculiarities in the formation of
the bones of the foot that the different species of extinct horses are
recognized by geologists.

Wild horses still exist in the wilds of Russia. Remains of the same
species have been dug out of the soil and found in caves in rocky
regions. Deeper in the earth are found the bones of horses differing
from those now living. The