K>S EVOLUTION [Chap. II 



Letter 63 rule that the small genera vary less than the large. What do 

 you think ? Hypothetically I can conjecture how the Labiatae 

 might fail — namely, if some small divisions of the Order were 

 now coming into importance in the world and varying much 

 and making species. This makes me want to know whether 

 you could divide the Labiata: into a few great natural divi- 

 sions, and then I would tabulate them separately as sub- 

 orders. I see Lindley makes so many divisions that there 

 would not be enough in each for an average. I send the 

 table of the Labiatae for the chance of your being able to 

 do this for me. You might draw oblique lines including and 

 separating both large and small genera. I have also divided 

 all the species into two equal masses, and my rule holds 

 good for all the species in a mass in the six volumes ; but it 

 fails in several (four) large Orders — viz. Labiatae, Scrophu- 

 lariaceae, Acanthaceas, and Proteacea?. But, then, when the 

 species are divided into two almost exactly equal divisions, 

 the divisions with large genera are so very few : for instance, 

 in Solanaceae, Solarium balances all others. In Labiata; seven 

 gigantic genera balance all others (viz. 1 1 3), and in Proteacea: 

 five genera balance all others. Now, according to my 

 hypothetical notions, I am far from supposing that all genera 

 go on increasing for ever, and therefore I am not surprised 

 at this result, when the division is so made that only 

 a very few genera are on one side. But, according to my 

 notions, the sections or sub-genera of the gigantic genera 

 ought to obey my rule {i.e., supposing a gigantic genus had 

 come to its maximum, whatever increase was still going on 

 ought to be going on in the larger sub-genera). Do you think 

 that the sections of the gigantic genera in D.C. Prodromus 

 arc generally natural: i.e. not founded on mere artificial char- 

 acters? If you think that they are generally made as natural 

 as they can be, then I should like very much to tabulate the 

 sub-genera, considering them for the time as good genera. 

 In this case, and if you do not think me unreasonable to ask 

 it, I should be very glad of the loan of Vols. X., XI., XII., and 

 XIV., which include Acanthaces, Scrophulariaceae, Labiata;, 

 and Proteaceae, — that is, the orders which, when divided quite 

 equally, do not accord with my rule, and in which a very few 

 genera balance all the others. 



I have written you a tremendous long prose. 



