CHAPTER XIII. 



LINEAR SERIES continued. 



MINOR SYMMETRIES : DIGITS. 



ALL the cases considered in the foregoing chapters have il- 

 lustrated Variation of parts whose repetition is disposed in Linear 

 Series along the chief axis of the body, being thus arranged 

 directly and immediately with reference to the Major Symmetry 

 of the body. We have now to consider cases of the Meristic 

 Variation of parts which are also repeated in Linear Series but 

 normally possess in some degree the property of symmetry partially 

 completed within the limits of their own series, thus forming a 

 Minor Symmetry. 



Of Linear repetitions thus occurring there is a great diversity, 

 and evidence will here be produced regarding two of the chief 

 examples, namely, the digits of vertebrates and the segmentation 

 of antennaB and tarsi of Insects. 



In each of these groups of organs the parts are frequently 

 formed in such a way as to make an approach to symmetry, 

 about one or more axes within the limits of the appendage to 

 which they belong. This fact will be found to lead to conse- 

 quences apparent in the manner in which numerical Variation 

 takes place in limbs of the various types. 



In these Minor Symmetries Linear Repetition may occur in 

 two forms : there may be repetitions of digits or other parts in 

 lines forming an angle with the axis of an appendage ; and there 

 may be repetitions in the form of joints &c. along the axis of the 

 appendage itself. 



The cases of Variation in number of joints in the appendages 

 of Insects are chiefly interesting as examples of manifest Dis- 

 continuity in Variation, and from the conclusions which they 

 suggest as to the supposed individuality of segments. This latter 

 question arises also in considering the relation of the two pha- 

 langes of the pollex and hallux to the three phalanges of the 

 other digits, but the evidence which can be gained from a study 



