CHAP. XIIL] DIGITS: HORSE. 361 



Meristic Variation may take place by division of single members 

 of Series, a phenomenon well seen in the B group ; and we have also 

 seen many cases of numerical Variation by addition to the Series 

 associated with a reconstitution, or more strictly a redistribution 

 of differentiation amongst the members of the series thus newly 

 constituted ; but here in these rare examples of the C group the 

 nature of the parts is such that it cannot be predicated that 

 the change is accomplished by either of these methods exclusively. 

 From such cases it follows that the two processes are not really 

 separable, but that they merge into each other. (Compare the 

 similar facts seen in regard to teeth p. 269, and mammae p. 193.) 



A. EXTRA DIGITS BORNE BY DISTINCT METACARPAL OR 



METATARSAL. 



The cases in this group may be subdivided as follows : 



(1) Two digits, one being formed by the development of the 

 digit II. 



a. Only three metacarpals or metatarsals (II IV) as 



usual. Common form : fore and hind limb. 



b. Four metacarpals (? I IV). Common form : anato- 



mically described in fore limb only. 



c. Five metacarpals (? I V). Single case in fore limb. 



(2) Two digits, one being formed by development of the digit IV. 



Rare. 



(3) Three digits; the digits II and IV both developed. Rare. 



(4) Two digits; the digits II and IV both developed, III 



aborted. Rare. 



It will appear from the evidence that though the same varia- 

 tion is often present in the limbs of both sides this is not always 

 so. The fore and hind limbs also sometimes vary similarly and 

 simultaneously, but in other cases they do not. Different forms of 

 numerical Variation are also sometimes found on the two sides, 

 and not rarely the variation in the fore limb is different fi'om that 

 in the hind limb. 



(1) Two digits, one formed by development of the digit II. 

 a. Three metacarpals or metatarsals only. 



To this division and to the next, (1) b, belong the great majority 

 of cases of polydactylism in the Horse. Unfortunately most of the 

 records have been made from living animals and contain no 

 anatomical description : in the absence of such particulars it is 

 not possible to know whether a given case belongs to this division 

 or to the next, and it thus is impossible to determine the relative 

 frequency with which the two forms occur. 



The following are given as specimen cases. 



