424 GROWTH OF THE 'ORIGIN.* [1855. 



have "close species," and for other purposes too vague to 

 enumerate. I have attempted, by Hooker's help, to ascertain 

 in a similar way whether the different species of the same 

 genera in distant quarters of the globe are variable or present 

 varieties. The definition I should give of a " close species " 

 was one that you thought specifically distinct, but which you 

 could conceive some other good botanist might think only a 

 race or variety; or, again, a species that you had trouble, 

 though having opportunities of knowing it well, in discrimi- 

 nating from some other species. Supposing that you were 

 inclined to be so very kind as to do this, and could (which I 

 do not expect) spare the time, as I have said, a mere cross 

 to each such species in any useless proof-sheets would give 

 me the information desired, which, I may add, I know must 

 be vague. 



How can I apologise enough for all my presumption and 

 the extreme length of this letter? The great good nature of 

 your letter to me has been partly the cause, so that, as is too 

 often the case in this world, you are punished for your good 

 deeds. With hearty thanks, believe me, 



Yours very truly and gratefully, 



Ch. Darwin. 



C. Dana in to J. D. Hooker. 



Down, 18th [July, 1855]. 

 ... I think I am getting a mild case about Charlock 

 seed; * but just as about salting, ill luck to it, I cannot 

 remember how many years you would allow that Charlock 



* In the Gardeners Chronicle, 1855, p. 758, appeared a notice (half a 

 column in length) by my father on the " Vitality of Seeds." The facts 

 related refer to the " Sand-walk " ; the wood was planted in 1846 on a 

 piece of pasture land laid down as grass in 1840. In 1855, on the soil 

 being dug in several places, Charlock (Brassica sinapistrum) sprang up 

 freely. The subject continued to interest him, and I find a note dated 

 July 2nd, 1874, in which my father recorded that forty-six plants of Char- 

 lock sprang up in that year over a space (14 x 7 feet) which had been dug 

 to a considerable depth. 



