THE ARGUMENT FROM PALAEONTOLOGY 67 



phalanges, and the "coffin-bone" the terminal 

 phalanx of the toe. The " hoof" corresponds to the 

 nail. In the fore-limb the "knee" of the horse is 

 equivalent to the wrist. The "splint-bones" repre- 

 sent the metacarpal bones of the first and third 

 digits. 



Now the ancestors of the horse are Protohippus 

 or Hipparion, which is found in the Pliocene ; 

 Miohipptts and Mesohippus, found in the Miocene ; 

 Orohippiis, in the Eocene ; and Eohippus, at the base 

 of the Eocene. In Protohippus each foot has three 

 well-formed digits; Miohippus, in addition to this, 

 has a rudimentary metacarpal bone of a fourth digit 

 in the fore-foot ; in Mesohippus this rudimentary 

 metacarpal bone is more fully developed ; in Oro- 

 hipptcs there are four well-developed digits in the 

 fore-foot, three in the hind-foot ; while in Eohippus 

 five digits are present. Thus this series of fossil 

 forms furnishes a complete gradation from the older 

 tertiary forms with four or five toes up to the horse 

 with one toe. These forms differ not only as regards 

 the number of toes, but also in other respects, chiefly 

 in the gradual diminution and loss of independence 

 of the ulna and fibula, and in the gradual elongation 

 of the teeth and increasing complexity of their 

 grinding surfaces. 



An excellent series of gradational forms is shown 

 in the case of Paludina, of which six or eight dis- 

 connected forms were known first, and described as 

 distinct species ; later on, connecting forms were 

 discovered, and it was realised that we had a case of 

 progressive modification from the older geologic beds 



