A NEW WOEKING HYPOTHESIS 483 



types, i.e. of species, my object being to account for existing 

 distribution. 



These hypotheses square with all my facts, for from 

 them you would expect to find : — 



I. That, as regards extent of variation, all existing 

 plants are made up of two classes or assemblages, (1) A large 

 number of species so distinct from one another that no one 

 doubts their constancy or disputes their limits, and which 

 we cannot connect with others or with one another except 

 by intercalation of an immense series of intermediate forms 

 that do not now exist. (2) Of a vast assemblage that range 

 themselves in clusters of variable forms so slightly distin- 

 guished that no two Botanists agree as to their limits, and 

 any one admits that one, or a few, small characters alone 

 distinguishes each from its allies. 



II. That, as regards rate of variation, some forms have 

 remained specifically unchanged from the Oolite downwards, 

 others only generically, whilst others are more changed still. 



III. That Australian forms are found only in the old 

 rocks of Britain, 



IV. That the Floras of sinking (Volcanic) islands contain 

 a larger proportion of distinct types than those of continents. 



V. That some of those types are not at all represented 

 on the continents, others only on the nearest continents. 



VI. That the further the island is from the continent 

 the greater is the peculiarity of its Flora. 



VII. That the number and variety of ordinal types is 

 as great in the S. temperate Zone, where there is so little 

 land, as in the North. 



The numbers and proportions of orders (and numbers 

 of genera too ?) remaining the same in both. This I can 

 understand if you will allow me in the South as large and 

 varied an available surface as Europe, Asia, and America 

 present ; for if you were to destroy 2/3 of Europe, N. Asia, 

 and America you would not reduce materially the number of 

 genera, nor of orders at all, but a vast number of species 

 would be destroyed. 



To Harvey 



March 1859. 



I am delighted to hear of the progress in Thesaurus and 

 Flora [of the Cape]. You are a brave man indeed. I am 



