Shortly after leaving the Galápagos, Darwin noticed for the first time that the mockingbirds he had collected from different islands were not all alike. He would later find examples of birds that differed even more from island to island, such as the Galápagos finches, but it was the mockingbirds that "first thoroughly aroused" Darwin's attention to the peculiar distribution of species on the Galápagos.
Darwin's plant collections were all clearly marked and documented, as Henslow had taught him. But Darwin did not always record the exact island where he found each Galápagos bird.
"It never occurred to me, that the productions of islands only a few miles apart, and placed under the same physical conditions, would be dissimilar."
Too late, he realized that many organisms were unique to each island-a fact confirmed by his mockingbird specimens. Darwin sorely regretted the lost opportunity to do a systematic study of each island, writing,
"It is the fate of every voyager, when he has just discovered what object in any place is more particularly worthy of his attention, to be hurried from it."