458 LETTEKS TO DAEWIN, 1843-1859 
In the early summer, Hooker had read in MS. Darwin's 
discussion of ' what to call varieties.' Cheered by hie criticism, 
Darwin subsequently sent for further criticism what he had to 
say about genera, in the discussion of ' the " Principle of Diver- 
gence," which with " Natural Selection " is the keystone of 
my book.' 
Kew : July 13, 1858. 
I went deep into your MS. on variable species in big and 
small genera and tabulated Bentham after a fashion, but 
not very carefully. After very full deliberation I cordially 
concur in your view and accept it with all its consequences. 
Bentham's book confirms you, though with modifications. 
The larger genera I believe to be groups of more presently 
variable beings than the small and I think you have quite 
made good your point. Still I w^ould not abandon the argu- 
ments against, for I still think that the disposition or rather 
the necessity of making more book varieties in large genera 
than in small is a very important fact. 
I have also w^ell considered Bentham's Exceptional 
Orders, and am inclined to attribute that also partly to his 
idiosyncrasy ; upon thinking well over his method of writing 
I have often seen that he will make rather hastily a new species 
in a larger genus of which a vast number of good species have 
recently turned up. The mental process is : ' Such and such 
a country teems with Astragalus or Pedicularis (which he has 
himself first elaborated), here is a new province of that country 
just supplied us with a lot of specimens and the chances are 
that heaps of them are new, and that more specimens will 
rather tend to prove doubtful new species to be distinct than 
the contrary.' It is not easy to explain to you how fully I 
appreciate this tendency in another person — but I am con- 
vinced it is so, and that that is the key to the Benthamian 
Exceptional Orders. This does not, however, apply to 
Wed dell's Urticeae which I must tabulate more carefully. 
This was the case when Bentham and I did the Afghanistan 
and Thibetan Astragali and Pediculariae — he pronounced 
many new which I thought varieties, always saying : ' Oh 
that country is the headquarters of Astragali, you must 
expect heaps of novelty.' 
In some passages of your MS. you rather underrate I 
