28 GEOGRAPHICAL DISTRIBUTION [Chap. VII 



Letter 399 I quite agree about Wallace's position on the ocean and 

 continent question. 



To return to geographical distribution : As far as I know, 

 no one ever discussed the meaning of the relation between 

 representative species before I did, and, as I suppose, Wallace 

 did in his paper before the Linnean Society. Von Buch's is 

 the nearest approach to such discussion known to me. 



Letter 400 To W. D. Crick. 



The following letters are interesting not only for their own sake, but 

 because they tell the history of the last of Mr. Darwin's publications — his 

 letter \.o Nature on the "Dispersal of Freshwater Bivalves," April 6th, 1882. 



Down, Feb. 21st, 1882. 



Your fact is an interesting one, and I am very much 

 obliged to you for communicating it to me. You speak a 

 little doubtfully about the name of the shell, and it would be 

 indispensable to have this ascertained with certainty. Do 

 you know any good conchologist in Northampton who could 

 name it ? If so I should be much obliged if you would inform 

 me of the result. 



Also the length and breadth of the shell, and how much 

 of leg (which leg ?) of the Dytiscus [a large water-beetle] has 

 been caught. If you cannot get the shell named I could take 

 it to the British Museum when I next go to London ; but 

 this probably will not occur for about six weeks, and you may 

 object to lend the specimen for so long a time. 



I am inclined to think that the case would be worth 

 communicating to Nature. 



P.S. — I suppose that the animal in the shell must have 

 been alive when the Dytiscus was captured, otherwise the 

 adductor muscle of the shell would have relaxed and the shell 

 dropped off. 



Letter 401 To W. D. Crick. 



Down, Feb. 25th, 1882. 

 I am much obliged for your clear and distinct answers to 

 my questions. I am sorry to trouble you, but there is one 

 point which I do not fully understand. Did the shell rertiain 

 attached to the beetle's leg from the 18th to the 23rd, and 

 was the beetle kept during this time in the air ? 



Do I understand rightly that after the shell had dropped 



