 He examined beds high on the hill which had been taken as seashells showing that St. Helena had risen from the ocean in recent times, but Darwin identified them as extinct species of land-shells.He noted that woodland had been destroyed by goats and hogs which had run wild since being introduced in 1502, and native vegetation only predominated on high steep ridges, having been replaced by imported species.At this stage Darwin had an acute interest in island biogeography, and his description of St Helena as "a little centre of creation" in his geological diary reflects Charles Lyell's speculation in Volume 2 of "Principles of Geology" that the island would have acted as a "focus of creative force".