88 MERISTIC VARIATION. [part I. 



eye-spots of some Satyrid butterflies, &c, are each in themselves 

 nearly symmetrical. To these separate systems of Symmetry the 

 term Minor Symmetry will be applied. Minor Symmetries may 

 or may not be compounded into a Major Symmetry. Between 

 these there is of course no hard and fast line. 



In each class of Symmetry, Meristic Repetition may occur, and 

 the repeated parts then stand in either 



I. Linear or Successive Series. "j p 



II. Bilateral or Paired Series. / >»aa* \\ 



III. Radial Series. S ^ V 

 Parts meristically repeated may thus stand in one or more 



geometrical relations to each other, and the first part of the 

 evidence of Meristic Variation will be arranged in groups according 

 as it is in one or other of these relations that the parts are affected. 

 In each group cases affecting Major Symmetry will be given first, 

 and those affecting Minor Symmetries will be taken after. 



As it is proposed to arrange the facts of Meristic Variation in 

 groups corresponding with these three forms of Meristic Repetition, 

 it will be useful to consider briefly the nature of the relation in 

 which the members of such series stand to each other, and the 

 characters distinguishing the several kinds of series. Reduced to 

 the simplest terms, the distinction may be thus expressed. 



In the Linear or Successive series the adjacent parts of any 

 tivo consecutive members of the series are not homologous, but the 

 severally homologous parts of each member or segment form a 

 successive seines, alternating with each other. For example, the 

 anterior and posterior surfaces of such a series of segments may 

 be represented by the series 



^i , AP, AP, AP, P. 



The relation of any pair of organs in Bilateral Symmetry differs 

 from this, for in that case each member of the pair presents to its 

 fellow of the opposite side parts homologous with those which its 

 fellow presents to it, each being, in structure and position, an 

 optical image of the other. The external and internal surfaces of 

 such a pair may therefore be represented thus : 



E 1,1 E. 



If the manner of origin of these two kinds of Repetition be 

 considered, it will be seen that though both result from a process 

 of Division, yet the manner of Division in the two cases is very 

 different. For in the case of division to form a paired structure, 

 the process occurs in such a way as to form a pair of images, 

 of which similar and homologous parts lie on each side of the 

 plane of division ; while, in the formation of a chain of successive 

 segments, each plane of division passes between parts which are 

 dissimilar, and whose homology is alternate. The distinction 

 between these two kinds of Division is of course an expression of 

 the fact that the attractions and repulsions from which Division 



