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a theory. If the theory is proved wrong, it was not necessarily also altogether a bad theory. At least it will have stimulated new research, which adds to knowledge. That is why many theories, even if they fail, are said to be “heuristic”—they are good for the promotion of discovery. Incidentally, the source of the word eureka —“I have found it!”—descends from the legend of the Greek scientist Archimedes, who, while sitting in a public bath, imagined how to measure the density of an object regardless of its shape. Put it in water, measure its volume by the rise in the water level, and its weight by how fast it sinks in the water. The density is the amount of weight divided by the amount of volume. Archimedes is said to have then left the bath, running through the streets, hopefully in his robe, while shouting, Heurika! Specifically, he’d found how to determine whether a crown was pure gold. The pure substance has a higher density than gold mixed with silver, the lesser of the two noble metals. But of far greater importance, Archimedes had discovered how to measure the density of all solids regardless of their shape or composition.
    Now consider a much grander example of the scientific method. It has been commonly said, all the way back to the publication of Charles Darwin’s On the Origin of Species in 1859, that the evolution of living forms is just a theory, not a fact. What could have been said already from evidence in Darwin’s time, however, was that evolution is a fact, that it has occurred in at least some kinds of organisms some of the time. Today the evidence for evolution has been so convincingly documented in so many kinds of plants, fungi, animals, and microorganisms, and in such a great array of their hereditary traits, coming from every discipline of biology, all interlocking in their explanations and with no exception yet discovered, that evolution can be called confidently a fact. In Darwin’s time, the idea that the human species descended from early primate ancestors was a hypothesis. With massive fossil and genetic evidence behind it, that can now also be called a fact. What remains a theory still is that evolution occurs universally by natural selection, the differential survival and successful reproduction of some combinations of hereditary traits over others in breeding populations. This proposition has been tested so many times and in so many ways, it also is now close to deserved recognition as an established fact. Its implication has been and remains of enormous importance throughout biology.
    When a well-defined and precisely consistent process is observed, such as ions flowing in a magnetic field, a body moving in airless space, and the volume of a gas changing with temperature, the behavior can be precisely measured and mathematically defined as a law. Laws are more confidently sought in physics and chemistry, where they can be most easily extended and deepened by mathematical reasoning. Does biology also have laws?
    I have been so bold in recent years as to suggest that, yes, biology is ruled by two laws. The first is that all entities and processes of life are obedient to the laws of physics and chemistry. Although biologists themselves seldom speak of the connection, at least in such a manner, those working at the level of the molecule and the cell assume it to be true. No scientist of my acquaintance believes it worthwhile to search for what used to be called the élan vital, a physical force or energy unique to living organisms.
    The second law of biology, more tentative than the first, is that all evolution, beyond minor random perturbations due to high mutation rates and random fluctuations in the number of competing genes, is due to natural selection.
    A source of the ground strength of science are the connections made not only variously within physics, chemistry, and biology, but also among these primary disciplines. A very large question remains in science and