Science Secrets Page 12
On 20 February 1835, Darwin and company were at the coastal city of Valdivia, Chile, when suddenly, a great earthquake struck the region. Darwin wrote: “An earthquake like this at once destroys the oldest associations; the world, the very emblem of all that is solid, moves beneath our feet like a crust over a fluid; one second of time conveys to the mind a strange idea of insecurity, which hours of reflection would never create.”15 Unsettling—remember the supposed words of Pythagoras, that “Earthquakes were nothing else but Conventions of the dead.”16 Houses were shaken violently, and people ran in terror; but the wooden constructions at Valdivia survived. Yet the port of Talcahuano, where the Beagle docked afterward, and the adjoining town of Concepción, were utterly destroyed by the earthquake. Nearly every house was shattered to rubble, dozens of villages were devastated, and the port and ruins were wiped out by three terrible tidal waves.
For Darwin, the most remarkable effect of the earthquake was that the land had risen. The land at the Bay of Concepción was actually lifted by two or three feet; rocks that used to be underwater were now exposed. Moreover, FitzRoy noted that at the island of Santa María, one shore had risen by ten feet, such that beds of mussel shells for which the inhabitants used to dive underwater were beginning to rot in the sun. Darwin saw that hundreds of feet uphill, one could find the old remains of similar shells, and at the city of Valparaiso he had found seashells even at an elevation of 1,300 feet above sea level.
Such evidence, appearing in successive layers of differently weathered strata, showed that Lyell was right: major features of the landscape, such as the Andes Mountains, had not arisen abruptly in a single ancient catastrophe, but gradually, through many events such as the severe earthquake.
In 1835, the Beagle arrived at dark islands known to the British as “the Enchanted Islands,” just below the equator and about 550 miles west of South America. The Spaniards called them Galápagos. These towering volcanic islands were the last series of stops before completing the mission and then sailing around the world back to England. Far from anywhere, the foreboding shores of ugly black rocks, broken and jagged, swarming with red crabs and huge, hideous crested iguanas, reminded Captain Robert FitzRoy of the legendary capital of hell: Pandemonium.17
Since the Galápagos are volcanic islands, their geological makeup is thoroughly different from the continental coast. The jagged black rocks, arid soils, high elevations, and the unusually temperate climate, are all distinct. Darwin encountered many craters, including some more than three thousand feet high. The vegetation was ugly and nearly leafless, it smelled bad, the stunted trees seemed dead, and only some tall cacti were large enough to cast shade. Darwin agreed with FitzRoy that the deathly landscape seemed hellish, like “the cultivated parts of the Infernal regions.”18
The expedition members encountered massive tortoises and some odd birds. Only Charles Island was populated, mostly by people who had been banished there for political crimes. Chatham Island housed pigs and goats; the settlers had introduced them. There were mice at Chatham Island and rats at James Island, these being the two islands most visited by sailors, and Darwin surmised that these mammals likely came from ships that sailed there for over a century. Darwin collected samples of plants, fish, seashells, and insects, many showing unusual traits. For example, although tropical regions usually have many insects, many of them colorful, the Galápagos had very few kinds, of very small sizes and dull colors.
