The natural pastures
Where there is less than eight inches of rain one finds the range grass, of separate plants with the bare earth between. The three American kinds are the bunch grass of the hollows, a tall tussock with tap roots reaching down to moisture; the little buffalo grass from two to four inches high; and the gramma grass of the same size which inhabits Mexico.
One may presume that the tussock fed the oldest herds and that, as it failed, the pony took to eating the shorter grasses.
The horse in a meadow pasture does not eat the ranker growths, but grazes the shorter, smaller kinds of grass. From this we may reason that the little buffalo grass of the ranges is the typical food of the species. The leaves of this plant are green in the spring but soon cure to a golden tawny colour, which changes to brown in the autumn, and a washed-out, greyish brown in winter. As they cure, the leaves curl downwards one by one until the plant becomes a ball or tuft exceedingly springy underfoot, sweet as a nut in taste, and equal in food value to standing oats.
Conditions of the stockrange
As one approaches the desert the land is sprinkled with bushes which protect themselves from being eaten with a very strong nasty taste, or deadly thorns. Of these the sage brush comes first, a thousand miles wide followed by a thousand miles of greasewood and acacia varied with forests of cacti. The grass becomes more scanty as one forces a way onward into the heart of the desert, where there are regions of naked rock and belts of drifting sand.
As the annual rainfall varies from year to year the desert tracts expand or shrink by turns. As the winds swing from side to side, or wax or wane in their supply of moisture, a fertile region is made desolate for a few centuries as Palmyra, or a desert shrinks before the spreading pasture. In cycles the desert blossoms or withers, but with the millions of years it slowly widens.
Such, then, were the conditions of the stockrange to which the ancient herds had to adapt themselves, learning to dispense with the shrunken meadows, and make the most of varying crops of bunch grass.
The taste for green pasture is so far forgotten that range horses will swim rivers and break fences to escape from the richest of meadows and get to the desert hillsides which seem to grow nothing but stones. Where sheep tear the bunch grass out by the roots and leave stark desert, the horses' lips and teeth are so delicately adapted to this feeding that they never uproot the plant.