XXXIV OENEEAL INTRODUCTION. 



the liorse is still for a time tridactylous. lu a four- 

 weeks horse embryo there are no rudiments of digits in 

 the paddle-shaped limbs, but in a five-weeks embrj^o 

 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 horse, 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 asymmetrical. They thus, while 

 forming a nearly symmetrical pair, differ from the large 

 symmetrical middle digit — the only complete digit in 

 recent Equidse. Occasionally a foal is born with t^vo 

 hoofs on one or more of its limbs ; at very long intervals 

 a foal appears with three hoofs on one or more of its 

 limbs. Alexander's Bucephalus, e. g., was polydactylous, 

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

 four specimens showing extra digits in the horse. Poly- 

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

 still more common in the pig. In man the extra digits 

 seem to be always due to the splitting either of the 

 thumb or of one or more of the finger.-^. In one of my 

 polydactylous 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, iln my most complete specimen 

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

 smaller than the other ; it, like the inner digit in Hip- 

 parion, is asymmetrical, and the articulation between the 

 first and second phalanges (the first and second pastern- 

 bones) has been obliterated. The smaller inner digit is 

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

 hence it does not seem to have been formed by splitting 

 * Tlie liorse, hence, does not durinj^ development, as some imagined, 

 pass througli a five-toed staj^e, i.e. it is never, even when a very small 

 embryo, pentadactylous like its supposed remote ancestor Phenacodus, in 

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

 of the human hand. 



