CH. IX] DIVERGENCE OF VARIATION 87 



mutation that produced Silene itself. All later mutations come 

 within it, in the sense that the effects of this first mutation are 

 shown in them all. If we now follow only the tribe Lvchnideae, 

 Pax's key next splits off, by triple (or more probably by two 

 separate) divergences two genera, Ciicubalus, with one species in 

 Eurasia, and with berry fruit, and Drypis, with one species in 

 south-east Europe, and with capsule with lid; but as these are 

 small and rather local genera, and could evidently be split off 

 from any genus with a capsule, it is unlikely that they were 

 formed at this early stage. The next division in the key is more 

 probably that which split off Melandrium with eighty species in 

 the northern hemisphere. South Africa, and South America, 

 which differs from Silene by its fully unilocular capsule as against 

 a capsule multilocular at the base. In view of the great dispersal 

 of Melandrium, it is by no means improbable that it may have 

 been formed even earlier than Dianthns, and having met with 

 greater vicissitudes, such as the separation of Old and New 

 Worlds, has lost many more species than either Silene or Dian- 

 thus. In both Melandrium and Silene the capsule has two teeth 

 to a carpel, and each has a closely related genus with one tooth 

 per carpel, which was probably split off later {Viscaria near 

 Silene, Lychnis near Melandrium). Further mutations might give 

 the two genera Uebelinia and Agrostemma near to Melandrium, 

 by changing the relative position of carpels and calyx segments, 

 which are opposite in Melandrium and alternate in the two small 

 genera — a change which could only come by some mutation. They 

 might also give Heliosperma as a mutation from Melandrium, it 

 having only two rows of papillae on the seed, instead of having 

 them all over, and Petrocoptis as a mutation from Lychnis, the 

 latter having the teeth of the carpel twice as many as the styles, 

 the former once. It will be noticed that this phenomenon appears 

 (in Silenoideae-Lychnideae) in two places, and must have 

 appeared independently in these two, though the morphology 

 or the structural features are the same in each case. 



