What observations led Darwin to the idea of species variability?

The worldview of Charles Darwin, who was going to become a priest, completely changed the round-the-world voyage on the Beagle ship. On this voyage, he went as a naturalist, whose task was to replenish the collection of the British Museum of Natural History.

The following observations led him to the idea of the variability of species:

the diversity of insects and finches in the Galapagos Islands of volcanic origin;
the remains of fossil animals of South America;
marsupial animals of Australia.