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Birds Tortoises Although the Beagle only stayed five weeks in the Galpagos it turned out to be an important stop for Darwin. The Galpagos Islands are a desolate volcanic archipelago ruled by giant tortoises and lizards. Six hundred miles off the coast of South America the landscape is bleak and prehistoric covered with black sand and lava the islands cut in half by the Equator. After visiting the islands in 1841 Herman Melville wrote The chief sound of life is a hiss. But for Darwin the Galpagos held important secrets. One of the first things he noticed was that a great number of Galpagos birds hawks mockingbirds water-sails herons were clearly related to birds in South America but with significant differences. As for the finches Darwin wrote in the first edition of his Journal 1839 It is very remarkable that a nearly perfect gradation of structure in this one group can be traced in the form of the beak from one exceeding in dimensions that of the largest gross-beak to another differing but little from that of a warbler. In the second edition 1845 he expanded on his observations The most curious fact is the perfect gradation in the size of the beaks in the different species of Geospiza finch from one as large as that of a hawfinch to that of a chaffinch and even to that of a warbler. The largest beak in the genus Geospiza is shown in Fig. 1 and the smallest in Fig. 3 but instead of there being only one intermediate species with a beak of the size shown in Fig. 2 there are no less than six species with insensibly graduated beaks...Seeing this gradation and diversity of structure in one small intimately related group of birds one might really fancy that from an original paucity of birds in this archipelago one species had been taken and modified for different ends. But that all came later. Darwin did not grasp their significance while in the Galpagos. It was only after the ornithologist John Gould sorted them out back in England and Darwin managed to retroactively figure out which islands they came from he had not labeled them properly that he realized their importance. He did however notice that each species of mockingbird was found only on one island and it was this fact-along with a statement by a local dignitary that he could at once tell from which island any tortoise was brought that first caused Darwin to think about how species varied between the islands and what that might mean. As a result while still on the Beagle Darwin began to doubt the immutability of species. In his notes about the how the birds mockingbirds not finches and tortoises of the Galpagos varied island to island he speculated If there is the slightest foundation for these remarks the Zoology of Archipelagos will be well worth examining for such facts would undermine the stability of species. Finches from the 2nd edition of Darwins Journal. 21