Adventure classics – Billions of tiny architects of the reefs

Voyage of HMS Beagle

by Charles Darwin

***

One of the questions on Charles Darwin’s mind even before the Royal Navy’s survey ship, HMS Beagle, began its multi-year, round the world mission in 1831 was the origin of coral reefs, the only geological features formed by animals. Exactly how coral polyps by the billions formed their structures was a question of importance both for geological theorists and for the British naval strategists.

Among the instructions issued for the Beagle’s voyage was that “the circularly-formed Coral Islands in the Pacific occasionally afford excellent land-locked harbours. . . and would be well adapted to any nice astronomical observations which might require to be carried on in undisturbed tranquility. While these are quietly proceeding. . . a very interesting inquiry might be instituted respecting the formation of these coral reefs, ” including examination of the “modern and very plausible theory” that such reefs have been raised from the summits of extinct volcanoes.

Oceanic coral reefs often formed a circular island or group of islands with a central open area, or lagoon. Scottish geologist Charles Lyell, whose books Darwin studied, and to whom he would dedicate his own book about the Beagle’s voyage, believed that coral reefs grew around the summits of underwater volcanoes, with the circular atolls reflecting the opening of the volcanic crater.

Acting on the Admiralty’s orders, Beagle Captain Robert Fitzroy stopped at the Keeling (now Cocos) Islands in the Indian Ocean. Among other activities, he took soundings of the area to determine the depth at which coral formations began. These gave the depth limit for living corals as only about 20 fathoms (37 meters), but the island in question, rising more than 7200 feet (2100 meters) Darwin wrote, “forms a lofty submarine mountain, with sides steeper even than those of the most abrupt volcanic cone. . . and every single atom, from the least particle to the largest fragment of rock in this great pile, which however is small compared with very many other lagoon-islands, bears the stamp of having been subjected to organic arrangement. We feel surprise when travelers tell us of the vast dimensions of the Pyramids and other great ruins, but how utterly insignificant are the greatest of these, when compared to these mountains of stone accumulated by the agency of various minute and tender animals!”

If the sea depth had remained constant, corals could never have built their reefs on the bottom, far below their habitable limit. On the other hand, besides the fact that the Keeling Islands soundings showed they were not formed around volcanoes, how did coral reefs come into existence?

Although it pained Darwin to disagree with Lyell, he proposed a new theory: that reefs formed around the fringes of existing land, and that it was the subsequent subsidence or sinking of that land below sea level that formed the open, circular interior lagoon of atolls. (At the time, no one was aware of the additional role played by rising and lowering sea levels in the formation of coral reefs.)

Darwin would end by writing an entire book, The Structure and Distribution of Coral Reefs, to state his new theory of the place of the humble coral polyps in the formation of the Earth. Far from being offended by Darwin, Charles Lyell was delighted.

(Next Friday, Adventure classics begins a September of young adventurers with William Golding’s Lord of the Flies. And you thought your schooldays were bad. . . )

Adventure classics – Neglected finches of the Galápagos

The Voyage of HMS Beagle

by Charles Darwin

***

Charles Darwin’s mind was so far from formulating his theory of evolution by natural selection that he initially overlooked the significance of the Galápagos Islands’ now-famous finches. As the official geologist of the Royal Navy’s survey ship, HMS Beagle, he considered the obvious volcanic origin of the islands their most striking aspect when he first caught sight of them September 15, 1835.

The original plan of a two-year voyage to survey the coasts of South America had stretched to nearly four years by the time the Beagle rounded the continent’s southernmost tip and reached its western extremity in the archipelago that had first gained notoriety as a pirate hideaway.

Darwin evoked shivers in his readers with the tale of a ship’s captain murdered by mutineers, his skull still visible in the underbrush. And he was still young enough to thrill at catching a ride on the immense tortoises: “I frequently got on their backs, and then giving a few raps on the hinder part of their shells, they would rise up and away; -- but I found it very difficult to keep my balance,” he admitted in his journal of the Beagle’s voyage.

He noted that “there can be little doubt that this tortoise is an aboriginal inhabitant of the Galápagos” and was struck by the islands’ seagoing iguanas (but not realizing they were in fact, the only such species), the birds he described as “finches,"  later realized to be more closely related to tanagers, did not at first excite his interest.

