402 MERISTIC VARIATION. [part I. 



from the addition of a phalanx to the pollex up to the condition of 

 Nos. 488 or 490, and comparing them with the essentially similar 

 series of cases in the hind foot of the Cat, there is this remarkable 

 difference : that though both progressions lead up to a similar 

 kind of Symmetry in the series of digits, in the human manus 

 an approach is made to a system of Symmetry whose axis lies 

 internal to the index, while in the Cat's feet the axis lies external 

 to the index (see Section (4)). The series of forms in the manus 

 of the Cat is still more peculiar and is not like any case of poly- 

 dactylism in other animals. 



(3) Symmetry in digital Variation. 



From the evidence it will have been seen that digital Varia- 

 tion in most of its manifestations may be similar and simultaneous 

 in the limbs of the two sides of the body, though not rarely it 

 affects the limb of one side only ; and still more frequently the 

 form which it assumes on one side differs in degree from that found 

 on the other side. Considerable difference in kind between Varia- 

 tion on the right side and on the left is much rarer. 



Almost the same statement may be made respecting simul- 

 taneity of Variation between the manus and the pes, though in 

 the pes the manifestation of Variation is rarely identical with that 

 in the manus of the same individual. Some variations, as for 

 instance duplicity of pollex and hallux, or extra digit external to 

 minimus, are not rarely found simultaneously in both pes and 

 manus, but there are many cases in which no such agreement is 

 found. The frequency of this simultaneous variation in the case of 

 syndactylism in the Pig may be specially noticed. 



Certain variations in certain animals seem to be almost or 

 quite restricted either to hind limb or to fore limb. The form 

 taken on by the pes of the Cat upon increase in number of digits 

 is distinct from that assumed by the manus. The development of 

 the digit II in the Horse is much more common in the manus. 

 The extra digit (or pair of digits) in the Pig is so far as I know 

 seen only in the manus. On the contrary the three-toed state in 

 the Ox is found in the manus and also in the pes. Generally 

 speaking, Meristic Variation is much commoner in fore limbs than 

 in hind limbs. 



One fact here calls for special notice. Though general statements 

 are hazardous, we are perhaps justified in affirming the principle 

 that large Meristic Variation, involving great departure from the 

 normal, very rarely affects exclusively one side of a bilaterally sym- 

 metrical body. In cases of variation in vertebrae, in spinal 

 nerves, in teeth, in the oviducts of Astacus, and many more, it is 

 seen that on the occurrence of great variation the change is seldom 

 restricted wholly to one side of the body, though the condition 

 reached by the two sides is frequently of differing degree. Now in 



