4 Michael Ruse
Geology at Cambridge University. But probably in part because of the isolation on
theBeagle— Darwin took with him the first volume of Lyell, and the others were
sent out on the voyage — he became an ardent uniformitarian. This set the scene
for two major pieces of geological work that Darwin did – one successful and one
very much non-successful. Lyell wanted to deny not just catastrophes but also (for
reasons to be given shortly) the directional nature of the world. How then could
he account for the seeming effects of cooling? The fossil plants around Paris are
tropical, clear evidence that things are not as they once were. Lyell introduced his
“grand theory of climate.” It is a mistake to think that temperature is a function
of distance from the equator. It is rather a function of relative distributions of
land and sea. Consider Britain, much warmer than it should be, thanks to the
Gulf Stream. What happens is that the world acts rather like a water bed. (This
is my analogy, not his!) As the rivers carry silt from the mountains down to
the sea, where it is deposited, the sea bed starts to sink because of the weight,
whereas the mountains start to rise because they are not as heavy as previously.
Things shift around, and then natural processes affect the terrain in new ways.
The consequence is that, because of Gulf Stream effects, the temperature around
the globe keeps shifting. Never the same, always moving up and down, but within
limits, rather like a sine curve.
Darwin bought into this completely, and his two major geological theories were
in fact responses to challenges posed by Lyell in thePrinciples. First there is the
question of coral reefs [Darwin, 1842]. In the tropics, one often finds circular reefs
around islands, rings of coral. Sometimes, indeed, there are no islands and just
the reefs. How can this be? Somewhat optimistically, Lyell suggested that they
might be the tops of extinct volcanoes. The coral just forms on them. The great
weakness here — one that Darwin seized on — is that it is highly improbable
that so many volcanoes would have risen from the sea bed just to the point where
they break the water’s surface. Darwin therefore turned the problem on its head.
He pointed out that coral can only live at the surface of the water. Hence he
suggested that initially the coral grows around the edges of islands, and then its
weight causes everything to start sinking. The islands go down and perhaps vanish
beneath the sea, but the coral keeps building on what is there and so stays visible
at the surface.
This was a triumph of reasoning. Darwin admitted that he had thought it all
up even before he had seen a coral reef! Today it is accepted as true. Borings of
coral reefs bear out Darwin’s hypothesis that they go all the way down. Notice,
however, to what extent it was a corollary of Lyellian geology. Precisely what
Thomas Kuhn [1962] would have called normal science within the paradigm. The
same is true of Darwin’s [1839] second major piece of geologizing, the so-called
Parallel Roads of Glen Roy. Along the sides of one of the small valleys off the
Great Glen in Scotland, there are parallel tracks. What could have caused them?
General opinion was that they were not human-caused — old hunting tracks —
but natural, being the remains of shores of water once filling the glen. But where
is the water now?