MODERN HORSES AND THEIR ANCESTORS 85 



and proportions of the horse-ancestors changed until, 

 after being pig-like, then tapir-like, they acquired the 

 perfect form and size for fleet and prolonged movement 

 over firm, grass-grown plains. Horses and other large 

 animals have to run, not only to escape pursuit by 

 carnivorous enemies, but in order to travel, before 

 they die from thirst, from a region suddenly dried 

 up by drought to a region where water can be had. 

 Many thousands of wild animals perish every year 

 from local droughts in Africa. No small animals 

 can exist in regions liable to be affected by sudden 

 drought. 



Three-toed horses, like the upper Tertiary Hipparion, 

 are occasionally born as " monstrosities " from ordinary 

 horses at the present day. All horses have the remnant 

 of a toe on each side of the big central toe in the form 

 of splint-bones concealed beneath the skin. In some 

 breeds, for instance, in the " Shire " horses, which have 

 enormous hairy feet in proportion to their huge strength 

 and weight, these splint-bones tend to develop three 

 little toe-joints, which are immovable, but obviously are 

 "petti-toes." It is related by Suetonius that Julius 

 Caesar used to ride a favourite horse which, had several 

 toes on each foot with claws like a lion. This was one 

 of the "monstrosities" alluded to above, a throw-back 

 to the ancestral many-toed condition. Specimens illus- 

 trating these, and all else which I am here relating 

 concerning horses, and much more which I have not 

 space to tell, may be seen in the North Hall of the 

 Natural History Museum. 



The three-toed ancestral horse, Hipparion, attained a 

 fair size (that of a big donkey), and was shaped like the 

 recent fleet one-toed horses. In the skull in front of the 

 orbit, the Hipparion has a strongly marked depression in 

 the bone, as long and broad as a hen's egg, and in shape 



