Everybody in Darwin's time thought that the map of the world was pretty much a constant. Some of Darwin's contemporaries did countenance the possibility of large land bridges, now submerged, to explain, for example, the similarities between the floras of South America and Africa. Darwin himself was not greatly enamoured of the land bridge idea, but he surely would have exulted in the modern evidence that entire continents move over the face of the Earth. This provides by far the best explanation of certain major facts of animal and plant dispersion, especially of fossils. For example, there are similarities between the fossils of South America, Africa, Antarctica, Madagascar, India and Australia, which nowadays we explain by invoking the once great southern continent of Gondwana, uniting all those modern lands. Once again, our late-coming detective is forced to the conclusion that evolution is a fact.
The theory of 'continental drift', as it used to be called, was first championed by the German climatologist Alfred Wegener (1880-1930). Wegener was not the first to look at a map of the world and notice that the shape of a continent or island often matches the coastline opposite as if the two land masses were pieces of a jigsaw puzzle, even when the opposite coastline is far away. I'm not talking about little local examples, such as the Isle of Wight's neat dovetailing into the Hampshire coast, almost as though the Solent wasn't there. What Wegener and his predecessors noticed was that something along the same lines seemed to be true of the whole facing sides of the giant continents of Africa and America. The Brazilian coast looks tailor-cut to fit under the bulge of West Africa, while the northern part of Africa's bulge is a nice fit to the North American coast from Florida to Canada. Not only do the shapes roughly match: Wegener also pointed to matching geological formations up and down the east side of South America and corresponding parts of the west side of Africa. Slightly less clearly, the west coast of Madagascar forms a pretty good fit to the east coast of Africa (not the section of South African coast that is opposite it today but the coast of Tanzania and Kenya further north), while the long, straight line of Madagascar's east side is comparable to the straight edge of western India. Wegener also pointed out that the ancient fossils to be found in Africa and South America were more similar than you would expect if the map of the world had always been the way it is today. How could this be, given the width of the South Atlantic ocean? Were the two continents once much closer, or even joined? The idea was tantalizing, but ahead of its time. Wegener also noticed matchings between the fossils of Madagascar and India. And there are similarly telling affinities between the fossils of northern North America and of Europe.
Such observations led Wegener to propose his daringly heretical hypothesis of continental drift. All the great continents of the world, he suggested, had once been joined up in a gigantic super-continent, which he called Pangaea. Over an immense span of geological time, he proposed, Pangaea gradually dismembered itself to form the continents we know today, which then slowly drifted to their present positions and have not finished drifting yet.
One can almost hear Wegener's sceptical contemporaries wondering, to use the street-talk of today, what he had been smoking. Yet we now know that he was right. Or almost right. Far-sighted and imaginative as Wegener was, I must make it clear that his hypothesis of continental drift was significantly different from our modern theory of plate tectonics. Wegener thought that the continents ploughed through the oceans like gigantic ships, not quite floating in water like Dr Dolittle's hollow island of Popsipetl, but floating atop the semi-liquid mantle of the planet. Reasonably enough, other scientists erected fortresses of scepticism. What titanic forces could propel an object the size of South America or Africa for thousands of miles? I shall explain how the modern theory of plate tectonics differs from Wegener's theory before coming to its supporting evidence.
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