CHAP, xiv.] DISCONTINUITY IN DIGITAL VARIATION. 407 



(6) Homceotic Variation in terminal digits when a new member 



is added beyond them. 



This is a principle that has been several times seen in Meristic 

 Variation, and in Chapter x. Section 7, it was treated of at length 

 in the case of teeth. Some few illustrations of the same principle 

 occur among the evidence as to digits. It has been seen for in- 

 stance how that, upon the appearance of an extra digit on the 

 radial side, the digit which stands in the position of pollex may have 

 three phalanges and resemble an index (No. 485, &c.). Similarly 

 it was found that upon the formation of a large digit externally to 

 the minimus the digit standing in the ordinal position of the 

 minimus may have an increased proportional length (No. 509). 

 Still more important is MORAND'S case (No. 510), in which the most 

 external digit had muscles proper to a minimus, while the digit 

 standing in the ordinal position of the minimus was without them. 



The cases of extra digit in the Horse (No. 536, &c.) still more 

 clearly illustrate the principle, if the view of the nature of those 

 cases taken in the text be received. 



It should be expressly stated that in digits, as in teeth, it is 

 not always that the terminal member is promoted on becoming 

 penultimate. Such promotion is indeed rather exceptional in 

 digits, but the fact that it may occur is none the less a phenome- 

 non of great significance. 



(7) The absence of a strict distinction between duplicity of a 

 given digit and other forms of addition to the Series. 



This subject has been so often spoken of in connexion with 

 special cases that it is unnecessary here to make more than brief 

 allusion to it. The same principle was shewn to be true of teeth 

 (p. 270) and of mammae (p. 193), and there is little doubt that it is 

 true of Meristic Series generally. Facts illustrating the matter in 

 relation to digits will be found in the evidence as to duplication of 

 pollex and hallux in Man (p. 351), as to duplication of the hallux in 

 the Fowl (p. 391), in the evidence of cases in the Horse of variation 

 intermediate between division of III and development of II (p. 371), 

 and in the cases of three-toed Cows (p. 377). 



In almost all the animals in which any considerable range of 

 digital Variation is to be seen it is possible to find a series of cases 

 making an insensible transition from true duplicity, or division into 

 two equivalent parts whose positions and forms are such that they 

 maybe reasonably looked upon as both representing a normally single 

 member, up to the condition in which while the series contains a 

 greater number of members, each member nevertheless stands in 

 a regular Succession to its neighbour. 



Upon the proper understanding of this proposition and upon 

 the recognition of its truth hang those corollaries before enuntiated 



