CHAP, xiv.] SYMMETRY IN DIGITAL VARIATION. 405 



numerical change. The bones of this pes do not normally exhibit 

 any very clear bilateral symmetry l . Yet on the appearance of new 

 digits the foot is reconstituted and its parts are, to use a metaphor, 

 ' deposited ' in a system of bilateral symmetry ' 2 whose completeness 

 is proportional to the degree of development of the new digits. 

 What may be the meaning of this extraordinary fact one cannot 

 yet guess. The fancy is constantly presented to the mind that there 

 is in the normal foot a condition of strain, that the balance between 

 the right foot and the left is a condition of imperfect stability, and 

 that upon the introduction of some unknown disturbance this 

 balance is upset and each foot settles down as a separate 

 system. But I see no way of testing this fancy and no way of 

 following it further. 



Still more complex are the facts seen in the human hand. 

 There is here first the fairly complete series of conditions ranging 

 from the normal, through the three-phalanged thumb up to the 

 several Conditions in which extra digits upon the internal side of 

 the limb seem to have sprung up to balance the four normal digits ; 

 but on the contrary there is the exceptional case of the Macacque's 

 foot (No. 504) where the extra parts are, as I believe, external. 

 (Besides these there are the wholly distinct series of "double- 

 hands," which will be spoken of below.) The former cases taken 

 alone would certainly suggest that there is an imperfect balance 

 or system of symmetry subsisting between the thumb and the 

 four fingers of the normal manus, but to this suggestion there 

 are numerous difficulties which need hardly be detailed in this 

 preliminary glance at the phenomena. 



With more confidence it can be maintained that the pollex 

 and perhaps the hallux of Man is in itself a Minor bilateral Sym- 

 metry, apart from the four fingers, for it may divide into equal 

 parts related as images. The same is true of the hallux of the 

 Dorking (p. 390), and probably of the extra digit or digits some- 

 times arising from the tibio-tarsus of the Turkey for example (see 

 No. 603). 



Besides this the facts of the frequent syndactylism between the 

 digits III and IV of the human manus, taken in connexion with the 

 phenomena of the Pig and Ox, suggest that the four fingers may 

 have among themselves again a relation of the nature of Symmetry. 



1 In the normal pes, though all the claws are retracted to the outside of the 

 second phalanges, yet the claws of digits III and IV rest close together, that of III 

 being external to its pad, while that of IV is internal to it* pail, forming, so far, a 

 relation of images between these two digits. In the polydactyle foot it is a remark- 

 able feature that, though the bones are in symmetry about an axis passing between 

 II and III, the relation of the claws of III and IV to their pads remains almost 

 normal, still giving a superficial appearance of symmetry between these two digits. 

 (In the polydactyle pes the pads are mostly rather narrower.) 



- It will be remembered that this symmetry appears not merely in the lengths of 

 the several digits but in the manner of retraction of the claws and in the corre- 

 sponding form of the second phalanges, three digits being fashioned (in the case of 

 six perfect digits) as right digits and three as lefts. 



