306 NATURAL HISTORY. 



become progressively nearer and nearer to the normal 

 pentadactylous type. In comparatively modern strata, 

 we find the Hipparion, in which the two little splint-bones 

 of the living horses are so far developed that they project 

 externally and carry little hoofs at their ends. The foot 

 is, therefore, three-toed in the Hipparion, but the animal 

 still walked upon the great middle toe, and the lateral 

 toes were functionally useless, as they did not touch the 

 ground. In the still older Anchitherium, the two lateral 

 toes are sufficiently developed to touch the ground, but 

 the middle toe is still much the biggest, and is the toe 

 upon which the weight of the body is principally sup- 

 ported. In the still older Orohippus, the fore-feet are 

 four-toed, the fifth or little finger being now developed, 

 but the thumb is still wanting, and the hind-feet have only 

 three toes. Finally, in the Eohippus, the oldest type of 

 equine animal .yet discovered, the fore-foot possesses four 

 complete toes, with a rudimentary thumb (or first toe) in 

 addition, thus becoming morphologically five-toed. The 

 above gives, of course, an exceptionally striking instance 

 of how palaeontology enables us to trace the line of descent 

 of some particular living animal ; but there are innumer- 

 able instances in which fossil forms exhibit characters 

 which more or less extensively bridge over the gaps 

 separating groups apparently widely remote. Upon the 

 whole, therefore, the evidence of palaeontology, though 

 lending but a partial support to the theory of the origin of 

 species by means of natural selection alone, is overwhelm- 

 ingly in favour of a general theory of the evolution of 

 animals from other pre-existent types. 



