The voyage of the "Beagle" brought the survey ship HMS "Beagle", under captain Robert FitzRoy, to the Galápagos on 15 September 1835 to survey approaches to harbours. The captain and others on board, including his companion, the young naturalist Charles Darwin, made observations on the geology and biology on Chatham, Charles, Albemarle and James islands before they left on 20 October to continue on their round-the-world expedition. Primarily a geologist at the time, Darwin was impressed by the quantity of volcanic craters they saw, later referring to the archipelago as "that land of craters". His study of several volcanic formations over the five weeks he stayed in the islands led to several important geological discoveries, including the first, correct explanation for how volcanic tuff is formed. Darwin noticed the mockingbirds differed between islands, though he thought the birds now known as Darwin's finches were unrelated to each other, and did not bother labelling them by island. Nicholas Lawson, acting Governor of Galápagos for the Republic of Equator, met them on "Charles Island", and as they walked to the prison colony, Lawson told Darwin the tortoises differed from island to island. Towards the end of the voyage, Darwin speculated that the distribution of the mockingbirds and the tortoises might "undermine the stability of Species". When specimens of birds were analyzed on his return to England, it was found that many apparently different kinds of birds were species of finches, which were unique to islands. These facts were crucial in Darwin's development of his theory of natural selection explaining evolution, which was presented in "On the Origin of Species".