Final answer:
When Darwin left England in 1831, prevalent views of the natural world were based on the belief in the fixity of species. Darwin's observations and collections during his voyage on the HMS Beagle, particularly in the Galápagos Islands, challenged these views and led to his theory of evolution by natural selection.
Step-by-step explanation:
When Charles Darwin left Plymouth, England in 1831 on the HMS Beagle, prevalent views of the natural world were based on the belief in the fixity of species, meaning that species were thought to be unchanging and created in their present form. This view was influenced by religious beliefs and the work of naturalists who did not have access to the extensive evidence that Darwin would later gather on his voyage. Darwin's observations and collections during the expedition, especially in the Galápagos Islands, challenged these prevailing views and laid the foundation for his theory of evolution by natural selection.