The strange plants and animals of the Galápagos Islands puzzled Darwin. Many lived only on the Galápagos--and sometimes only on one specific island. How had these species gotten there? And why weren't they the same as those on similar islands around the world?
Instead, in countless details, they were more like species from the mainland of South America. That would make sense if they were migrants. But while many birds, plants, insects, reptiles, and even mollusks on the Galápagos resembled South American species, they were also slightly different.
Before the Beagle reached home, Darwin began to wonder if species from the mainland had reached the Galápagos, and then changed--as they adapted to this new environment. Was it possible? Over time, could species change?