458 LETTERS TO DARWIN, 1843-1859 



In the early summer, Hooker had read in MS. Darwin's 

 discussion of ' what to call varieties.' Cheered by his 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 would 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 well 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 



