To P. H. Gosse   27 April [1857]¹

My dear Sir

I have thought that perhaps in course of summer you would have an opportunity & would be so very kind as to try a little experiment for me.—² I think I can tell best what I want, by telling what I have done. The wide distribution of same species of F. Water Molluscs has long been a great perplexity to me: I have just lately hatched a lot & it occurred to me that when first born they might perhaps have not acquired phytophagous habits, & might perhaps like nibbling at a Ducks-foot.— Whether this is so I do not know, & indeed do not believe it is so, but I found when there were many very young Molluscs in a small vessel with aquatic plants, amongst which I placed a dried Ducks foot, that the little barely visible shells often crawled over it, & that they adhered so firmly that they c^d. not be shaken off, & that the foot being kept out of water in a damp atmosphere, the little Molluscs survived well 10, 12 & 15 hours & a few even 24 hours.—³ And thus, I believe, it must be that Fr. W. shells get from pond to pond & even to islands out at sea. A Heron fishing for instance, & then startled might well on a rainy day carry a young mollusc for a long distance.—

Now you will remember that E. Forbes argues chiefly from the difficulty of imagining how littoral sea-molluscs could cross tracts of open ocean, that islands, such as Madeira must have been joined by continuous land to Europe:⁴ which seems to me, for many reasons, very rash reasoning.— Now what I want to beg of you, is, that you would try an analogous experiment with some sea-molluscs, especially any strictly littoral species,—hatching them in numbers in a smallish vessel & seeing whether, either in larval or young shell state they can adhere to a birds foot & survive say 10 hours in damp atmosphere out of water. It may seem a trifling experiment, but seeing what enormous conclusions poor Forbes drew from his belief that he knew all means of distribution of sea-animals, it seems to me worth trying.—⁵

My health has lately been very indifferent, & I have come here for a fortnight’s water-cure.—⁶

I owe to using your name a most kind & most valuable correspondent, in M^r Hill of Spanish-Town.—⁷

I hope you will forgive my troubling you on the above point & believe me, | My dear Sir | Your’s very sincerely | Ch. Darwin

P.S. | Can you tell me, you who have so watched all sea-creatures, whether male Crustaceans ever fight for the females: is the female sex in the sea, like on the land, “teterrima belli causa”?⁸

I beg you not to answer this letter, without you can & will be so kind as to tell about Crustacean Battles, if such there be.—