ITS PLACE IN NATUKE. 9 



cause it results from adaptation to particular con- 

 ditions, which may become changed in course of 

 time, and then the animals which have become 

 adapted exclusively for life under those conditions 

 perish, while those animals that retain more general 

 characters readily adapt themselves to the altered 

 circumstances. The commonplace, average sort of 

 creatures are thus often the longest lived as species, 

 while such very strangely modified forms as Uinta- 

 therium* Machairodus, \ and Thijlacoleo, \ passed rap- 

 idly over the stage and then vanished from sight. 



It is proposed in this little work to treat of the 

 horse, not as an isolated form, but as one link irra 

 great chain, one term in a vast series, one twig of a 

 mighty tree ; and to endeavor to trace, as far as our 

 present knowledge permits, what its relations are to 

 the rest, and by what steps of modification in its 



* A huge beast from the Eocene of North America, with 

 limbs resembling those of an elephant, and a rhinoceros-like 

 skull, but with great descending flattened tusks in the upper 

 jaw, and three pairs of bony prominences, like horns, on the 

 top of the head. 



t An animal allied to the tiger, with enormous saber-like 

 upper canines, found in the later Tertiaries of both Europe 

 and America. 



t A marsupial of the late Tertiary period of Australia, as 

 large as a sheep, allied to the phalangers and kangaroos, but 

 with one huge cutting cheek-tooth (premolar), and one great 

 incisor on each side of each jaw, all the other teeth being ex- 

 tremely reduced in size and almost functionless. 



