90 THE UNFINISHED BOOK. [1857. 



curious relation in itself, and is very much so, if my theory 

 and explanation are correct.* 



With hearty thanks, your most troublesome friend, 



C. Darwin. 



C. Darwin to J. D. Hooker. 



Down, April 12th [1857]. 



My DEAR HOOKER, Your letter has pleased me much, 

 for I never can get it out of my head, that I take unfair 

 advantage of your kindness, as I receive all and give nothing. 

 What a splendid discussion you could write on the whole 

 subject of variation ! The cases discussed in your last note 

 are valuable to me (though odious and damnable), as showing 

 how profoundly ignorant we are on the causes of variation. 

 I shall just allude to these cases, as a sort of sub-division 

 of polymorphism a little more definite, I fancy, than the 

 variation of, for instance, the Rubi, and equally or more 

 perplexing. 



I have just been putting my notes together on variations 

 apparently due to the immediate and direct action of external 

 causes ; and I have been struck with one result. The most 

 firm sticklers for independent creation admit, that the fur of 

 the same species is thinner towards the south of the range of 

 the same species than to the north that the same shells are 

 brighter-coloured to the south than north ; that the same 

 [shell] is paler-coloured in deep water that insects are 

 smaller and darker on mountains more livid and testaceous 

 near the sea that plants are smaller and more hairy and with 

 brighter flowers on mountains : now in all such, and other 

 cases, distinct species in the two zones follow the same rule, 

 which seems to me to be most simply explained by species, 

 being only strongly marked varieties, and therefore following 



* See ' Origin,' ed. i. p. 100. 



