222 SPREAD OF EVOLUTION. [1865. 



send lately by post a paper on climbing plants, as an experi- 

 ment to see whether it would reach you. One of the points 

 which has struck me most in your paper is that on the dififer- 

 ences in the air-breathing apparatus of the several forms. 

 This subject appeared to me very important when I formerly 

 considered the electric apparatus of fishes. Your observa- 

 tions on Classification and Embryology seem to me very good 

 and original. They show what a wonderful field there is for 

 enquiry on the development of Crustacea, and nothing has 

 convinced me so plainly what admirable results we shall ar- 

 rive at in Natural History in the course of a few years. What 

 a marvellous range of structure the Crustacea present, and 

 how well adapted they are for your enquiry ! Until reading 

 your book I knew nothing of the Rhizocephala ; pray look at 

 my account and figures of Anelasma, for it seems to me that 

 this latter cirripede is a beautiful connecting link with the 

 Rhizocephala. 



If ever you have any opportunity, as you are so skilful a 

 dissector, I much wish that you would look to the orifice at 

 the base of the first pair of cirrhi in cirripedes, and at the 

 curious organ in it, and discover what its nature is; I suppose 

 I was quite in error, yet I cannot feel fully satisfied at 

 Krohn's * observations. Also if you ever find any species of 

 Scalpellum, pray look for complemental males ; a German 

 author has recently doubted my observations for no reason 

 except that the facts appeared to him so strange. 



Permit me again to thank you cordially for the pleasure 

 which I have derived from your work and to express my sin- 

 cere admiration for your valuable researches. 



Believe me, dear Sir, with sincere respect. 



Yours very faithfully, 



Ch. Darwin. 



P.S. — I do not know whether you care at all about plants, 

 but if so, I should much like to send you my little work on 



* See vol. ii., pp. 138, 187. 



