Black on Black
Of all the exotic new worlds that Darwin saw on his travels, none was more alien than the Galápagos Islands:
"Nothing could be less inviting than the first appearance. A broken field of black basaltic lava, thrown into the most rugged waves, and crossed by great fissures, is everywhere covered by stunted, sunburnt brushwood, which shows little signs of life."
Darwin noticed that many species seemed a perfect match for their environment, even down to their coloring. Describing "most disgusting clumsy Lizards" that were "as black as the porous lava rocks over which they crawl," Darwin mused, "They assuredly well become the land they inhabit."
Though iguanas were common throughout South America, two species from the Galápagos Islands especially interested Darwin. These strange creatures appeared specially adapted for life there--and indeed could be found nowhere else. The marine iguana was especially distinctive: No other iguana swims or feeds in the ocean. Intrigued, Darwin opened the stomachs of several and found nothing but seaweed.
The other Galápagos iguana, Darwin observed, lived on land. It ate prickly pear cactus, spines and all. He was told it could go a year without drinking, getting all its water from the cactus plant. Darwin broke off a cactus branch and threw it to a small group, observing, "It was amusing enough to see them trying to seize and carry it away in their mouths, like so many hungry dogs with a bone."
A Tree-Sized Cactus
On the Galápagos, Darwin found that normally smaller plants could be found in enormous sizes. The stems of some prickly pear cactus, Darwin noted, grew six to ten feet high and one foot in diameter.
The Galápagos marine iguana is well adapted to feeding in the ocean. It can stay underwater for an hour or more, and with its legs at its sides uses its flattened tail to swim like a crocodile.
Darwin engaged in a curious experiment with one of these iguanas. He repeatedly tossed it into the water, but instead of staying safely in the sea, it always returned and let him do it again. Marveling at this "apparent stupidity," Darwin came up with an explanation: given the complete lack of natural predators on land, and the danger of sharks at sea, it instinctively fled to dry land whenever threatened.
Encounter with an Iguana
"Their limbs and strong claws are admirably adapted for crawling over the rugged and fissured masses of lava, which everywhere form the coast."
Galápagos land iguanas often sit in the sun or bury themselves to hide or stay cool. Darwin wrote,
"I watched one for a long time, till half its body was buried; I then walked up and pulled it by the tail; at this it was greatly astonished, and soon shuffled up to see what was the matter; and then stared me in the face, as much as to say, 'What made you pull my tail?'"