Whenever one of my fellow Darwin groupies is asked what they would tell Charles Darwin about, in the unlikely event of his miraculous return to the Land of the Living, their almost inevitable single-word response is genetics. It's an obvious and sensible answer: Darwin would have given his back teeth to understand the mechanism of heredity. It was a major missing link in his theory of evolution, and he knew it.
But I should like to suggest an alternative scientific field which would be of extreme interest to the resurrected Darwin. I don't for one second claim that it's a more appropriate topic than genetics to explain to the great man, but it's one that would fascinate him: I would tell Mr Darwin about plate tectonics.
Darwin first made his name in the world of science as a geologist. Having received some practical experience geologising with Adam Sedgwick in North Wales shortly before he set off on HMS Beagle, he picked up much of the latest revolutionary geological thinking by devouring Charles Lyell's recently published Principles of Geology during the voyage. Darwin later wrote that Lyell's book ‘altered the whole tone of one's mind & therefore that when seeing a thing never seen by Lyell, one yet saw it partially through his eyes’.
Darwin put his new Lyellian eyes to good use. By the time he returned to Blighty in 1836, he had gathered considerable evidence to show that much of South America is gradually rising, and had come up with what proved to be the correct explanation for the formation of coral reefs. We now know that the underlying mechanism behind both of these phenomena is plate tectonics. Darwin would have been intrigued to hear the modern take on his geological theories.
But it wouldn't just be Darwin the geologist who would be want to learn about plate tectonics; Darwin the naturalist would be all ears too. Darwin and his friends (most notably Hooker) spent much time thinking about how species came to be distributed in the way that they are. They hypothesised former land-bridges, and Darwin brilliantly suggested how changes in global temperatures associated with the former glacial period (he did not know that there had been more than one ice age) would have allowed temperate species to relocate to tropical areas before being forced into the mountains as warmer temperatures returned. The following extract from a letter Hooker send to Darwin in 1858 is typical of their correspondence on the subject:
[I] want you to [go into] print that I may take up your refrigeration doctrine, to which I think I should have come clumsily at last by myself as the only way of accounting for the spread of European species to Australia.
It is curious—that so many more Europ. sp. should be in Australia than in Fuegia & S. Chili! Especially considering the enormous distance of Europe to Australia & no continuous mountains.
Put end of string on globe on England & other end on V[an] D[ieman's] L[and (i.e. Tasmania)] & it will run through the most continuous masses of Land on globe—it is the greatest stretch of all but [sic, presumably he meant by] dry land that you can find, & I can connect the Botany the whole way by mountains of 1. Borneo; 2, Java & Ceylon & Penins Ind. 3 Khasia; 4 Himal 5 Caucasus, 6 Alps. 7 Scandinavia.— I can thus connect Botanically England with VDL. better than I could Canada with Fuegia!
Had they known about plate tectonics, Darwin and Hooker might have understood better why the flora of Canada and Fuegia (which are nowadays connected by one huge, continuous landmass) are so different. We now know that North and South America were not always joined at the hip, and once formed separate continents with their own distinct species, divided by a wide ocean.
Charles Darwin would have had great fun working out how the modern theory of plate tectonics might be applied to his own theory of evolution. Perhaps he might have realised how it can be used to explain the mysterious Wallace Line which separates the Asian and Australasian zoogeographical regions. No doubt, he would have got many things wrong in his theorising, but knowledge of plate tectonics would have opened up a whole new line of enquiry for Darwin's species work. It would have been yet more grist to his cerebral mill.
See also: Books - Charles Darwin, Geologist