At this stage, Darwin had an acute interest in the 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". He later recalled believing in the permanence of species, but "as far as I can remember, vague doubts occasionally flitted across my mind". When organising his "Ornithological Notes" between mid June and August, Darwin expanded on his initial notes on the Galapagos mockingbird "Mimus thenca":