chap, xiii.] DOUBLE-FOOT : PECORA. 379 



For, as will be hereafter shewn at length in the case of Insects, super- 

 numerary appendages may grow out from a normal appendage and are 

 then a pair, being formed as a right and a left, composing a separate 

 Secondary Symmetry. 



On the first view the digits of each group are in symmetry with 

 each other like those of the normal limb, the two groups also balancing 

 each other like the halves of a double-hand : on the other view one of 

 the groups would be supposed to be made up of a right and a left digit 

 III, or of a right and a left digit IV. The possibility of the second 

 view being true arises of course in the Artiodactyles from the fact that 

 in them the normal digits compose a bilateral Minor Symmetry. 



There is nevertheless little doubt that the former account is the 

 right one and that neither group is a Secondary Symmetry ; for were 

 either of the groups really in Secondary Symmetry the supposed super- 

 numerary group should contain at least parts of four digits. Lastly, 

 some of the cases, as No. 566, are clearly of the nature of double limbs, 

 both groups having a common axis of Symmetry. 



A further difficulty arises from the fact that most of these double 

 limbs are old specimens cut off from the trunk. There is therefore 

 no proof that such a limb is not that of a polymelian in Geoffroy St 

 Hilaire's sense. In other words, though it is practically certain that 

 neither of the groups of digits is itself a system of Secondary Symme- 

 try it is quite possible, and in some cases likely that the whole limb is 

 of this nature. In cases of duplicity, especially of posterior duplicity, 

 the two limbs of one or both of the united bodies frequently form a 

 compound structure somewhat resembling one of the double limbs here 

 under consideration. Hence it is not possible to include with confid- 

 ence great numbers of cases of double limbs described by various 

 writers or preserved in museums, for it is rarely that particulars re- 

 garding the rest of the animal are to be had. This difficulty applies to 

 almost all cases known to me and they are therefore given with this 

 caution. This objection of course does not apply to such a case as No. 

 564. 



The following few cases will sufficiently illustrate the different forms of limbs 

 included in this section. They consist of two chief kinds ; first, limbs like Nos. 563 

 and 566, in which both groups contain two digits, and secondly, cases like No. 567, in 

 which one of the groups contains three digits, recalling the state described in the 

 last section (cp. Nos. 558 and 559). Besides these there are some cases of amor- 

 phous extra digits not here related. 



63. Cow, full-grown, right fore foot with four digits arranged in two groups of two, 

 as shewn in fig. 116. The carpus not preserved. No particulars as to the rest of the 

 animal. This specimen is in the Museum of Douai and is described in detail by 

 Delplanque, Etudes Teratologiques, n. Douai, 1869, p. 30, PI. I. [The possibility 

 that this may be a limb of a pygomelian is not excluded.] 



64. Cervus dama (Fallow Deer). A female having each hind foot 

 double. The division occurs in the upper part of the tarsus, which 

 gradually diverges into two separate tarsi [? metatarsi] and two 

 separate feet. This doe had for several successive years dropped a 

 fawn with the same malformation, though she had been served by 

 several bucks. Ward, Edwin, Proc. Zool. Soc, 1874, p. 90. 



55. Two cases, a Roebuck and a Deer, mentioned by Geoffroy St 

 Hilaire (Hist, des Anom., I. p. 697) are probably of this nature. 



