1867.] FRITZ MULLER. 253 



C. Darwin to F. Mutter. 



Down, July 31 [1867]. 



MY DEAR SIR, I received a week ago your letter of 

 June 2, full as usual of valuable matter and specimens. It 

 arrived at exactly the right time, for I was enabled to give a 

 pretty full abstract of your observations on the plant's own 

 pollen being poisonous. I have inserted this abstract in the 

 proof-sheets in my chapter on sterility, and it forms the most 

 striking part of my whole chapter.* I thank you very sin- 

 cerely for the most interesting observations, which, however, 

 I regret that you did not publish independently. I have been 

 forced to abbreviate one or two parts more than I wished. 

 . . . Your letters always surprise me, from the number of 

 points to which you attend. I wish I could make my letters 

 of any interest to you, for I hardly ever see a naturalist, and 

 live as retired a life as you in Brazil. With respect to mi- 

 metic plants, I remember Hooker many years ago saying he 

 believed that there were many, but I agree with you that 

 it would be most difficult to distinguish between mimetic 

 resemblance and the effects of peculiar conditions. Who 

 can say to which of these causes to attribute the several 

 plants with heath-like foliage at the Cape of Good Hope ? 

 Is it not also a difficulty that quadrupeds appear to recognise 

 plants more by their [scent] than their appearance ? What I 

 have just said reminds me to ask you a question. Sir J. Lub- 

 bock brought me the other day what appears to be a terres- 

 trial Planaria (the first ever found in the northern hem- 

 isphere) and which was coloured exactly like our dark- 

 coloured slugs. Now slugs are not devoured by birds, like 

 the shell-bearing species, and this made me remember that I 

 found the Brazilian Plan arise actually together with striped 

 Vaginuli which I believe were similarly coloured. Can you 

 throw any light on this? I wish to know, because I was 

 puzzled some months ago how it would be possible to ac- 



* In ' The Variation of Animals and Plants.' 



