Charles Darwin, whose 200th birthday the world has just celebrated, may have been the last complete biologist. His research interests spanned the entirety of the life sciences as they were known in his day, from his "Monograph on the Subclass Cirripedia" (1851) through his "Descent of Man" (1871) to "The Power of Movement in Plants" (1880). Darwin’s first publication (1839) was a ripping-good adventure story featuring "atmo-spheric dust with infusoria (1)." And his last publication, a four-paragraph communication appearing in 1882 just two weeks before his death, was a work of freshwater malacology (2).
In fact, Darwin first touched on freshwater mollusks in his (1859) "Origin of Species." Early in the chapter he entitled "Geographical Distribution – Continued," Darwin observed, "Some species of fresh-water shells have very wide ranges, and allied species which, on our theory, are descended from a common parent, and must have proceeded from a single source, prevail throughout the world. Their distribution at first perplexed me much." But Darwin then went on to relay a number of anecdotes regarding the attachment of juvenile freshwater mollusks to the feet and feathers of waterfowl, concluding his lengthy paragraph with "Sir Charles Lyell informs me that a Dytiscus (3) has been caught with an Ancylus firmly adhering to it."
Darwin’s fascination with dispersal and biogeography brought him back to the subject of freshwater malacology again in 1878 (4), with a charming anecdote about a surprisingly large unionid mussel found attached to the toe of a duck shot in Danversport, Massachusetts (figure at left). And it reached full flower in 1882, with his "On the Dispersal of Freshwater Bivalves (5)."
Darwin opened this, the last paper he would publish before his death, with "The wide distribution of the same species, and of closely-allied species of freshwater shells must have surprised every one who has attended to this subject." After reviewing his observations of 1859 and 1878, Darwin wrote, "I am now able to add, through the kindness of Mr. W. D. Crick, of Northampton, another and different case. On February 18 of the present year, he caught a female Dytiscus marginalis, with a shell of Cyclas cornea (6) clinging to the tarsus of its middle leg." Darwin went on to relay additional data about this now most illustrious of all fingernail clams, which was large (0.45 inch), viable (dropping from the bug only after five days) and fertile (bearing two juveniles). He then added several anecdotes about other individual sphaeriids found attached to the digits of amphibians, and finished with the charming observation that "my son Francis, while fishing in the sea off the shores of North Wales, noticed that mussels were several times brought up by the point of the hook."
Darwin concluded his 1882 work, "there can, I think, be no doubt that living bivalve shells must often be carried from pond to pond, and by the aid of birds occasionally even to great distances." This point may seem a bit trivial to us today, perhaps even quaint (7). But Darwin’s central thesis, that all these creatures have diverged from a single common ancestor, required that they have originated at a single point, and dispersed throughout the world. If a convincing case could be built for freshwater mollusks, surely to be ranked among the most disadvantaged of the world’s dispersers, perhaps the remainder of the worldwide biota might fall into line.
There’s an interesting postscript to the story of Charles Darwin’s career as a freshwater malacologist. The "Mr. W. D. Crick of Northampton" who sent Darwin his report of the fingernail clam pinched on the water bug leg was Walter Drawbridge Crick (1857-1903), the grandfather of Francis H. C. Crick, who (with James Watson & Maurice Wilkins) shared the 1962 Nobel Prize for elucidating the structure of DNA (8).
At the outset of this essay, I characterized Charles Darwin as “the last complete biologist.” Chief among the reasons that there can be no more such protean figures must be the 20th century explosion of molecular biology, which has expanded our discipline in directions Darwin could never have imagined. It is a source of some inspiration to me that one can trace a path from Darwin to DNA through the great man’s last paper, and the humble discipline of freshwater malacology.
(1) Yes, Chapter 1 of Darwin’s “Voyage of the Beagle” included a passing note about “infusoria” (primarily diatom frustules) in dust accumulated while on shipboard.
(2) I’ve taken a bit of license with this paragraph. Darwin had a couple publishing credits prior to his (1839) “Voyage,” and several posthumous papers after his 6 April 1882 paper on freshwater bivalve dispersal. Darwin’s complete bibliography, including PDF downloads of the papers mentioned here, is available at Darwin Online.
(3) Dytiscus is a genus of large, predatory water bugs. Although spending the great majority of their lives swimming gracefully through the water column, they may on occasion take to the wing, flying like balsa-wood airplanes with old rubber bands.
(4) Darwin, C. (1878) Transplantation of shells. Nature 18: 120-121.
(5) Darwin, C. (1882) On the dispersal of freshwater bivalves. Nature 25:529-530.
(6) The genus Cyclas has since been synonymized under Sphaerium. Today this common European "fingernail clam" is generally referred to as Sphaerium corneum.
(7) It is not, however. See the classic paper:W. J. Rees (1965) The Aerial Dispersal of Mollusca, Proc. Malac. Soc. London 36: 269 - 282.
(8) We must acknowledge an article in the February 2009 issue of National Geographic for calling our attention to this remarkable coincidence: Ridley, M. (2009) Modern Darwins. National Geographic 215: 56 - 73.