148 THE GENESIS OF SPECIES. [CHAP. 



ship in which the horse was to stand to the human inhabit- 

 ants of this planet. These extinct forms, as Prof. Owen 

 remarks, 6 " 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 physiologi- 

 cally, i. e., they cannot produce a race of hybrids fertile 

 inter se. 



As to the mere action of surrounding conditions, the 

 same professor remarks : a *' 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 Palaeothere, to sink less deeply into swampy 

 soil, and be more easily withdrawn than the more concen- 

 tratively simplified and specialized foot of the horse. Rhi- 

 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 

 Pliocene period, has no support from observation or anal- 



ogy." 



Not only, however, do we fail to find any traces of the 

 incipient stages of numerous very peculiar groups of ani- 

 mals, but it is undeniable that there are instances which 

 appeared at first to indicate a gradual transition, yet which 

 instances have been shown, by further investigation and dis- 

 covery, not to indicate truly any thing of the kind. Thus 

 at one time the remains of Labyrinthodonts, which, up till 

 then, had been discovered, seemed to justify the opinion 

 that, as time went on, forms had successively appeared 



6 " Anatomy of Vertebrates," vol. iii., p. 792. 6 Ibid., p. 793. 



