Final answer:
Charles Darwin's observations of finch and tortoise species' adaptations in the Galápagos Islands greatly contributed to his formulation of the theory of evolution by natural selection.
Step-by-step explanation:
Charles Darwin's observations during his voyage on the HMS Beagle led to the development of his theory of evolution. Darwin noted that Galápagos finches on different islands exhibited varying beak shapes and sizes, adapted for specific types of food available on their respective islands. This suggested that species adapt to their environment over time, which is a cornerstone of his theory. Additionally, Darwin's study of the giant Galápagos tortoises and their varied shell shapes depending on the island they inhabited shed light on how environmental factors can influence physical traits and lead to speciation.
Furthermore, the dramatic variations in species across different but proximate geographic locations demonstrated to Darwin that species underwent modifications over generations to better suit their habitats. This concept culminated in his evolutionary principle known as natural selection. Darwin's conclusions were strengthened by geological observations indicating that Earth's surface and the life upon it change over time, paired with his understanding of the work by scholars like Charles Lyell and Thomas Malthus.