Rather than being “brilliantly colored, as might have been expected in an equatorial district. . . the males of all, or certainly of the greater number, are jet black; and the females (with perhaps one or two exceptions) are brown. The most curious fact is the perfect gradation in the size of the beaks. . .”

He was an avid collector (his days spent shooting while at Cambridge were paying off), but still so far from forming a theory of evolution of species that he neglected to keep records of which “finch” came from which of the archipelago’s numerous islands.

The Beagle finally returned to England in October 1836, after circumnavigating the globe, and in January 1837 Darwin presented the preserved “finches” along with his other bird and mammal specimens to the Geological Society of London. Not until he received a report from the society’s ornithologist did the possibility of the birds being unique to each island – and the implications of that fact for the transmutation of species – occur to him.

Fortunately, the Beagle’s Captain Fitzroy had also collected bird specimens, labelled according to island by his steward Harry Fuller and Darwin’s servant Syms Covington. These specimens helped him reconstruct the locations where his own birds had been shot.

Seventy years after Darwin set foot on the Galápagos islands and nearly a half-century after the publication of his landmark volume, On the Origin of Species, a more comprehensive collection of birds gathered by the 1905-06 Galápagos expedition by the California Academy of Sciences (link) would further confirm the significance of the little birds immortalized as the Galápagos finches.

(The picture illustrating this post is a facsimile of the original illustration in The Voyage of HMS Beagle, engraved for a 1957 edition by Ralph Beedham, although a version is also available on Wikipedia.)

(Next Friday, Adventure classics sees Darwin across the Pacific, concluding an August of adventures at sea.)

Adventure classics – The mystery of the vanished giants

The Voyage of HMS Beagle

by Charles Darwin

***

As the Royal Navy’s HMS Beagle made its way around South America in the mid-1830’s, frequently dropping off its on-board geologist, Charles Darwin, to make inland excursions inland, Darwin had time to wonder about the strange fossils he was collecting along river banks. What were they? And what had happened to sweep them from the earth?

The sheer numbers of fossils troubled him. He struggled to accommodate the huge quantities of bones, seemingly produced by the catastrophic deaths of large numbers of animals, with his enthusiasm for Scottish geologist Charles Lyell’s theory of gradual change over time.

“The number of the remains embedded in the grand estuary deposit which forms the Pampas and covers the granitic rocks of Banda Oriental, must be extraordinarily great. . . Besides those which I found during my short excursions, I heard of many others, and the origin of such names as ‘the stream of the animal’, ‘the hill of the giant’, is obvious,” he wrote in his journal of the voyage of the Beagle.

Darwin published his records in two editions, in 1839 and 1845. The 1957 collectible edition I’m working from is based on the 1845 edition, when Darwin was well on his way to puzzling out his theory of evolution based on natural selection. Some of the South American fossils, such as the extinct giant sloths, were obviously related to still living relatives. But what to make of creatures such as the Macrauchenia, looking like a humpless camel with a long snout, whose 1913 reconstruction illustrates this post? Or the Toxodon, with a rhinocerous-like body and rodent-like teeth, whose skull he bought for 18 pence, which when found “was quite perfect; but the boys knocked out some of the teeth with stones, and then set up the head as a mark to throw at.”

It would require 21^st century science to place the giant grazers in the family trees Darwin was starting to create, Scientific American magazine reported March 20, 2015, in “Mystery of Darwin’s ‘Strange Animals’Solved.”

Initially researchers trying to place the animals’ relationships were stumped by a lack of DNA, which degrades quickly in the South American climate. Instead, an international team used a different tactic, extracting collagen protein, to at last give two of Darwin’s mysterious mammals their place in the family tree of mammals.

“Part of a group of more than 250 mammals known as the South American ungulates,” Scientific American reported, “the creatures lived on the continent for around 60 million years before disappearing around 12,000 years ago.”

Purely on the basis of their fossils’ physical appearance, some had been suspected them of being related to elephants. Darwin himself suggested a possible link between the camel-like Macrauchenia and surviving llama-relatives, the wild guanacos of the pampas. But after sequencing proteins from two museum specimens of Toxodon and two of Macrauchenia, scientists finally placed them in a group that includes horses, tapirs and rhinos.

And what caused their disappearance after millions of years of survival? I’m going to suggest a catastrophic cause undreamed of by Darwin – the arrival of humans on the South American continent.

(Next Friday, Adventure classics puts Darwin back on board ship, to continue an August of adventures at sea.)