CHAPTER XVIII. 

 Bilateral series. 



Of the organs repeated in Linear Series whose variations have 

 been illustrated, many are bilaterally repeated also ; but thus far we 

 have considered them only in their relations as members of Linear 

 Series. It now remains to examine the variations which they 

 exhibit in virtue of their relation to each other as members of 

 a Bilateral Series. 



Meristic Variation in this respect is manifested in two ways. 

 A normally unpaired organ standing in the middle line of a bi- 

 lateral symmetry may divide into two so as to form a pair of 

 organs ; and conversely, a pair of organs normally placed apart 

 from each other on either side of a middle line may be com- 

 pounded together so as to form a single organ in the middle 

 line. 



In animals and plants nothing is more common than for 

 different forms to be distinguished from each other by the fact 

 that an organ standing in the middle line of one is in another 

 represented by two organs, one on either side. The facility there- 

 fore with which each of these two conditions may arise from the 

 other by discontinuous Variation is of considerable importance. 



Admirable instances of the bearing of this class of evidence upon 

 the question of the origin of Species are to be seen in zygomorphic 

 flowers. Veronica for example differs from the other Scrophulariacese 

 especially in the fact that it has only one posterior petal, instead of 

 two posterior petals one on each side of a middle line. But there is 

 evidence not only that forms having normally two posterior petals 

 may as a discontinuous variation have only one such petal, placed in 

 the middle line, but also that the single posterior petal of Veronica may 

 as a variation be completely divided into two. Similarly the single 

 anterior petal of Veronica may also as a variation be divided into two, 

 thus giving three posterior and two anterior petals as in for example 

 Salpiglossis 1 . In these cases, which might be indefinitely multiplied, 



1 An account of several discontinuous variations in the structure of zygomorphic 

 corollas was given by Miss A. Bateson and myself. Jour. Linn. Soc, 1892, xxvni., 

 Botany, p. 386. 



