CHAPTER XVI. 

 Radial Series. 



Little need be said in preface to the facts of Meristic Varia- 

 tion in Radial Series. In them phenomena analogous to those 

 of the Variation in Linear Series are seen in their simplest form. 

 Just as in Linear Series the number of members may be changed 

 by a reconstitution of the whole series so that it is impossible 

 to point to any one member as the one lost or added, so may 

 it be in the Meristic Variation of Radial Series: and again as 

 in Linear Series, single members of the series may divide. Be- 

 tween these there is no clear line of distinction. 



Next, as in Linear Series, Variation, whether Meristic or 

 Substantive, may take place either in single segments (quadrants, 

 sixths, «fec), or simultaneously in all the segments of the body. 

 For instance, a single eye may be divided into two, or there 

 may be duplicity simultaneously occurring in all the eyes of the 

 disc (see No. 634) and so on. 



These phenomena are here illustrated by facts as to the 

 Meristic Variation of Hydromedusas and of Aurelia. The latter 

 is exceptionally variable and in its changes exhibits important 

 features. 



Together with these facts as to Variation in Major Symmetries 

 is given an instance of similar Variation in the pedicellariae of an 

 Echinid, and it will be seen that in this case of a Minor Symmetry 

 the change is perfect and altogether comparable with those found 

 in Major Symmetries of similar geometrical configuration. 



The best field for the study of the variations of Radial Series 

 is of course to be found in plants ; and in the Meristic Variations 

 of radially symmetrical flowers precisely similar phenomena may 

 be easily seen. 



