Final answer:
Charles Darwin doubted the fixity of species during his 1831 voyage on HMS Beagle, leading to his theory of natural selection.
Step-by-step explanation:
Charles Darwin began to doubt the fixity of species during a voyage around the world in 1830.
In 1831, Darwin embarked on a historic voyage aboard the HMS Beagle, which would eventually lead him to formulate the theory of natural selection. While traveling, he meticulously observed and collected samples of the diverse wildlife he encountered, especially on the Galápagos Islands where he studied various bird species, like the finches with their notably differentiated beak structures adapted to specific environmental niches. It was these observations that laid the groundwork for his groundbreaking work, 'On the Origin of Species by Means of Natural Selection' published in 1859, changing our understanding of biological diversity permanently.
Previously, other thinkers like Robert Hooke had also suggested that nature and life forms evolved over time, challenging the view that species were unchanging. However, it was Darwin's detailed observations, coupled with Alfred Russel Wallace's independent conclusions, that solidified the concept of evolution by natural selection.