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CHAPTER XXIX. 



ASSES. 



Differences between the Ass and Horse — Hybrids between the Horse and 

 Ass — Varieties of Asses — The Domestic and Abyssinian Wild Ass — 

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

 Chapman's Zebra — Tlie Grevy or SomaUland Zebra — The Quagga. 



Differences between the Ass and Horse. — The followine 

 are the chief differences between the two animals : — 



I. The ass has castors (see p. 216) only on the fore legs. 



M. Chauveau is of opinion that the castor is the rudiment 

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

 are the followino- 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 which have either disappeared or 

 have become rudimentary, began from below ; it is unreason- 

 able 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. 



(d.) 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 onager, I have seen them much nearer 

 the elbow thaii the knee. 



