322 THE HIND LIMB. 



Professor Ewart points out that the callosities of the 

 horse agree, as to their inconstancy, with other vestigial 

 structures, which, being no longer factors in the struggle 

 for existence, are very variable. In some cases in which 

 the fore chestnuts are very small and the hind ones 

 absent, there are long spar-like ergots ; and in other 

 instances the ergots are absent, although there are well- 

 formed chestnuts on both fore and hind legs. 



Some zoologists maintain that the chestnuts are vestiges 

 of scent or sebaceous (waxy or oily) glands, which are found 

 in certain species of deer and other animals. Professor 

 Ewart shows that the chestnuts contain no trace of glands. 

 This glandular theory of the origin of chestnuts is not 

 supported by any valid argument. 



Chauveau and other writers are of opinion that the 

 chestnut is the vestige of the first digit (thumb or big 

 toe). Against this idea we have the following facts : — 



(fl). The chestnut has no special connection, bony, 

 ligamentous, 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 first, second, fourth and 

 fifth digits began from below, it is unreasonable to 

 assume that a vestige of the first digit would 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 

 are represented only by minute vestiges at the end of 

 the respective splint bones, but by no callosities. 



