EVOLUTION OF THE HORSE 



(Fig. 153, 6'), 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 

 r\ 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 



IV 



IV 



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

 Foot of OroMppus ayilis, X %. 

 (From Lull, after Marsh.) 



descent. Collateral branches will have been given off from the' >v 

 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. 



