EVOLUTION OF THE HORSE 



311 



(Fig. 153, e), which continued on into Pliocene times and attained 

 a height of 48 inches, had the second and fourth digits of each 

 foot represented by mere splint bones as in modern horses, 

 and had therefore already attained to the single-toed condition 

 (Fig. 158). 



In Pliocene times, however, we still find a three- toed horse 

 Hipparion surviving in Europe, but the modern one-toed genus 

 Equus (Fig. 153, /) also makes 

 its appearance both in the old and 

 new worlds, becoming extinct in 

 the new world in Post-Pleistocene 

 times until re-introduced from 

 Europe by the agency of man. 



The time occupied in the evolu- 

 tion of the genus Equus from 

 its remote ancestor Eohippus is 

 estimated by Professor Sollas at 

 five or six millions of years. This 

 period is sufficient to allow of a 

 very slow and gradual change 

 from one condition to the other. 

 Allowing five years for each 

 generation, Sollas arrives at the 

 conclusion that somewhere about 

 a million generations intervene 

 between the two extremes. The 

 total increase in height during 

 this time has been 53 inches, and 

 if this increase were spread fairly 

 uniformly over the whole period 

 it would only mean about 0*00005 inch for each successive 

 generation an amount which would be quite imperceptible to 

 human observers. 



In reconstructing such a pedigree as that of the horse from 

 palaeontological evidence it is of course necessary to bear in 

 mind that the great majority of extinct forms which come to 

 light will almost certainly not be actually in the direct line of 

 descent. Collateral branches will have been given off from the 

 phylogenetic tree in various directions, and it is much more likely 

 that any particular form discovered will belong to one of these 

 branches than that it will belong to the main stem. This fact, 



FIG. 155. a, Fore Foot and b, Hind 

 Foot of OroJiippus agilis, X ! 

 (From Lull, after Marsh.) 



