The archipelago of 19 islands was and is being created by a "hotspot" where magma bursts through Earth's crust. The hotspot stays in one place, but the oceanic plate through which it erupts is creeping steadily eastward. So the islands on the eastern side of the archipelago were built up earlier--a few million years ago--than the ones on the west, most of which are still erupting.
As if Darwin didn't do enough for science with the theory of evolution by natural selection, he also figured out the life history of oceanic islands. He realized that such islands start as volcano peaks, erode into flatter islands as their volcano ceases spewing lava, and eventually sink into the sea surrounded by an atoll of coral that grew on the flanks of the volcano.
This caldera at the top of Sierra Negra volcano on Isabela last erupted in 2005.
So the islands emerged from the sea as molten rock--perfectly sterile.
Today the islands are filled with bizarre plants and animals. How'd they get here, 600 miles from the nearest land? By air, sea or rafts of floating debris.
Plants on the Galapagos tend to have seeds that float long distances in water (like mangroves), are light enough to be blown there, or are able to hitch a ride on birds. In the background of this photo of a Galapagos mockingbird you can see a forest of scalesia trees. They look like typical trees--they can be more than 20 feet tall and they grow in forests--but they're in the same taxonomic family as sunflowers and daisies!
These trees are a great example of how organisms will find a way to fill any unfilled ecological niches. Typical trees (until humans started planting them) didn't colonize the Galapagos. But lightweight flower seeds landed there and did pretty well for themselves, and now there are 15 scalesia species on the island, many of which have evolved the structure of trees or shrubs.
One of the thrills of visiting the Galapagos is getting up close to animals. Most of the species you encounter evolved there in the absence of predators, in a sort of peaceable kingdom, so they're not particularly concerned when someone takes a picture.
Unfortunately for the giant tortoises, they didn't have any particularly useful defenses against human hunters, who wiped out some populations. (Tortoises can live for many months without food or water, so sailors stored them in ships' holds to eat later.) One of Darwin's inspirations came from an off-hand comment that sailors could tell which island a tortoise came from based on the shape of its carapace.
On islands with dense vegetation, like here in Santa Cruz, tortoises are built like tanks. They can crash through scalesia bushes, munching greenery all the way.
The Galapagos is no place for a mammal. But it's a great place to be a reptile. Land animals had to make the trip here via rafts of vegetation that broke loose from the mainland, which isn't so bad if you have scaly skin, are cold-blooded and can go for a long time without fresh water. A few rodents managed to colonize the islands, and there are some native bats, but reptiles rule.
One of the weirdest reptiles is the marine iguana, the world's only seagoing lizard. It basks on lava rocks to warm up in the morning, then swims around in the surf eating seaweed. They get to be four feet long or more and look for all the world like Godzilla. Like other Galapagos creatures, they aren't particularly bothered by humans gawking at them.
The Galapagos hosts plenty of migrant birds, species that spend the summer in North America, say, but prefer to winter at the equator. But the full-time resident birds are the weird ones.
Depending on the time of year, ocean currents and winds can come to the Galapagos from the north, south, east or west. Flamingoes, normally found in the Caribbean, were blown here from points north. And penguins were cast away here, probably swept north in a strong current. (This one is swimming near a fishing boat on Isabela.) They evolved into their own species, the Galapagos penguin, and their range stretches across the equator. (In case it ever comes up in a trivia contest, this is the one species of penguin that is not restricted to the Southern Hemisphere.)
Seeing Caribbean-style flamingoes and Antarctic-style penguins within several hundred meters of each other seemed just so wrong in so many ways.
That's another thing that made me shake my head in admiration for Darwin--the plants and animals here are so unexpected and so confusingly adapted that it's amazing he managed to make sense of it all.
The mascot of the islands is probably the blue-footed booby, shown here with more penguins. I missed the mating dance, unfortunately, in which the boobies shake their blue feet at each other and press their upturned bills together. They tend to be oblivious to people, and they're so goofy looking that they win the most-likely-to-be-depicted-on-a-T-shirt contest.
The Galapagos isn't unique in having fearless animals. Other birds evolved similar behavior, especially on islands: moas, dodos, great auks... notice a pattern? Yeah, most of them are extinct. Either the adults or the eggs were eaten to oblivion by human hunters, with an assist from introduced dogs or rats that attacked the birds' nests.
When Darwin got to the Galapagos, the tortoises had been hammered by whalers and there were some introduced species. But because the islands were so inhospitable to humans, the species there were still fairly untouched.
Today Darwin is the patron saint of the islands. This statue of him looms over a cove on San Cristobal where the H.M.S. Beagle is thought to have first set anchor. The arch is along the road to the Charles Darwin Research Station Santa Cruz, and the mural is near the main dock on the same island.
He could have figured out evolution with natural selection without the help of the Galapagos, but the islands inspired him. The islands were young, had been periodically sterilized by lava, and were far from the mainland, so Darwin realized that whatever species lived there had arrived from elsewhere and had subsequently adapted to the strange conditions.
The islands also provided simple and compelling examples of evolution--the finches with beaks adapted to different types of seeds, for instance--that would help Darwin make the case for evolution to the rest of the world.
And that's one of the main reasons tourists come to the islands--to see the origin of Origin of Species, to see the creatures that inspired his insight. And, this year, to celebrate his 200th birthday.