6$ INTRODUCTION. 



acted with decisive power to the destruction of all 

 the other Mammals in question.* 



These considerations, and more particularly the 

 presence of horse-hones upwards to the surface, 

 seem to indicate the original residence of the pre- 

 sent domestic horse to have extended over the same 

 surface of the Old World as the anterior fossil ani- 

 mal ; we say the domestic horse^ without therefore 

 excluding the Hemionus, which once resided as far 

 west as Prussia, or denying that the Koomrah 

 existed in northern Africa, which is of the true 

 form of Eq. Cahallus, though the specifical identity 

 may be doubted. We are also inclined to question 

 the positive unity of species in the Tangums and 

 Kiangs of the central high' ridges of Asia, and even 

 that of the wild horses originally indigenous in the 

 British Islands : possibly the Sarans of the great 

 Indian chain may be distinct, although the homoge- 

 neous character of their structure cannot be doubted : 

 they, and other varieties hereafter to be mentioned, 

 appear to be different forms of one type, very closely 

 allied, yet distinct. 



We do not as yet know the limits of what con- 

 stitutes a genus, nor have we a satisfactory definition 

 of species, since it is admitted that hybrids derived 

 even from assumed distinct genera, are not without 

 the power of procreating a fertile offspring, with 



* From this view burrowing Canidae and Rodentia are 

 purposely excluded, because, from their habits, they may be 

 found in the same localities, without belonging to the same 

 era. 



