40G MKKISTIC VARIATION. [PARTI. 



It has been mentioned that there is some evidence to shew 

 that in the human ]- it is tin- digits II and III which are most 

 frequently syndactvlf, .-ven up to the point of being (in No. ">l) 

 a|]iaivntly ivpiv-eutrd by a single digit, and in this connexion 

 it will be remembered that in the polydactyle pes of the Cat 

 it is also between these digits that the new axis of Symmetry 

 falls. 



Th.'Sf >r;mty allusions to the possible influences which Sym- 

 iiiftrv niav exercise over Meristic Variation of digits will suffice to 

 indicate the nature of the problem to those who may care to 

 f\aniiiM' it. It is with hesitation that so indefinite a matter is 

 >|iok,-n "t at all. Nevertheless it is likely that if any one can find 

 a way of interpreting these indications the result will be con- 

 siderable. 



(")) Duplicity of limbs. 



In the evidence as to the digits of Man facts were given 

 iv>pecting the state known as Double-hand, and some similar 

 'asfS were referred to in Artiodactyles. In these instances the 

 digital series, and to some extent the limb, is in its new shape 

 made up of the external parts of a pair of limbs compounded 

 together in such a way that there is a partial duplicity of the limb, 

 the two halves being more or less exactly complementary to each 

 other and related as images 1 . 



This phenomenon in its perfect form must be essentially 

 distinct from the other cases of increase in number of digits; 

 for in the double-hands the limb developes an altogether new 

 bilateral symmetry (see especially No. 4-92). Between cases of 

 duplicity in limbs and the other forms of polydactylism confusion 

 can only arise when the nature of the parts is ambiguous. 



As has been stated, in all certain cases of double-limbs the 

 two are compounded by their internal or pra-axial borders, but 

 the case of Macacque No. 50-i was peculiar in the fact that there 

 \\as in it a presumption that the two limbs were not a pair but in 

 Succession. 



In Arthropoda there are a very few cases of true duplicity in 

 appendages comparable with the double-hands. These cases will 

 be dealt with hereafter. 



The fact that a structure naturally hemi-symmetrical, needing the limb of the 

 other side to balance it, may on occasion develop as a complete symmetry is most 

 ]>;u,i lexical, but no other interpretation <>f the facts seems possible. The phenome- 

 non is of course comparable with that observed by DRIESCH in the eggs of J-lchinus, 

 where each half-ovum developed into a whole larva on being separated from the 

 other half-ovum (see p. 3", \'nt<'). It will be shewn that in almost every case in 

 which such un ;i|p|ir:ii.-in<v i^ found iii tlie extra appendages of Insects this appear- 

 ance is misleading, and that the extra parts have a Secondary Symmetry of their 

 own : but no such way through the dith'culty is here open. 



