In The Seashell on the Mountaintop, Alan Cutler recounts biographical details of Niels Stenson (known as Nicholas Steno). Steno was a Danish anatomist in the 1600s who was known for his brilliant dissections. And he solved, to his satisfaction, one of the controversies of the day: was the heart a mere pump or was it the central source of the body’s heat? He examined the human heart and observed that it is made of muscle fibres exactly like those in the arms and legs. He concluded that it was not a heat engine, but a pump. Steno was invited to Florence and spent much of his career there.
In those days, the whole history of the earth was considered to be brief and static: the Earth was as God had created it. Mountains were regarded with dread as ruins. Shells were thought to be stones that had formed in place by God’s whim or as dim shadows of Platonic ideal forms. Steno’s acute observations and reasoning came to his aid again in forming a theory of the origins of fossil shells. For one thing, if sediments molded around the shells, the shells must have been hard and the sediments soft and later in forming.
Steno’s observations of soil and water led him to three geologic principles: that the oldest layers in a landscape are laid down first; that water deposits sediments throughout its extent; and that when sediments are laid down, they are horizontal. Within a layer, or stratum, of water-deposited sediments the larger pieces sand to the bottom first. So even when a layer had been tilted or overturned he could tell which side had been the top. Thus he perceived that the earth had a history that required ages of time; and that changes had occurred during that history.
He never completed his Big Book on the earth’s history; but he did write a preliminary sketch of his ideas, a mere 78 pages long with a few diagrams, in the 1660s. It took more than sixty years of controversy and counterclaims for his ideas to become established, but they were never entirely lost; and in 1720 another book was published by Reaumur, describing four ages in the earth’s history as visible in Tuscany. Those ages are still being used. Of course, it took another 200 years to assign time scales and to firm up the mechanism; but the idea of a changing earth had taken hold, at least in some minds, with Steno.
But the idea that the earth has a history and is formed by natural forces; or indeed that fossil seashells were formed by living creatures, certainly did not have a smooth ride. This is worth reading for anyone interested in the history of science or of ideas or in the current debate about evolution vs. creationism. In fact, a lot of the arguments that were used and discarded 300 years ago are still being trotted out now by some writers.