90 MERISTIC VARIATION. [part I. 



that in any study of this subject reference to it cannot be omitted. 

 The evidence in question relates first to abnormal repetition of 

 limbs or other peripheral structures, (which in the normal form are 

 grouped into and form part of a system of Symmetry,) such ab- 

 normal repetitions occurring in such a way as to lie outside this 

 normal system of Symmetry and unbalanced by any parts within 

 it. This phenomenon occurs in many forms, especially in bilateral 

 animals, and may be exceptionally well studied in the case of 

 supernumerary limbs in Insects and in supernumerary chelae in 

 Crabs and Lobsters. It will be shewn that such extra parts 

 generally, if not always, make up a Secondary system of 

 Symmetry in themselves ; and the way in which such a 

 Secondary system is related to the normal or Primary system 

 of Symmetry of the body from which they spring, constitutes 

 an instructive chapter in the study of Meristic Variation. 



More extensive repetitions of this class, when affecting the 

 axial parts of the body, give rise to the well-known Double and 

 Triple Monsters, which, as has often been said, reproduce in the 

 higher animals phenomena which, under the name of fission, 

 are commonly seen in the lower forms. The general evidence as 

 to these abnormalities is so accessible and familiar that it need 

 not be detailed here, and it will therefore be enough to give an 

 outline of its chief features and to point out the bearing of this 

 class of evidence on the subject of Meristic Variation in general. 



