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 

 ■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 sp. 



Equus 



Ple5ippu5 

 isp 



8 



one -toed 



The 40,000,000 year old pedigree of the horse, involving in a direct line at least 

 ten genera and over one hundred species. 



