The pristine wildlife of the Galapagos Islands, which are located nearly 600 miles off the coast of Ecuador, has long appealed to tourists with an eye for nature. In recent years, the number of such visitors has steadily increased: there were 200,000 of them in 2013, compared to fewer than 12,000 in 1979. But the attraction of the islands is far from new. That same wildlife also attracted a young naturalist to the islands in 1835, and his findings there would inform one of the most important scientific theories ever posited.
Charles Darwin had been sailing for three years on the H.M.S. Beagle, helmed by captain Robert FitzRoy, before arriving at the Galapagos. During the voyage's five weeks at the islands, he observed giant tortoises and marine iguanas, mockingbirds and finches. His observations of the variations between species on different islands sparked the idea that would lead to his theories of evolution and natural selection, published more than two decades later in On the Origin of Species.
When LIFE sent photographer Alfred Eisenstaedt to photograph the islands in 1957, his camera found many of the same species Darwin had written about. In a lengthy essay accompanying Eisenstaedt's photographs, LIFE summarized Darwin’s amazement upon finally embarking on the most anticipated leg of his journey:
The Beagle arrived on Sept. 15, and Darwin gazed with awe at the forbidding scene—black basaltic mountains, pitted, cratered, blistered, seamed with lava flows, littered with slag heaps, strewn with cinders, parched and prostrate beneath a smoldering, sullen sky. To his eyes the landscape suggested “what we might imagine the cultivated parts of the Infernal regions to be.”
Liz Ronk, who edited this gallery, is the Photo Editor for LIFE.com. Follow her on Twitter @lizabethronk.