Darwin described the lack of transitional fossils as “the most obvious and gravest objection which can be urged against my theory”, but explained it by the extreme imperfection of the geological record.1 And so the hunt was on for intermediate forms which would demonstrate the transition between one species and another. Lyell used the term “missing link” in 1863,2 a common but inaccurate term, although still in popular use today. Here are the main discoveries of the 19th century that were used to support Darwin’s theory.
The Beagle visited the Galápagos Islands, off the coast of Ecuador. There Darwin collected many specimens of birds, animals and plants and, while there, learned from the local people that they could identify the island from which a tortoise had come by its shell. In writing up his notes as he left the islands he observed that different species of mockingbird occurred on different islands.1 The detailed identification and publication of the birds was undertaken by John Gould, curator of the Zoological Society Museum in London 2,3 and this led to the discovery that different species of finch occurred on the different islands too. These differences between species led Darwin to write at the time ‘such facts would undermine the stability of species’.4 In other words, perhaps these different species on the different islands had at one time been the same, before they were isolated from one another. He wrote later this ‘seemed to me to throw some light on the origin of species’. He started a notebook on the “Transmutation of Species” in 1837 and his first known evolutionary tree is depicted there.5
Charles Darwin set sail on HMS Beagle on 27 December 1831 as a gentleman companion to Captain Fitz Roy. There was a crew of seventy-three. The ship’s naturalist left the ship early in the voyage and so Darwin assumed this role. The voyage circumnavigated the world and lasted almost five years. Darwin spent much of his time investigating the geology and natural history of the areas visited.1 During the voyage Darwin’s specimens were sent back to Cambridge for examination together with his journals. As a result he was already well known and respected by the scientific community by the time he returned in HMS Beagle on 2 October 1836.2