In 1831, Charles Darwin served as the naturalist aboard HMS Beagle during its five-year mission to survey the coastline of South America. This journey allowed Darwin to observe and collect a vast array of specimens and fossils. His detailed observations laid the groundwork for scientific theories that would reshape the understanding of the natural world.
The Galápagos Finches
Darwin’s collection of finches from the Galápagos Islands became famous after his voyage. Initially, Darwin did not recognize their significance and even misidentified some specimens. Their importance became clear upon his return to London when ornithologist John Gould examined them.
Gould identified the birds as a unique group of 12 distinct species, all new to science. His analysis revealed that the main variation among these closely related species was the shape and size of their beaks. This diversity was directly linked to the birds’ different diets and the ecological niches they occupied on their respective islands.
Some finches possessed robust beaks for cracking hard seeds, while others had slender beaks for probing for insects. Another species developed a beak that functioned like pliers to grip cactus spines. This process, known as adaptive radiation, provided a clear example of how species could change and adapt over time.
Galápagos Tortoises and Iguanas
Darwin also studied the giant tortoises of the Galápagos, observing that those from different islands had distinct shell shapes. The local vice-governor informed Darwin that he could identify a tortoise’s home island by its carapace. Tortoises from islands with abundant ground vegetation had dome-shaped shells.
In contrast, those from drier islands with taller vegetation had saddle-backed shells, which allowed for greater neck movement to reach food. These variations suggested the tortoises had adapted to the specific conditions of their home islands.
Darwin was also fascinated by the marine iguana, a species unique to the islands. He noted its ability to forage for seaweed in the ocean and documented its adaptations, like a flattened tail for swimming. He also studied land iguanas, noting differences that supported his ideas on how isolated populations could diverge.
Darwin’s Fossil Discoveries
On the South American mainland, Darwin made several fossil discoveries. In Patagonia, he unearthed the remains of enormous extinct mammals, including:
- Megatherium, a giant ground sloth the size of an elephant.
- Glyptodon, a massive armored creature whose shell resembled a modern armadillo’s.
- Toxodon, a large herbivore with a build resembling a rhinoceros.
- Macrauchenia, a camel-like animal with a long neck.
Darwin noted that these extinct giants resembled the smaller, living species that inhabited the same region. This connection between extinct and living species was an important observation.
This suggested a “law of succession of types,” where modern species arise in the same areas as their fossilized predecessors. The evidence indicated that life forms change over generations, with new species descending from ancestral ones.
Other Significant Animal Observations
Darwin’s work during and after the voyage was extensive. After returning to England, he spent eight years studying barnacles (Cirripedia), which identified new species, clarified their life cycles, and established his reputation as a biologist.
During the voyage, he identified two distinct species of rhea in Patagonia occupying separate territories. In Australia, he observed the platypus and the rat-kangaroo, noting how the unique marsupials fit into their environment. His work on coral reefs led him to theorize that atolls form from coral growing on sinking volcanic islands.