Why did darwin’s observations of galápagos tortoises cause him to wonder how species originate?

While at the Galapagos islands, he noticed that animals on the different islands differed. 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 the species. He wondered how each island came to have its own type of tortoise. 
Please mark brainliest thank you! c: