274 ASSES 



CHAPTER XXVIII. 



Differences between the Ass and Horse — Hybiids bet^^een the Hoi se and 

 Ass — Vaiieties of Asses — ^The Domestic and Abyssinian Wild Ass — 

 The Onager — The Kiang — The Mountain Zebia — Burchell's Zebia — 

 Chapman's Zebia — The Gr^vy or Somahland Zebia — The Quagga — 

 Points of the Ass, 



Differences between the Ass and Horse. — The follow- 

 ing are the chief differences between the two animals — 



I. The ass has castors (p. 192) only on the fore legs. 



Chauveau is of opinion that the castor is the rudiment 

 of the thumb, or rather first digit- Against this idea there 

 are the following facts : — 



(a.) The castor has no special connection, bony, liga- 

 mentous, or otherwise, with either the bones of the knee or 

 those of the hock. As, in the evolution of the horse, the 

 wasting away of the digits (p. 289, e^ seg.), which have either 

 disappeared or have become rudimentary^ began from below, 

 it is unreasonable to assume that a vestige of the first digit 

 should have remained after all trace of its metacarpal or (as 

 the case may be) metatarsal bone had been lost. 



{b) The second and fourth metacarpal and metatarsal 

 bones still remain strongly in evidence , yet their digits have 

 entirely vanished. 



{c.) The castors are situated below the hock and above 

 the knee. In the onasrer I have seen them much nearer the 

 elb^w than the knee. 



I think the foregoing facts are sufficient to prove that 

 the castors are not rudiments of the first digits. I have no 

 theory to offer respecting their origin. 



