CH. IX] DIVERGENCE OF VARIATION 85 



A. Ovules on both sides of ventral nerve; follicle — 



(1) Outer integument of ovule Sub-fam. I. Paeonieae 



longer. 



(2) Outer integument of ovule Sub-fam. II. Helleboreae 



not longer. 



B. Ovule solitary at base ventral Sub-fam. III. Anemoneae 



nerve; achene. 



As Paeonia is comparatively small, it is extremely probable that 

 it is much vouno-er than the Helleboreae, which include three of 

 the first seven very large genera; and this is confirmed by its 

 small distribution as compared with them. 



It is clear that if we suppose the big genera of a family to be 

 the first formed, and that by the most divergent variation that 

 (on the whole) occurs in the family, whilst the intermediate and 

 smaller genera are younger, we can get a satisfactory picture of 

 what seems to have gone on. The big genera, formed by early and 

 divergent variation, mark out the outer limits (or nearly so) of 

 the familv, the intermediate and small ones, which are on the 

 whole the younger, coming later and filling in the outline thus 

 made. In the later stages of the family, the divergences tend to 

 become smaller and smaller, especially as the possibilities of large 

 divergences have become somewhat used up. At each stage the 

 divergence is probably limited by what has already occurred, and 

 with comparatively few exceptions keeps within the limits thus 

 marked out. If, as in Annonaceae, the commencing mutation, 

 which gave rise to the family, includes a berry fruit, then this may 

 be a family character; if, as in Myrtaceae, it is produced in the 

 second mutation, the berry may characterise the sub-family 

 resulting from that. It may even be produced in later and later 

 mutations, and be the mark only of a tribe, a sub-tribe, a group 

 of genera, a single genus, or it may even mark only some of the 

 species in a genus. 



The key to a family, if well constructed, in all likelihood gives 

 a clue to the mutations by which that family evolved into its 

 present condition. But one must remember that while a group of 

 the largest genera will doubtless be older than a similar group of 

 smaller ones in the same family, those that are actually largest, 

 or those that are the most widely distributed, need not necessarily 

 be the oldest, for there are so many accidents that may befall 

 plants in the shape of geological and other changes. Once a genus 

 becomes so large and important that it has many species and 