The most striking native animals were reptiles: sea turtles, giant tortoises, countless ugly lizards, and several kinds of snakes. The massive tortoises were nearly deaf, and to Darwin they seemed like ancient survivors of the biblical flood: “they appeared most old-fashioned antediluvian animals; or rather inhabitants of some other planet.” One kind of hideous and strong lizard, “imps of darkness,” lived exclusively on the rocky sea-beaches, their claws admirably fitted for crawling about, and their flat tails suited for swimming. Remarkably, it was the only known lizard that subsists by eating marine vegetables. Another species of lizards was not aquatic at all, but remained inland, moving sluggishly and looking “singularly stupid.”19 Darwin recorded strange behaviors for all of these reptiles. For example, the large aquatic lizards entered the sea often to eat submarine seaweed, but when Darwin confronted one on them, as if threatening it, cornering it on the rocks, he found “one strangle anomaly,” namely, that:
when frightened it will not enter the water. From this cause, it is easy to drive these lizards down to any little point overhanging the sea, where they will sooner allow a person to catch hold of their tail than jump into the water. They do not seem to have any notion of biting; but when much frightened they squirt a drop of fluid from each nostril. One day I carried one to a deep pool left by the retiring tide, and threw it in several times as far as I was able. It invariably returned in a direct line to the spot where I stood. It swam near the bottom, with a very graceful and rapid movement, and occasionally aided itself over the uneven ground with its feet. As soon as it arrived near the margin, but still being under water, it either tried to conceal itself in the tufts of seaweed, or it entered some crevice. As soon as it thought the danger was past, it crawled out on the dry rocks, and shuffled away as quickly as it could. I several times caught this same lizard, by driving it down to a point, and though possessed of such perfect powers of diving and swimming, nothing would induce it to enter the water; and as often as I threw it in, it returned in the manner above described. Perhaps this singular piece of apparent stupidity may be accounted for by the circumstance, that this reptile has no enemy whatever on shore, whereas at sea it must often fall a prey to the numerous sharks. Hence, probably urged by a fixed and hereditary instinct that the shore is its place of safety, whatever the emergency may be, it there takes refuge. 20
Overall, the Galápagos were unusual compared to other places visited by Darwin because instead of there being large quantities of herbivorous mammals, there were large quantities of herbivorous reptiles. As for birds, Darwin collected sixty-four specimens, including water-birds, unusual finches, mockingbirds, flycatchers, a weird hawk, and owls.
Despite such oddities, there were obvious major similarities among the animals of the Galápagos and those of South America. There were close resemblances in their appearance, voices, gestures, and habits. Darwin wrote: “almost every product of the land and water bears the unmistakable stamp of the American continent,” adding that the “naturalist feels as though he's standing on American land.”21
A common story about the birth of evolution is that Darwin based it on the unique giant tortoises of the Galápagos. The vice governor of the islands told Darwin that he could identify the origin of any giant tortoise by the shape of its shell. The story goes that Darwin then reflected on that statement, analyzed the tortoises' shells, and realized that those unique tortoises had adapted to distinct environments.
But this myth too was dispelled by Frank Sulloway.22 Actually, back then, Darwin did not heed that one might distinguish tortoise species based on their shells: dome or saddle shaped (galápago meant saddle). He assumed that the tortoises he saw in the Galápagos were of same species found in islands in the Indian Ocean. Hence, he did not care to collect the shells of any tortoises to analyze them. FitzRoy did take some thirty giant tortoises as their ship departed the islands, but just as food. Darwin and company ate every one of those giant, tasty tortoises before they returned to England, and they threw out their huge shells and bones in the oceanic waters. Darwin kept two young tortoises as pets, but all of the giant tortoises brought on board were destroyed.
On the way back to England, Darwin began to organize his notes. While writing about his four specimens of Galápagos mockingbirds, he noted that two of them were distinct kinds exclusive to two islands. At that point he noted that some Spaniards claimed to know from which island came any tortoise by looking at the animal's size, shell, and scales. He briefly remarked that such statements “undermine the Stability of species,” and should t
herefore be examined.23 This does not mean that he had a sudden conversion to transmutation, but rather, as he later recalled, that during the voyage he experienced some “vague doubts” about the immutability of species.24
What was so puzzling? The oddity was that Darwin had not expected that islands so close together, having the same rocks, climate, and elevations, could yet have different inhabitants. Species were supposed to match their environments, so why did different kinds inhabit equivalent environments? He had not pondered this question early enough to label and segregate all specimens by location. Therefore, he could hardly make any inferences about finches in particular, and about tortoises he only had hearsay. At least for the four specimens of mockingbirds, he had noted the four islands where he obtained them, and moreover, he had collected more mockingbirds on the continent—east, south, and west—in Uruguay, Patagonia, and Valparaiso.
Trying to identify specimens, Darwin was plagued by confusions. Were diverse specimens different species, genera, or merely different varieties? Where had he obtained some of them? Which animals were new species? He had not visited the northwestern coast of South America, above Lima, so he could not judge whether animals in the Galápagos were unique to the islands. He could hardly identify the large fossils from South America, either.
