After graduating from Cambridge, Darwin received a letter form his botany professor John Stevens Henslow inviting him on a scientific expedition of the ship HMS Beagle to explore the coasts of South America.
The voyage began in December 1831.
In South America, Darwin studied geology and collected specimens.
Darwin studied many mockingbirds of the same species. He noticed slight variations in the sounds, feathers and beak of birds from different islands. He concluded that different environments had different effects on the birds, and so the weak would be weeded out while the strong reproduced; over time creating different species altogether. This is how all species were formed.