In 1831 , Charles Darwin visited the Galapagos Islands . While observing the giant land tortoises that lived on these islands , Darwin noted that the shape of the tortoise shell varied depending on which island the tortoise lived. Tortoises on one island had round shells , for example , whereas tortoises on a neighboring island had more flattened , saddle -shaped shells . Which statement BEST summarizes Darwin's explanation for these differences ?