78 THE HOESE. 



Darwin* collected a number of cases of horses of 

 various breeds and countries so marked, and from 

 them came to the conclusion of the " probability of 

 the descent of all the existing races from a single 

 dun-colored, more or less striped primitive stock, to 

 which our horses occasionally revert." 



Fossil remains of true horses, differing but very 

 slightly from those now existing, are found abun- 

 dantly in the most recent geological ages in almost 

 every part of America, from Escholtz Bay in the north 

 to Patagonia in the south. Whether any of these 

 remains should really be referred to E. cdballus or 

 not — that is, whether they belonged to animals which 

 possessed all the external characters attributed above 

 to that species — is, of course, doubtful. Our knowl- 

 edge of existing forms teaches us that closely similar 

 and perhaps identical skeletal and dental characters 

 may be associated with considerable external differ- 

 ences, especially in the character, distribution, and 

 color of the hair. If zebras were only known from 

 such portions of their structure as could be preserved 

 in a fossil state, we should never have guessed how 

 greatly they differed in outward aspect from horses 

 and asses. All that we can do with a fossil bone or 

 tooth is to assign it to any known species which it re- 



* TJie Variation of Animals and Plants under Domestication, 

 vol. i. chap. ii. (1868). 



