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nature is so strong that it was not recognised as an illusion until the nineteenth century. In Europe at the time, there was a pretty much universal belief that living things had been created and put on the Earth in their present forms by a Supreme Being. The scientists of the day were mostly religious and the very last thing they wanted to do was question such an idea and bring down on themselves the wrath of the Church. However, scientists have no choice but to go with the evidence. And the evidence was overwhelming that the bewildering diversity of life on Earth – everything from bacteria to blue whales, fungi to flying foxes, gorillas to giant sequoias – is the consequence of a purely natural mechanism.
    An important clue came from fossils. These appeared to be the relics of ancient creatures, buried by sediments settling to the bottom of lakes and seas, and somehow – nobody knew exactly – turned to stone. Fossils reveal that the creatures that inhabit theEarth today are not the same as the ones that once inhabited the planet. Some ancient creatures such as the dinosaurs have dis appeared entirely whereas other vanished creatures appear related to creatures today. The simplest, most primitive creatures appear fossilised in the oldest sediments. As the rock layers became progressively younger, the fossils became ever more complex and sophisticated.
    The idea dawned on scientists that the fossil record was a
time sequence
of life on Earth. It was telling us that, over vast tracts of time, species of creatures gradually change their appearance, morphing from one into another and eventually becoming the species we see around us today. Life was not created on Day One by a Creator, remaining frozen and static forever after. Instead, it has evolved, gradually, from simpler ancestral forms.
    Such evolution explains the striking similarities between creatures living today such as humans and chimpanzees. If all life on Earth descended from a common ancestor in the distant past, it is obvious that all creatures today are related. But what drives evolution? What causes species to change over the generations? And how have all creatures ended up doing what they do so incredibly well that they give every appearance of being designed? The man who found the answer was Charles Darwin.
    Darwin embarked on HMS
Beagle
in 1831. During his five years as the ship’s naturalist, he made some tantalising observations of the biological world. On the Galápagos archipelago, 1,000 kilometres off the west coast of South America, the finches on different islands had different-shaped beaks. In all cases, the beaks were perfectly shaped for exploiting the nuts available locally: short, stubby beaks for cracking open big nuts, slender beaks for less formidable seeds.
    An explanation began to form in Darwin’s mind when he also noticed that the birds and animals on the Galápagos were but slight variants of those common on the mainland of South America. The Galápagos, it seemed, had been colonised by creatures from the nearby continent. Some birds and animals from South America that could easily have made a living on the Galápagos were conspicuous by their absence. Only a small subset had made it across the ocean barrier on winds or mats of floating vegetation. It had been these hardy creatures that had radiated to fill all the empty niches – a single type of finch spreading to all islands and evolving beaks best suited to exploit the seeds found locally.
    Darwin was now in possession of new and important clues about evolution. But he did not know what was driving the changes in species – what was pushing each to an apparent perfect fit with its environment. Back in England in 1836, and still only twenty-seven, he sat down at his desk, laid out the facts he had collected before him, and began to think.
    Darwin was aware of one common way that creatures change their forms over the generations: by deliberate breeding. Plants and domestic