1 867.] MIMICRY. 71 



What I have just said reminds me to ask you a question. 

 Sir J. Lubbock brought me the other day what appears to be 

 a terrestrial Planaria (the first ever found in the northern 

 hemisphere) 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 Planarise 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 account 

 for the bright colours of the Planariae in reference to sexual 

 selection. By the way, I suppose they are hermaphrodites. 



Do not forget to aid me, if in your power, with answers to 

 any of my questions on expression, for the subject interests 

 me greatly. With cordial thanks for your never-failing kind- 

 ness, believe me, 



Yours very sincerely, 



CHARLES DARWIN. 



C. Darwin to C. Lyell. 



Down, July 18 [1867]. 



MY DEAR LYELL, Many thanks for your long letter. I 

 am sorry to hear that you are in despair about your book ; * 

 I well know that feeling, but am now getting out of the lower 

 depths. I shall be very much pleased, if you can make the 

 least use of my present book, and do not care at all whether 

 it is published before yours. Mine will appear towards the 

 end of November of this year ; you speak of yours as not 

 coming out till November, 1868, which I hope may bean error. 

 There is nothing about Man in my book which can interfere 

 with you, so I will order all the completed clean sheets to be 

 sent (and others as soon as ready) to you, but please observe 

 you will not care for the first volume, which is a mere record 



* The 2nd volume of the loth edit, of the ' Principles.' 



