vi] DISTRIBUTION OF SELECTED GROUPS 135 



America where it changed into Tapir avus and con- 

 tinued well into the Miocene when it died out. But 

 in Eurasia Protapirus changed into Paratapirm 

 which in turn produced the Pliocene Tapirus proper. 

 Formerly also in Europe its only descendant in the 

 Old World is now the Indo-Malay species, none 

 having made their way into Africa. Others however 

 got, in Pleistocene times, into North America, soon to 

 die out there, except those two or three kinds which 

 are now in tropical America, including South-East 

 Mexico. 



Thus has come about the striking case of dis- 

 continuous distribution of a small and very natural 

 family at two opposite parts of the equator. 



Horses. 



The story of the horse tribe is based upon such 

 an abundant material that pedigrees with dozens of 

 generic names have been constructed, and its very 

 intricate story has been so often condensed and 

 popularised that the horse has become the show- 

 animal of evolution. Yet, although the evolution of 

 the tribe as such is well understood, often in minute 

 detail, that of the genus Equus, the horses, asses and 

 zebras proper, is not known. The main reason is that 

 since early Tertiary times there existed in America, 

 Asia and Europe a bewildering mass of odd-toed 



