The powerful algorithm that has populated the earth with 10 million species, each occupying a different ecological niche, is an example of what computer scientists call “random generate and test.”

Start with the DNA alphabet, then blindly shuffle the letters to produce a kaleidoscope of living forms. The fittest, selected by the demands of the environment, will multiply and fill their habitats.

The Darwinian principle is also at work inside the body, though in very different ways. Through random variation and selection, the immune system spins out the endless diversity of antibodies that it uses to stop microscopic invaders.

But cancer also thrives through this evolutionary imperative as, mutation by mutation, a normal human cell transforms into a deadly tumor, which becomes fitter and fitter at the expense of its host. Among the advantages it evolves is the ability to outwit our immunological defenses.