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The idea that the ranges of species was due to the their spreading from a point of origination had several precursors before Darwin developed it in 1837. Buffon drew attention to the fact that similar species occupy the same position in different ecologies (although he did not use those terms¹). The French zoologist Peter Simon Pallas discovered that similar forms were often connected by a graded chain of intermediate forms, and in 1825, Leopold von Buch drew the logical conclusion that varieties become segregated species.

The first biologist to have suggested that species were independently created all over the world, however, was the botanist J G Gmelin in 1747, and following his work, as Mayr says, "[t]he Biblical story of the Garden of Eden and of Noah's Ark was quietly superseded by various theories of centres of creation."². And Buffon ("the father of zoogeography"³), in 1779 proposed that a fauna (the complete ecology of an area) was the product of the conditions of the district where it originated. This was not a theory of common descent, but of special creation, however. "The earth makes the plants; the earth and the plants make the animals", he wrote⁴.

Darwin's friend Joseph Hooker had done considerable work on geographic distribution from his travels to the Antarctic, Australia and New Zealand in 1839-1843, and had carried out experiments to show that species could spread beyond their "allocated" domain, published in 1853. This clearly influenced Darwin's thinking on the subject. Geographic (also now known as "allopatric") speciation - the idea that isolation due to geographic barriers is a cause of speciation - was something Darwin held to be important on islands, but also where rivers, mountains and other impediments prevented species split into separate breeding populations from back-crossing. According to Mayr, Darwin prevaricated on its importance, and eventually also accepted the possibility of "sympatric" speciation - speciation due to a move into new ecological or behavioural niches.

A number of other botanists and zoologists had noted the defined spread of species, especially Alexander von Humboldt (1805), but including EAW Zimmerman (1778-1783) and CF Willdenow (1798), and it was this that gave the death knell to the idea that species had spread from a central point, the landing site of the Ark. However, only Alfred Russel Wallace, in 1855, had published the principle that species are always found close in space and time to an allied species that precedes it in the geological record. Wallace did his work in the Amazon region and the Malay Archipelago, while Darwin's own observations twenty years earlier had been in the Galápagos Archipelago and the plains of Patagonia during the Beagle voyage. Darwin was spurred into publication of his ideas when Wallace, not knowing of Darwin's views, sent him a paper on the topic (and on natural selection) in 1858. Darwin had just suffered the death of his son Charles, and in his grief, he passed it on to Lyell and Hooker, with whom he had previously discussed his views (his discussions with Lyell were spurred on by Lyell's excitement by Wallace's 1855 paper), who submitted it to the Linnean Society of London with extracts of Darwin's unpublished 1844 Essay on Natural Selection and a letter Darwin wrote to Asa Gray in the US on 5 September 1857. It is clear that Wallace and Darwin independently discovered geographical speciation, and are due the joint credit they now are imputed.⁵

¹ Mayr 1982, p 411

² Mayr 1982, p 440

³ Mayr 1982, p 440

⁴ Mayr 1982, p 441

⁵ But see Quammen 1996 chapters 2 and 3 for a review of the dissenting views of various authors.

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