XXXIV Ci-ENUKAL INTKODUC'i'lON. 



tlie horse is still for a time tridactylous. lu a four- 

 weeks liorse eiiil)iyo there are do i-uclimeuts of digits m 

 the puddle-shaped limljs, but in a five-weeks embryo 

 there are rudiments of three digits, and at six weeks the 

 foot is a miniature of that of the rhinoceros.* The 

 second and fourth digits in the embryo liorse, though 

 small, are almost as complete as in Hipparion, the three- 

 toed fossil horse preserved in large numbers in the 

 Pikermi beds near Athens. As in Hipparion, the second 

 and fourth digits are asyui metrical. They thns, while 

 forming a nearly symmetrical pair, differ from the large 

 symmetrical middle digit — the only complete digit in 

 recent Equida:. Occasionally a foal is born with tn'o 

 hoofs on one or more of its limbs; at v^ery long intervals 

 a foul a]ipenrs witli three hoofs on one or more of its 

 liml)s. Alexander's Bucephalus, e. g., was polydactylous, 

 as was Cassar's favourite horse. I have in my possession 

 four sjiecimeus showing extra digits in the horse. Poly- 

 dactylism is not uncommon in man, and it seems to be 

 still more common in the yno:. In man the extra dibits 

 seem to be always due to the splitting either of the 

 thumb or of one or more of the fingers. In one of my 

 jiolydactylous horse limbs the extra digit is without doubt 

 due to the splitting of the third or middle digit. The 

 extra digit in the three remaining specimens is, however, 

 not so easily accounted for. When the large middle 

 digit splits, the two resulting digits are almost identical 

 in form if not in size. In my most complete specimen 

 one of the digits — the inner — is not only very much 



ip- 



smaller than the other ; it, like the inner digit in Hi 



G^ 



parion, is asymmetrical, and the articulation between the 

 first and second phalanges (the first and second pastern- 

 bones) has been obliterated. The smaller inuev dio-it is 

 thus far from being an image of the large functional one • 

 hence it does not seem to have been formed b_y splittino- 

 * The liorse, iieuce, does not duriiij^ development, as some imao-iued 

 pass through a five-toed staije, i.e. it is never, even when a verv small 

 embryo, pentadactylous like its supposed remote ancestor Plieiiacodiis in 

 which the skeleton of the fore-foot is, in many ways, wonderfullv like that 

 of the human hand. 



