404 MERISTIC VARIATION. [part I. 



but its own external parts may more or less balance its own inter- 

 nal parts. This relation differs greatly in different animals, the 

 Minor Symmetry being nearly complete in the Artiodactyles and 

 in the Horse, but much less so in the human manus and pes, &c. 

 The matter now for consideration is the influence or consequences 

 of the existence of this symmetry in the Meristic Variation of 

 digits ; and conversely the light which the observed phenomena of 

 Variation throw on the nature of that relation of symmetry. It 

 will be seen that in some points the two halves of a bilaterally 

 symmetrical limb behave just as do the two halves of the bilaterally 

 symmetrical trunk, while in other points their manner of Varia- 

 tion is different. 



Thus, the digit III of the Horse may divide into two halves 

 related to each other as images, bearing hoofs flattened on their 

 adjacent edges ; that is to say, the two resulting parts are formed 

 not as copies of the undivided digit, but as halves of it, a condition 

 never seen in division occurring anywhere but in the middle line of 

 a bilateral Symmetry. 



In the syndactyle feet of the Pig or the Ox the converse pheno- 

 menon exists ; for the digits III and IV, which normally stand 

 as images of each other, are here wholly or in part compounded to 

 form a digit to which the uncompounded digits are related as 

 halves. 



Thus far the connexion between the geometrical relations of 

 the digits and the modes of their Variation is clear and simple, 

 and does not differ from that maintained in the Major Symmetry. 

 But in proceeding further there is difficulty. 



If, for instance, the manus or pes of a Horse possesses within 

 itself the properties of a bilateral Symmetry, then the splint-bone 

 II may be supposed to be in symmetry with the similar bone IV. 

 It would therefore be expected that on the occasion of the develop- 

 ment of II to be a full digit, the splint-bone IV would at least not 

 unfrequently develop, thus exhibiting that similarity and simul- 

 taneity of Variation which we have learnt to expect from parts in 

 symmetry with each other. Nevertheless such an occurrence 

 seems to be extremely rare. Then arises a further question: if 

 the digit II develop simultaneously, say in the two fore feet, would 

 the mechanical conditions of which Symmetry is the outward 

 expression be satisfied without a corresponding change in the 

 digit IV of the fore feet ? Is the frequent absence of symmetry 

 in the variation of the halves of the Minor Symmetry in any way 

 connected with the possibility that the two Minor Symmetries 

 together may be maintaining their relations to each other as parts 

 of a Major Symmetry ? Of course as to this we know nothing, but 

 the existence of this double relation should be remembered. 



In several other phenomena of digital Variation the influence 

 of Symmetry is to be suspected. Reference may first be made to 

 the series of changes seen in the Cat's hind foot in correlation with 