Finally, he delivered his specimens to specialists in London. Richard Owen analyzed the fascinating collection of mammal fossils. John Gould analyzed the collection of birds. Owen found that the mammal fossils resembled animals living presently in South America. Darwin's presumed mastodon was actually a kind of giant llama, big as a camel. The apparent ant eater resembled also an armadillo, as did the huge fossil with the massive armor shell. Another giant skeleton, larger than a bear, resembled the sloth.
Other naturalists, too, were concerned with the puzzle of how new species replace old species. Did God repeatedly create new species whenever bad weather wiped out an entire species? In 1836, John Herschel had written to Charles Lyell, “Of course I allude to that mystery of mysteries, the replacement of extinct species by others. Many will doubtless think your speculations too bold, but it is as well to face the difficulty at once.”25 Did God operate instantaneously or through a series of intermediate causes? By abrupt sporadic miracles or by systematic laws?
Lyell became eagerly interested in Owen's findings on Darwin's specimens, because they confirmed a “continuity of types”: that animals presently living in South America resembled ancient extinct species. Darwin also was impressed. As for the birds, John Gould painstakingly realized that the specimens from the Galápagos were especially interesting, because most of them were absent on the continent. Darwin's inferences in attempting to classify his birds were mostly erroneous. Whereas he identified only one species of mockingbird in the Galápagos specimens, Gould distinguished three. Whereas Darwin presumed that his mockingbirds from the continent were all the same species, Gould concluded that they were not. Likewise, Gould organized the finches as species with various affinities, whereas Darwin had thought that they were more diverse. Gould also found that the weird hawk was an “intervening link” between rather disparate groups: common buzzards and the South American caracaras.26
By 1837, Darwin had privately inferred that species can change in time. In July he began a notebook on “Transmutation of Species.” He began to collect and organize facts about many species and their relations in places and times. By 1840, he was fully convinced that species were mutable.27 He had been struck by the overall character and distribution of the South American fossils and especially the Galápagos species. The distribution of animals in space and time, geography and fossils, suggested that species evolved. The differences and similarities between the ancient extinct animals and the living were stunning. There was a “continuity of types.” But why would God replace species with similar but distinct species? Having designed all species to live in harmony and to fit in perfect balance with their environments, why would the benevolent Creator allow entire species to die?
The environment did not always suit all species. Thinking about what he had seen regarding the distinct species of rheas in southern regions, Darwin considered the possibilities: perhaps the climate favored the more southern species, but one could also infer that it hurt the more northern species. If each species was perfectly suited to its territory, Darwin thought, then neither was perfectly suited to the region in between. The two species might be competing to occupy territory. How would this match the theory that every creature fits perfectly in its environment? The assumption that ecology is perfectly balanced became doubtful.
Some animals did not seem to match their environment. Animals at the Galápagos resembled American animals, yet the soils and environment of the Galápagos were quite distinct from those of the continent. Darwin realized that geologically the Galápagos islands resemble instead the Cape de Verde Archipelago, volcanic islands off the coast of West Africa, near the equator. He exclaimed: “there is a considerable degree of resemblance in the volcanic nature of the soil, in climate, height, and size of the islands, between the Galapagos and Cape de Verde Archipelagos: but what an entire and absolute difference in their inhabitants!”28
Instead, the Galápagos animals resembled the American animals, and the animals of Cape de Verde resembled the African animals. Why? This seemed to be an “almost universal rule”: island populations resemble those of nearby continents. Darwin reasoned that “this grand fact” could not be explained by the usual theory of multiple independent creations.
As Darwin thought about transmutation, he reflected on the words of the Galápagos Spaniards and the vice governor about the tortoises. Darwin tried to compare his two pet tortoises to other specimens, but their shells only become distinctive when they grow big—so unfortunately, he could infer nothing. Darwin trusted the words of the Galápagos settlers, yet they had exaggerated; even experts on tortoises today can scarcely guess the island origin of a Galápagos tortoise by just examining it. There are gradations between the dome and saddleback shells. When zoos returned fifty tortoises to the Galápagos in the 1970s to breed them with locals, specialists failed to identify the likely origin of any but one such tortoise. In short, Darwin just did not reach his theory of transmutation by inspecting tortoise shells.
