64 ISOLATION [ch. vii 



mutation to be sudden, and as most of the mountain endemics 

 are well-marked Linnean species, it was perhaps very definite 

 also, the principle of divergence of character coming into 

 operation. 



Another well-known series of facts is that families which are 

 widely distributed chiefly or only in the more broken southern 

 hemisphere, have rarely any genera that cover the whole of their 

 area of distribution. In the plants that are more marked in the 

 northern hemisphere, on the other hand, there is very often a 

 genus that does cover the whole area, even if that also includes 

 the southern hemisphere, such for example as Ranunculus^ 

 Senecio, or Solanu7n. Whether this difference has anything to do 

 with the isolation of so many areas in the south, we do not know, 

 but the fact is suggestive. 



We are still very far indeed from any proper understanding of 

 the operations that have been concerned in evolution, except 

 that natural selection must evidently play a less conspicuous, or 

 at any rate, a less direct part. It looks as if, more especially under 

 certain circumstances such as elevation or isolation, evolution 

 must go on, and this supposition is borne out by such things as 

 the progressive change that shows in such plants as Stratiotes 

 described by Miss Chandler (3), where a whole series of species 

 differing in characters of no conceivable functional importance, 

 have succeeded one another in successive geological horizons. If 

 these changes had been a little more marked, we should have had 

 two or more genera succeeding one another, and this point must 

 always be borne in mind, together with the tendency to diver- 

 gence, in considering extinct genera. 



