Charles Darwin was born 200 years ago, on February 12th 1809. Possibly the most famous scientist ever, his fundamental and beautiful theory of evolution has stood the test of time. Darwin formed his theory as a result of his observations during the voyage of the Beagle, but it was many years from then until he actually published his ideas.
It is less well-known that, in the same voyage, Charles Darwin produced another theory of evolution, not of life but of coral reefs. The formation of coral reefs was a matter of some debate at that time, in fact, one of the mission objectives for the Beagle voyage was to study their nature. Darwin’s theory was published in his book The Structure and Distribution of Coral Reefs in May 1842, some 6 years after the Beagle came home, and 17 years ahead of his more famous work, On the Origin of Species.
Darwin’s theory of coral reef formation is all the more remarkable for having been thought out completely before he had ever seen a reef. In his own words:
Such a statement could imply that Darwin was biased in his views, and it would be natural to believe that he sought only such evidence as supported his theory. That this is not the case becomes apparent on reading the book. Darwin was a meticulous and thorough observer, seeking as much information as he could from a wide variety of sources. The appendix of his book, fully one third as long as the main text, contains a detailed description of a large number of coral reefs, all of which he examined in the light of his theory. The book also includes a map of the world, showing the locations of all these reefs.
Darwin accumulated such a wealth of information on reef structure and history that his theory emerges as almost an inevitable consequence. He tests his theory for its ability to explain the features of a variety of reefs and atolls, as well as the distribution of different types of reef in different parts of the world.
The theory states that, in conditions where coral can survive, it will form fringing reefs (reefs close to shore) around islands and other land masses. If that land is rising over time, the reef will grow outwards as the land rises, and the fringing reef will be stable. If the land is neither rising nor sinking, the reef will expand outwards over time, forming a barrier reef (a reef enclosing an island, at some distance from it). If the land is subsiding, the coral will grow upwards in consequence, and eventually a barrier reef, then an atoll (a more or less circular reef with no land enclosed), will be formed.
Darwins evidence was drawn from a great many sources. Using plumb-lines, he measured the depth of the sea around many reefs, at different distances from the reef. This showed him a slope too steep to be consistant with reefs being formed on volcanic craters, as was thought at the time. By using soft wax on the bottom of the lead weight, he was able to gain clues to the nature of the sea-bottom. Sand and other fragments would become embedded in the wax, hard rock would simply leave impressions in it. This helped him deduce the limits of depth at which coral can grow.
Some of his soundings were as deep as 7200 feet (2160 metres), so he was definitely doing more than just scratching the surface!
Darwin noted the importance of sediment, both in forming islands as it is washed up by the waves, and in inhibiting coral growth by smothering corals or preventing them from getting a good anchorage inside lagoons. He dissected several parrotfish to investigate their stomach contents, verifying that they do indeed eat coral, pass it through their bodies, and excrete fine sand which contributes to the sediment.
Darwin saw that the types of coral on the outer margins of a reef were different from those inside the lagoon. On the outside of the reef, larger corals grow well. Inside the lagoon, corals are less vigorous. This is largely because the outside margins have the full force of the current, bringing nutrients to the coral. Inside the lagoon, the currents are weaker, so the coral does not grow as fast. He was able to identify some of the corals in the outer margins by walking the beaches after storms and looking at the new types of coral fragments that were washed ashore.
Darwin also investigated the channels that cut through many reefs. He saw that the channels tend to form on the side away from the current. Channels facing the current would tend not to accumulate as much sediment as those on the other side, so corals could take hold and grow into the channel, closing it up over time.
Darwin was able to deduce the growth-rate of corals from many sources. He used accounts of one ship that grew a 2 foot (60 cm) thickness of coral on its hull in 20 months, as well as information about ship-anchors that had become embedded in reefs over the years, and direct observations from other naturalists.
Having been led to his theory by his intuition, and having backed it up with observation, Darwin then goes on to test his theory in many ways.
Many atolls have deep lagoons, which can be best explained by upward growth at the rim while the whole of the land subsides. If the land were rising, there is no reason for the rim to rise faster than the interior, so a deep lagoon would not be formed.
Many atolls occur in the open ocean, rising steeply from depths beyond which coral cannot grow. There is no obvious explanation that could cause such a reef to rise from the depths, but if the reef were growing upwards at the same pace as the land is sinking, these isolated reefs are to be expected.
There are smaller, irregular reefs in many places, and these too can be accounted for. As a land-mass sinks and its peaks become separated islands, linear reefs may form between them, which may persist as irregular reefs long after the land subsides.
The larger reefs of the Maldives appear to be slowly separating into smaller reefs. Darwin accounts for this by noting that particularly large atolls may be breached by wide channels. Coral can grow at the edges of these channels, and if the channel is wide enough the coral growth may eventually cut the atoll in two.
Fringing reefs occur near land that is stable or rising, but atolls and barrier reefs are formed where the land is sinking. The map shows clearly that fringing reefs tend to be apart from atolls and barrier reefs. For the same reason, volcanoes should not often be close to atolls and barrier reefs, and indeed they are not.
Darwin’s theory of reef formation and development, like his other theory of evolution, has stood the test of time. Unlike his other theory, it was well received by the scientific community and the public from the moment it was published. If it had been the only thing he published, it would still be an important work. Darwin produced many other books and papers, you can find his complete works online at darwin-online.org.uk. I think I’ll read a few more, though the one on fossilised barnacles will not be near the top of my list. You can read more about Darwin and his work at Blog for Darwin, the site that inspired this post.
Happy birthday, Charles Darwin.