Darwin and Wallace

On 1 July 1858 two papers on natural selection were read in London, one by Charles Darwin and one by Alfred Russel Wallace, who was on the other side of the world.

What Darwin saw on the islands

The Galápagos birds did not hit Darwin with a sudden flash of insight. According to the Natural History Museum in London, the first clue came from mockingbirds. On San Cristóbal Darwin saw a mockingbird he recognised, and on nearby Floreana the mockingbirds were clearly different. The differences between islands a few kilometres apart were bigger than the differences he had seen across a whole continent. The finches only became part of the story later, after the ornithologist John Gould identified them as separate species. Some had stout beaks for cracking seeds and others thin beaks for catching insects, and all looked like relatives of one finch species from the South American mainland. Darwin began to wonder whether the island species were the changed descendants of a few birds that had arrived from the mainland long ago.

Darwin did not start from nothing. Earlier in the century Jean-Baptiste Lamarck had proposed that changes an animal acquires during its life, by using or not using a body part, are passed to its young. The geologists James Hutton and Charles Lyell argued that the Earth was shaped by slow processes still going on today, which meant it had to be very old and gave species time to change. The economist Thomas Malthus wrote that populations grow faster than their food supply. Both Darwin and Wallace read Malthus, and both took from him the idea that in every generation more young are born than can survive.

Wallace, the collector

Alfred Russel Wallace was born in 1823 in Usk, in what is now Monmouthshire in Wales. He worked as a surveyor and then as a teacher in Leicester, where in 1844 he met the beetle collector Henry Walter Bates. In 1848 the two sailed to the Amazon, paying their way by selling specimens. After more than four years Wallace headed home, and the ship caught fire and sank in the Atlantic with almost all his collection. Within a year he set off again, this time to the Malay Archipelago, the islands of today's Malaysia and Indonesia, where he travelled from 1854 to 1862. Over eight years he gathered 125,660 specimens, including more than 5,000 species new to Western science. He also noticed that animals on islands a short distance apart could belong to two different worlds, one Asian and one Australian. The boundary he drew between them is still called Wallace's Line.

The letter from Ternate

In 1858, lying in his hut with a fever on the island of Ternate, Wallace worked out how species could change: the individuals best suited to their surroundings survive and breed and pass their useful features on. He wrote it up and sent it to Darwin, who he knew was interested. Darwin had been working on the same theory for about 20 years without publishing. He was struck by how closely it matched his own work and wrote that Wallace "could not have made a better short abstract". On the advice of his friends Charles Lyell and Joseph Hooker, papers by both men were read together at the Linnean Society of London on 1 July 1858. A year later, in 1859, Darwin published On the Origin of Species.

The argument rests on three observations. Most characteristics are inherited from parents, even though nobody then knew how. More offspring are born than can survive, so they compete for limited food and space. And offspring vary, and much of that variation is inherited too. Put those together and the individuals whose inherited features help them get by will, on average, leave more young, so those features become more common in the next generation. Darwin called the process natural selection and the long-term result descent with modification. He also bred pigeons at home, picking birds with the features he wanted, to show how far a species could change under selection by people. After his experiments he gave all 120 of his pigeon specimens to the Natural History Museum. Darwin died in 1882 and Wallace in 1913. Wallace never resented being in Darwin's shadow and called himself Darwin's greatest fan, though the two disagreed about whether natural selection could explain the human mind.