A NEW WOKKING 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 
