Leonardo’s Mountain of Clams and the Diet of Worms by Stephen Jay Gould Read Free Book Online Page A

scholars recognized—and had known since antiquity—that the earth was spherical and not flat.)
    But the heavy earth is far from homogeneous. The interior of our planet is a complexly marbled mass composedof solid earth, liquid water running through veins in the rocks, and even air, where water has hollowed out caverns in the rocks. Therefore, as a result of this unequal distribution of earth, one hemisphere must always be heavier than the other.
    Now the planet also has a center of mass (called by Leonardo, in a terminology that we would not use today, a “center of gravity”). On a homogeneousplanet, this “center of gravity” will coincide with the geometric center of the world. But on our actual planet, with one hemisphere heavier than the other, the “center of gravity” will lie below the geometric center and within the heavier hemisphere. The planet, as a living body seeking balance, must strive to bring the center of gravity closer to the geometric center. The earth pursues thisgoal in a manner known from time immemorial to all riders on seesaws (the Leicester Codex contains a picture of such a seesaw, albeit for a different purpose). To balance a seesaw, the heavier person must move toward the fulcrum at the center, while the lighter person must move farther away. In exactly the same manner, the solid masses of the heavier hemisphere must sink toward the center of the world,while the rocks of the lighter hemisphere must rise. The emergence of mountains from the seas, and the consequent placement of marine fossils on high hills, records this rising of land in the earth’s lighter hemisphere.
    Leonardo succinctly describes the general process in Manuscript F (in the Institut de France):
    Because the center of the natural gravity of the earth ought to be in the centerof the world, the earth is always growing lighter in some part, and the part that becomes lighter pushes upwards, and submerges as much of the opposite part as is necessary for it to join the center of its aforesaid gravity to the center of the world; and the sphere of the water keeps its surface steadily equidistant from the center of the world.
    Leonardo must then find a general mechanismfor ensuring planetary balance by lightening one hemisphere, while making the other heavier—and he succeeds with two principles, both based on erosion by water: one mode operating in the earth’s interior, the other at the earth’s surface. In the interior, internal veins of water carve out caverns, which eventually become unstable. Their tops finally collapse, and enormous blocks of rock fall all theway to the center of the world. There, the blocks distribute themselves about the center with approximately equal volume in each hemisphere—thus adding weight to one hemisphere and subtracting from the other (for the entire block had previously resided in one hemisphere alone). Leonardo includes a striking illustration of this process in the Leicester Codex—although scholars have failed to recognizethe meaning of this figure—showing a fallen block as a large arch neatly draped about the center of the world (see accompanying figure). In describing this internal mechanism in the Leicester Codex, Leonardo explicitly cites the rising of fossiliferous strata as a consequence:
    The fact of the summits of the mountains projecting so far above the watery sphere may be due to the fact that a verylarge space of the earth which was filled with water, that is the immense cavern, must have fallen in a considerable distance from its vault towards the center of the world, finding itself pierced by the course of the springs, which continually wear away the spot through which they pass . . . Now this great mass has the power of falling . . . It balances itself with equal opposing weights roundthe center of the world, and lightens the earth from which it is divided; and it [the lightened earth] removed itself immediately from the center of the world and rose to the