. Biology; the story of living things. THE EPIC OF EVOLUTION 511 There are forty-five fossil species of Equus, and seven wild species now living in Asia and Africa, from which domesticated horses and donkeys have been derived. Between the extremes of Eocene Eohip- pus and modern horses, eight other genera have been found, con- taining over one hundred species, and forming a continuous series with no gaps of importance. The arrangement of this series, and the approximate duration in time of each type, is shown in the accompanying table. In addition to the main line that eventuated in Equus, the
. Biology; the story of living things. THE EPIC OF EVOLUTION 511 There are forty-five fossil species of Equus, and seven wild species now living in Asia and Africa, from which domesticated horses and donkeys have been derived. Between the extremes of Eocene Eohip- pus and modern horses, eight other genera have been found, con- taining over one hundred species, and forming a continuous series with no gaps of importance. The arrangement of this series, and the approximate duration in time of each type, is shown in the accompanying table. In addition to the main line that eventuated in Equus, there have been various side lines which became extinct. The original home of Eohippus, and other genera of fossil horses, was North America, and here for millions of years they w^orked out their evolutionary salvation. In Pleistocene times, they made repeated migrations back and forth across the Bering Sea bridge to Eurasia and Africa, w^here their descendants, the wild asses and zebras, carry on today. Meanwhile, all the horses of North America became extinct, not suddenly but gradually over a stretch of thou- sands of years. What caused their extinction is unknown. Perhaps it was the Pleistocene glaciers, or it may be that they were finally wiped out by their carnivorous enemies. The suggestion has even been made that the deadly tsetse flies, fossils of wiiich have been found in the Florissant shales of Colorado, might have caused their downfall. These villainous flies, with the aid of parasitic protozoans which they transfer to mammalian hosts, have made it impossible for any except native cattle and horses to live in considerable river bottom areas of Africa today. million 50 years side lines AQ_ 30 20 PLEISTOCtNE^ 7 mnnline EOCENE OLlGOCEMt. losp. Orobippus lohippus â ibsp. Epibippus 2sR Caenertx four-toedt horses 16 sp. nssohippus "Miobippus nsp. tbree-toed: all toes used MIOCENE ^ PLIOCENE 16 Sp. Pambippus Pli|ohippus Msiychippii three-toed: only central toe^ used 17
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