ITS NEAREST EXISTING EELATIONS. 73 



munication between the two continents in the neigh- 

 borhood of Behring's Straits, and, having prevailed 

 over the allied forms they found in possession, totally 

 disappeared from the country of their birth until 

 re-introduced by the agency of man. This supposi- 

 tion, based upon the great abundance and variety 

 of the possible ancestral forms of the horse which 

 have lately been discovered in America, may be at 

 any time negatived by similar discoveries in the Old 

 World, the absence of which at the present time can- 

 not be taken as any evidence of their non-existence. 

 In a popular exposition of this subject it would 

 be out of place to give an account of the views, more 

 or less crude, which have been put forth by the vari- 

 ous zoologists who have lately exercised much labor, 

 patience, and thought in endeavoring to investigate 

 the exact lines of descent of the different species and 

 even breeds of the existing horses from those of 

 earlier periods. In the first place, they would only 

 be intelligible to any one possessing a full knowledge 

 of the minute anatomical characters on the compari- 

 son of which the results are based j but what is of 

 still more consideration, the conclusions from all 

 these researches can be looked upon at present as 

 provisional only, being founded upon such imperfect 

 materials as exist as yet in our collections, and liable 

 to be modified at any moment by fresh discoveries 



