On the ship "Beagle". He studied the life forms on many islands. The Galapagos Island were important because the islands had diverse life that were altered from island to island. A finch on one island differed in ways from a finch on a neighboring island as did the tortoise.
Many of the species on the Galapagos Islands evolved in isolation from their ancestors on other land masses. They adapted different traits and eventually evolved into new adaptations and in some cases new species. This is part of what made this area appealing to him.