236 



ANNALS NEW YOUK \<\\]>i:\IY OF SCIEXChs 



sequence. They arc i(lriiti\-al or closrlv allicil in the European and North 

 American sequeDce. In North America, the series is more complete, the 

 approximation to a direct genetic sequence is ch)ser and the successive 

 stages api)ear earlier in time. This is reasonably interpreted by suppos- 

 ing that the center of dispersal was intermediate between Europe and the 

 western United States but nearer to the lattei-. That is to say, it was 

 either in boreal North America or in nortlieastci-n Asia. The absence of 



S)nall 4-7bai Norse<> S7na// 5-Totd Larger d'Toed Lar,^e. /~ToedHorsei 



TERTIARY PERIOD 



Paleocen; eocene epoch 



OLrCOCENE 



M I OCENE 



QUATERNARY 



PLIOCENE PLEISrOCENE RECENT 



Eo/iippus . 



Orokippus 



Epihippus 



Mcsohippui 



M iontppus 



"Anc/iith'ni 



AncnMerium 



Hupohippus 



Paranippui 



Merucnippui 



Protohippus 



Pliohippus 



Hippidcum 



[Eu-ropt and North Americ^j 



'_ -'ft-^^B^- 1 (North }/^merica) \ 



j - ■ -^H {AfortM 4mfnca) | 



__) -^^1^^^^ ^Nort/i Am^r/ca.) 



__4- |. __ '^-^1^^ (North A\77er/caJ 



{North America)} 4\i<^ 



__!_-_ ^Europc)_ J ^^- Hi ' 



[North America ^Asia and Euro pe.)\. _^\ 



. ! (North A77iertea.) 



' (North Amertca.)_:_A^^ 



' (North A7verua)_^ : 3m_ 



(North America.J_\_ _\ ^-^ 



{South Am\ertca)^_ L _ ii- 



I 



r 



Onohippidiun ' _ 



Hipparion 

 E^uus 



(North America. , Ana , Europe and North yifriea)ju\ 



^orth a^nd South America. ,Asia. , Europe .and Africa)_ Jf^-^ 



J I L_ 



_ (South Amhtca.)_:. 



Fig. 18. — Oeoloyk- range and phylogeiielic relations of fossil Eqi;iii(r 



The overlap in geologic range of the genera, and the sudden appearance of each new 

 stage, indicate that our record is not derived from the center of disper.«al uf the race; 

 although the American series is sufficiently direct to indicate that it was not very remote. 



primitive survivors of the race in the East Indies is natural; as the horses 

 were very early adapted to open plains, unfitted Tor mountain or forest 

 habitat, the great transverse Himalayan chain would form an almost im- 

 passable barrier and the heavily forested regions of the East Indies would 

 have no attractions to tempt the ancestral liorses to pass around its east- 

 ern end. 



