National Geographic : 1949 Jul
Shells Take You Over World Horizons The chimney of the Chateau of Blois is wreathed with scallops. The volute or scroll was used by the Greeks in their Corinthian and Ionic capitals. Famous at Blois is the double winding staircase, which reproduces with mathematical precision the double curves in the columella of certain turbinate shells. While the artist finds in shells rousing ex amples of the dynamic spiral, it would be shortsighted to assert that shells are the only source and origin of this spiral. You can see the same curves, with the identical propor tions, in the florets of a sunflower, in the scales of a pine cone, in the leaves of an artichoke, in the horns of the Rocky Mountain goat, in the cyme of forget-me-nots, in fern croziers, and in elephants' tusks. The lowly mollusk simply uses a funda mental law of growth, together with economy of structure, for building a strong house. The architect builds spiral staircases and columns, using a fundamental law of beauty and an economy of structure that are synonymous with the mollusk's law of growth. The curve of the shell is one of the simplest of all known curves. Its proportions may be defined by a mathematical formula. Its dis cipline is rigid. The diameter of the coiled tube will grow in exact proportion to its length. You can see a beautiful diagram of this fact when you refer to the picture of the cham bered nautilus (page 65). The control of its proportions is so perfect that each new coil is exactly three times the width of the coil preceding it. Shell Growth Mathematically Exact Another type of shell may increase at a much slower rate. For example, one terebra may increase the diameter of its coil at the rate of one and one-quarter times at each complete turn. At the other extreme, the abalone (page 56) multiplies its coil diameter by ten at each complete revolution. This is purely a mathematical proportion, because the abalone curve widens so fast that the shell never gets around itself. It is merely a short segment of a wide spiral. This type of spiral has a remarkable prop erty. It can increase by growing at one end only and always retain without change the form of the entire figure. A little shell grows into a big one and both look the same. This is a marvelous fact, because the shell grows only at one terminal end. Compare this with most growth. When a boy grows into a man, he grows proportion ately all over. How could a shell grow all over? The animal builds a house of which each increment is forever dead, rigid material. Yet this structure, added to at one end only, continues to grow as if by magic, appearing to become larger all over!* The diversity, which is so bewildering as we look over a collection of shells, is due to the endless combinations of this dynamic spiral. For instance, the growing edge, or aperture, of the shell may be round, triangular, oval, wide or narrow ellipses, or countless other shapes. The shape of this growing edge will vary the form of the completed shell. An additional variation is seen in the way this aperture-that is, the cross section of the shell tube-is set at various angles in relation to the axis of the shell. One angle will cause it to go round and round, approximating a coiled rope. Another will pull out the coil into a long, steep corkscrew. Still another influence comes from the va rious velocities of growth in relation to angle of rotation. These factors of the figure of the growing lip, its angle, rate of growth in relation to twist, and so on, may be likened to the few notes of music by combinations of which all the countless varieties of tunes are produced. Thus the tens of thousands of shell forms are rooted in the simplicity of a curve with a simple formula governing its proportions. * For a fuller description of this spiral see On Growth and Form, by D'Arcy W. Thompson. Exploring in Our Color Plates THE 32 PAGES of color illustrations in this issue take the reader on a world-wide hunt for shells, from the cold shores of New England to the humid beaches of the Tropics. Color Page 41 The rock-bound coast of New England, with its intervening beaches, teems with mollusks offer ing an opportunity for many thousands of summer visitors to become acquainted with the beauty of shells. In this region the tides have a great rise and fall, sometimes 10 feet or more. When the At lantic breakers draw back they uncover jungles of seaweed and limpid pools among the rocks. Color Page 42 All the shells on page 42 are from the collec tion of the Boston Museum of Science. The Thais in the upper corners is a sharp-pointed snail with colorful stripes and a marblelike texture. Just below these is the familiar Periwinkle, one of the commonest shells on the New England coast. The larger round snails are Moon Shells.