Later, he realised he had made a big mistake in not noting which islands some specimens from the Galapagos Islands were found on. He knew this was a mistake because he had sensed how islands were rather like the higher parts of mountains, which also have many unique species. The idea we now know as geographical speciation was beginning to form in his mind. Darwin opened his secret 'transmutation' notebook (Notebook 'B') in July 1837, less than a year after the "Beagle" returned to England.