VI.] SPECIES AND TIME. 161 



fication by minute and infinitesimal steps ; a fortiori no 

 approach to a proof of modification by " Xatural Selection " 

 acting npon indefinite fortuitous variations. On tlie con- 

 trary, the series is an admirable example of successive 

 modification in one special direction along one beneficial 

 line, and the teleologist must here be allowed to consider 

 that one motive of this modification (among probably an 

 indefinite number of motives inconceivable to us) was the 

 relationship in whicli the horse was to stand to the human 

 inhabitants of this planet. These extinct forms, as Pro- 

 fessor Owen remarks,^ " differ from each other in a greater 

 degree than do the horse, zebra, and ass," which are not 

 only good zoological species as to form, but are species 

 2')liysiologically — i.e. they cannot produce a race of hybrids 

 fertile inter se. 



As to the mere action of surrounding conditions, the 

 same Professor remarks : ^ " Any modification affecting the 

 density of the soil might so far relate to the changes of 

 limb-structure, as that a foot with a pair of small hoofs 

 dangling by the sides of the large one, like those behind 

 the cloven hoof of the ox, would cause the foot of Hip- 

 parion, e.g., and a fortiori the broader based three-hoofed 

 foot of the Palaeotliere, to sink less deeply into suampy 

 soil, and be more easily withdrawn than the more concen- 

 tratively simplified and specialized foot of the horse, lihi- 

 noceroses and zebras, however, tread together the arid 

 plains of Africa in the present day; and the horse has 

 multiplied in that half of America where two or more 

 kinds of tapir still exist. That the continents of the 

 Eocene or Miocene periods were less diversified in respect 

 of swamp and sward, pampas or desert, than those of the 



1 "Anatomy of Vertebrates, " vol. iii. p. 792. " Ibid. ]). 793. 



