CHAPTER XX. 

 Supernumerary Appendages in Secondary Symmetry. 



Introductory. — The Evidence as to Insects. 



Of all classes of Meristic variations those consisting in repeti- 

 tion or division of appendages are by far the most complex and 

 the most difficult to bring into system. There is besides no 

 animal which normally presents the condition seen in the varia- 

 tions about to be described, though there may be a true analogy 

 between them and phenomena found in colonial forms. It has 

 nevertheless seemed well to introduce some part of this evidence 

 here for two reasons. First the subject is a necessary continuation 

 of the evidence as to digits, which would otherwise be left incom- 

 plete ; secondly it will be shewn that though many of the cases are 

 irregular and follow no system that can be detected, there remain 

 a large number of cases (being, indeed, the great majority of those 

 that have been well studied) whose form-relations can be put in 

 terms of a simple system of Symmetry. Thus not only are we 

 introduced to a very remarkable property of living bodies, but also 

 the way of future students of Variation may be cleared of a mass 

 of tangled facts that have long been an obstacle ; for on apprehen- 

 sion of the system referred to it will be seen that cases of repeti- 

 tion in Secondary Symmetry are distinct from those of true 

 Variation within the Primary Symmetry and may thus be set 

 apart. 



Arrangement of evidence as to Repetition of Appendages. 



In the first instance I shall give the evidence as to Secondary 

 Symmetries in Insects and Crustacea, prefacing it with a prelimin- 

 ary account of the system of Symmetry obeyed by those cases 

 which I shall call regular, and explaining the scheme of nomen- 

 clature adopted. Besides the regular cases of extra parts in 

 Secondary Symmetry there are many irregular examples which 

 cannot be shewn to conform to the system set forth. Of all but a 

 few of these, details are not accessible, and of the rest many are 



