In his preface to the third edition of the work, Darwin notes that the French naturalist Jean-Baptiste de Lamarck had suggested in 1815 that the changes in organic life visible over the eras happen in accordance with biological laws, rather than as the result of some chance intervention by supernatural forces. Other precursors receive due acknowledgement too, particularly a pair of British researchers, Dr William Wells (writing in 1813) and Patrick Matthew (1831).

Although all these writers, and others, seemed to be pointing the way towards the theory of natural selection, none of them had quite put it on a systematic footing. Some who held the view that modern life-forms had developed from different types also took the view that that development had probably now stopped, and that, say, the elephant of the mid-19th century would represent the way that all elephants would now look forever more.

What we now refer to as the theory of evolution is not the work of any one man, but of many researchers in the field of natural history. Darwin remarks how curious it is that certain theoretical breakthroughs should be made at the same time by different theorists working in different countries, as happened in the 1790s, for example, and again in the period leading up to the writing of his own book.

Darwin’s great colleague in the enterprise was Alfred Russel Wallace, with whom he gave a paper on the origin of species to the Linnaean Society in London in July 1858. It was only when Wallace sent Darwin a memoir in that year, outlining his own ideas about evolution, that Darwin was emboldened to go public with conclusions that he had himself been coming to over the course of the preceding 15 years.

Survival of the fittest

The Origin Of Species establishes the principle that all forms of life are in a state of constant flux. They adapt to their environments, and those individual members of each species that adapt the best are the ones that become the model for succeeding generations. The sabre-toothed tiger developed massive incisors so that it could tear its prey apart more easily, and help itself to survive. The hairstreak butterfly has false antennae and spots on the undersides of its wings, so that birds preying on it and going for its head will attack the wrong end, allowing the butterfly time to get away. Disguise can assist the predator as well as the prey, which is why the polar bear developed a white coat so that it can’t easily be seen by its quarry in the Arctic environment.

If all this proved controversial enough for those who believe that life was created fully formed by an all-powerful deity, what was especially unpalatable was that this theory had logically to be extended to human life too. Perhaps surprisingly, apes and humans are hardly mentioned in the book, but the inference would follow anyway that we are descended from higher primates who came down from the trees and learned how to walk upright. This was their particular strategy in the struggle for life.

Darwin nudges us towards this thought in his closing passage: “Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows.” Outright elaboration of this had to wait until Darwin’s late work The Descent Of Man (1871), when it was still held to be tantamount to blasphemy.

The doctrine of evolution – for a doctrine is how it came to be seen – became known as the “survival of the fittest”, with the implication that nature is a cruel and ruthless force that is content to let the weakest perish. “Fittest” was misunderstood, and to some extent still is, to mean exclusively “strongest” (rather in the way that we get fit if we put in lots of time at the gym), whereas Darwin intends it in the sense of “most suitable”, or “most appropriate”. That butterfly that manages to fool its predator isn’t physically stronger for having developed a confusing appearance. Its confusing appearance makes it more able, literally fits it better, for survival in a hostile environment.

Hovering over the whole book is the awkward presence of God the creator, who is hardly named but whom the author feels bound to acknowledge, like the uninvited guest at a polite soirée. Among the book’s rebukes to creationism is the argument that evolution proceeds by very gradual steps with no great and sudden changes. Thus, there is a huge diversity of species, but hardly any major innovation within each one. If the species were independently created, there would be nothing to stop sudden unexpected changes from taking place.

Although evolutionary theory has lost some ground among scientists in the United States, and here too in recent years, as the theory of intelligent design has gathered supporters, it remains one of the great breakthroughs of 19th-century science. That its principal exponent may well have died an atheist as the result of his findings has not stopped others from being able to reconcile a belief in God with the evidence all around us that life has evolved, and continues to do so.