448 THE UNFINISHED BOOK. [1857. 



C. Darwin to /. 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 per- 

 plexing. 



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 sea 

 — that plants are smaller and more hairy and with brighter 

 flowers on mountains : now in all such, and other cases, dis- 

 tinct 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 the 

 same laws as recognised and admitted varieties. I mention 

 all this on account of the variation of plants in ascending 

 mountains ; I have quoted the foregoing remark only gener- 

 ally with no examples, for I add, there is so much doubt and 

 dispute what to call varieties ; but yet I have stumbled on so 

 many casual remarks on varieties of plants on mountains 

 being so characterised, that I presume there is some truth in 

 it. What think you ? Do you believe there is any tendency 

 in varieties^ as generally so called, of plants to become more 



