86 DIVERGENCE OF VARIATION [ch. ix 



covers great areas, the chances of its complete disappearance, 

 unless mere age, or further (probably universal) mutation can do 

 it, are small. The intermediate genera, on the other hand, may 

 often have suffered complete extinction, and still more the 

 smallest genera. 



What has been said is also strongly supported by the facts of 

 distribution. There can be no doubt that in any given family, the 

 distribution of the genera goes on the whole with their size, as 

 has been shown in Age and Area, chap, xii, p. 113 (Size and 

 Space). Age, size of genus, and area occupied by it, all go 

 together. 



It is clear that this analysis of the Ranunculaceae fully sup- 

 ports the theory of differentiation as against that of natural 

 selection, upon which no prediction can possibly be made as to 

 the size or composition of a family. 



As another example, let us take the sub-family Silenoideae in 

 the Caryophyllaceae. It contains eighteen genera, whose numbers 

 of species, from the latest monograph (35), where the numbers in 

 the large genera are evidently rounded off, are 



400, 300, 90, 80, 30, 30, 25, 10, 8, 7, 5, 5, 4, 4, 4, 1, 1, 1. 



The first two genera, Silene (400) and Dianthus (300), which 

 contain 700 out of the total number of 1005 species in the sub- 

 family, are instantly picked out (supposing these to be the only 

 genera in the group) by the very first dichotomy that is given in 

 the key, which splits the Silenoideae into two tribes. All the 

 Lychnideae, headed by Silene, show a calyx with commissural 

 ribs; the Diantheae, headed by Dianthus, not so. The other 

 Lychnideae contain 80, 10, 8, 7, 5, 5, 4, 1, 1 species, and the other 

 Diantheae show 90, 30, 30, 25, 4, 4, 1, adding up, the one to 121 

 species, the other to 184, or in both cases much fewer than in the 

 big genus at the head of the group (400-121 and 300-184). Each 

 tribe is headed by a big genus, and the one tribe adds up to 521, 

 the other to 484, showing a difference just as indicated in Test 

 Case II, p. 94. The figures seem to indicate that in the Diantheae 

 there were more genera produced of intermediate size, so that 

 perhaps the stimulus of genus formation came earlier, and 

 resulted in the greater number of species shown by the smaller 

 Diantheae than by the smaller Lychnideae. 



As the divergence just considered includes all the Silenoideae 

 on one side or the other, it is not unlikely that it was the first 

 mutation to appear after the first formation of the group by the 



