Darwin developed his theory through decades of careful observation, particularly during his voyage on the HMS Beagle, where he collected specimens and noted variations among similar species in different locations. He combined these observations with insights from geology, breeding practices, and population dynamics.

The book challenged the prevailing religious and scientific beliefs of the time by suggesting that species weren't immutable creations but rather evolved from common ancestors, contradicting literal interpretations of biblical creation and the established scientific consensus.

Darwin presented multiple lines of evidence, including comparative anatomy, embryology, geographical distribution of species, and the fossil record. He also drew parallels with artificial selection in domestic breeding to demonstrate how natural selection could work over longer time periods.