Thus far I have mentioned various animals that Darwin encountered in the Galápagos. But what about those that he did not find? Those also had a puzzling significance. Presumably, God had placed the many unique species of birds and reptiles on the islands. But what about amphibians? Darwin knew that a French naturalist and cartographer, Bory de St. Vincent, had pointed out that on volcanic islands off the coast of Africa, strangely, there was a complete absence of Batrachians—the whole order of animals consisting of frogs, toads, newts, and the like.29 Yet some oceanic islands' conditions are ideally suited for such animals. In particular, certain areas of the Galápagos were appropriate. Therefore, Darwin painstakingly searched for such creatures. He concluded, “Of toads and frogs there are none. I was surprised at this, considering how well the temperate and damp woods in the elevated parts appeared adapted for their habits.”30
Likewise, he noted that there were no frogs in the Canary Islands near North Africa, none at the Sandwich Islands (Hawaii), none at St. Jago in the Cape de Verde, none at St. Helena Island (between Africa and South America).31 But why not? Why did God choose not to place any frogs in the oceanic islands? Darwin commented: “The absence of the frog family in the oceanic islands is the more remarkable, when contrasted with the case of lizards, which swarm on most of the smallest islands. May this difference not be caused, by the greater facility with which the eggs of lizards, protected by calcareous shells, might be transported through saltwater, than could the slimy spawn of frogs?”32 And Darwin was right. Frogs, similar animals, and their spawn have semipermeable skins that let their inner fluids seep out in saltwater, which kills them. Such animals cannot float across a stretch of salty oceanic waters without promptly dying. By contrast,
the eggs of some lizards can well be transported by oceanic currents or in floating tufts of plants and dirt.
Darwin also contemplated the lack of native mammals in the Galápagos. On land, only some kinds of rats seemed to be native, and Darwin imagined that they had somehow arrived by ships or otherwise. There were also seals and sea lions. And there was another distinctive kind of mammal on the Galápagos, one kind of mammal found in virtually all islands: bats. The Galápagos had two species, the hoary bat and the red bat. Many other islands, such as New Zealand, Norfolk Island, the Viti Archipelago, Hawaii, Mauritius, also possessed unique bats. Why would the Creator make unique bats but no terrestrial mammals or frogs on most islands? Darwin knew that bats fly across great bodies of water, and Darwin noted that some bats were seen wandering stranded over the ocean in daytime, and some bats were known to visit Bermuda, six hundred miles from the continent.
Darwin realized that all the various kinds of species on the Galápagos were only such as could have arrived there by water or air: birds, reptiles, insects, bats. He conjectured that the diverse inhabitants were not created in these islands—they were colonists. Their ancestors all originated somewhere else, such as in South America. In sum, the only kinds of native animals on the Galápagos were those kinds that could have gotten there by themselves or by accident without an independent miracle.
Yet many of these animals were distinct from those on the continent. So Darwin reasoned: they first arrived at the Galápagos and afterward they changed. They somehow adapted to their new environments. But how? Years later Darwin reflected that in the Galápagos, “both in space and time, we seem to be brought somewhat near to that great fact—that mystery of mysteries—the first appearance of new beings on this earth.”33
Darwin also tested his inferences. Was it possible that eggs and seeds could really float across hundreds of miles of saltwater and still come to life on land? Darwin found that timber drifting in the ocean, as well as icebergs can carry clusters of trapped soil and seeds in such a way that the seeds remain able to produce vegetation. Further, he tested whether a dead pigeon, floating on saltwater for thirty days, could bear seeds in its stomach that would afterward germinate. He found, “to my surprise,” that indeed such seeds could then germinate. Also, by studying pigeons' excrements he found that some kinds of seeds passed undigested and could then germinate. Darwin also tested whether seeds can germinate after being immersed in saltwater for weeks. He found that out of 87 kinds, 64 did germinate after being immersed in water for 28 days. A few germinated even after being immersed for 137 days. Most seeds sink, but fruits float for a long time. Dried hazelnuts floated for 90 days, and when Darwin planted them later they did grow. He found that 18 of 94 plants with ripe fruit floated for more than 28 days. He estimated that 14 out of 100 plants from a given country might float for 28 days and retain the power to germinate.