Many of the published eulogies to the recently departed Francis Crick, mine included, have compared him to other scientific greats, such as Charles Darwin. I discovered only recently that Charles Darwin's last publication was, in effect, a joint publication with Francis Crick's grandfather.

On 18 February 1882, Walter Drawbridge Crick, an amateur malacologist and professional shoe manufacturer living in Northampton, wrote to Darwin to say that he had found a small freshwater cockle attached to the leg of a water beetle. Darwin was always interested in how freshwater animals, and molluscs in particular, dispersed by hitch-hiking on other animals.

The issue mattered because freshwater invertebrates vary surprisingly little from one region of the world to another. This could mean either that the diffusion of freshwater shells “took place before the present distribution of land and water”, as suggested by John Gwyn Jeffreys in his British Conchology (van Voorst, London, 1862–1869) — or that there is frequent dispersal and population mixing, as argued by Darwin.

Crick's grandfather guessed rightly that Darwin would be interested in the water beetle's passenger. Darwin replied with a barrage of questions. Crick sent him the beetle and the shell, both of which survived the journey. Darwin sent the shell to Gwyn Jeffreys for identification, but Gwyn Jeffreys was away from home and the shell was returned by a servant, broken. Crick, who knew his molluscs, had already identified it as Sphaerium corneum, which Darwin knew by its synonym of Cyclas cornea. Crick, meanwhile, had returned to the pond where he caught the beetle and found a dead frog with a bivalve of the same species clamped to its foot.

On 6 April 1882, Darwin's note “on the dispersal of freshwater bivalves” appeared in Nature. As well as describing Crick's peripatetic cockles, Darwin recalled the extraordinary fact that he had caught a freshwater beetle (of a different genus) while on HMS Beagle, 45 miles from land.

Thirteen days later, Darwin died. Crick died in 1903 at the age of 46, fifty years before his grandson co-discovered a cosmopolitan, universal code shared by all living creatures.