Darwin’s most important observations were made on the Galapagos Islands. This is a group of 16 small volcanic islands situated off the west coast of Ecuador, South America.
Darwin observed that organisms on the Galapagos were similar to, yet different from, one another and from similar species in South America. For example, the giant tortoises on one island had saddle-shaped shells, while those on another island had dome-shaped shells. People who lived on the islands could even tell the island a turtle came from by its shell. This started Darwin thinking about the origin of species. He wondered how each island came to have its own type of tortoise.