6 8o EVOLUTION OF THE HORSE. 



In examining the body of the horse, we find many 

 vestigial structures which are more or less useless to the 

 present possessor, but which appear in a better developed 

 condition in certain horse-like fossils. Thus, the splint 

 bones extend lower down in the Pliohippus (Fig. 643), 

 than in the horse of to-day ; in the Protohippus of the 

 Lower Pliocene, each of these bones is furnished with a 

 pastern and hoof, which, according to Marsh, did not 

 come to the ground ; and in the earlier Mesohippus, the 

 digits of the splint bones are larger, and evidently took 

 part in progression. The general anatomical characters 

 of these fossil animals were evidently horse-like. Von 

 Baer wisely suggested that the history of an individual 

 is the history of its species ; although, as Bateson points 

 out, "if it be generally true that the development of a 

 form is a record of its descent, it has never been suggested 

 that the record has been complete." The elaborate 

 investigations of Cossar Ewart show us that a horse, 

 during the early (embryonic) stages of his life, has, like 

 the Mesohippus, three digits on each limb. The ves- 

 tigial (second and fourth) digits are best developed when 

 the foetus is about 22 weeks old, at which time it is about 

 14 inches long. The first joint (that between the meta- 

 carpal bone and the long pastern bone) of these digits 

 persists till after birth ; th second joint (that between the 

 long and short pastern bones) is the first to go ; and the 

 third joint (that between the short pastern bone and the 

 pedal bone) is the next to disappear, which it does, 

 when the foetus is about five months old. Although 

 the digits of the splint bones gradually degenerate 

 as the foetus increases in size, they can still be inden- 

 tified at birth, in the bulb (" button ") at the end 

 of their respective splint bones. Professor Ewart 

 (Experimental Contributions to the Theory of Heredity) 

 points out that " this warrants the deduction that, as 

 one-hoofed recent horses are related to three-hoofed 

 Pliocene forms, the latter are related to the still older 



