90 M ERISTIC VARIATION. [PART I. 



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

 'I'll-- t-vidt-nce in question relates first to abnormal repetition of 

 liinl>- MI- other peripheral structures, (which in the normal form are 



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

 normal n-pftitions occurring in such a way as to lie outside this 

 /".///oil xi/sti-in of Symmetry and unbalanced by any parts within 

 1 phenomenon occurs in many forms, especially in bilateral 

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

 -npernunirrary limbs in Insects and in supernumerary chel* in 

 < 'iabs and Lobsters. It will be shewn that such extra parts 



rally, it' not always, make up a Secondary system of 

 Symmetry in themselves ; and the way in which such a 



Hilary system is related to the normal or Primary system 

 of Symmetry of the body from which they spring, constitutes 

 an iiiMrnrthv chapter in the study of Meristic Variation. 



M"H- .-.\ tensive repetitions of this class, when affecting the 

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

 Tri|>k Mi.n-ti-rs. which, as has often been said, reproduce in the 

 higher animals phenomena which, under the name of fission, 

 are commonly seen in th<- lower forms. The general evidence as 

 to tln-M- abnormalities is so accessible and familiar that it need 



!> di-taili-d ln-iv, and it will therefore be enough to give an 

 iH- of ii> chief fi'atuivs and to point out the bearing of this 

 la - of rvid.'iiri- mi tlii- -ul)jf<-t of Meristic Variation in general. 



