Charles Darwin may be safely called the greatest biologist of the 19th century, but his ideas about evolution were neither entirely new nor complete. Indeed, it is one of the famous stories of science that Darwin’s theory of evolution was made public in 1858 only because Alfred Wallace had reached the same conclusions as Darwin and mailed these to Darwin himself. Ever generous, Darwin arranged to present his work and Wallace’s in the same session, so that neither would have priority, although Darwin had formulated the basic ideas many years earlier. It is unlikely, however, that evolution would have had the immediate impact that it did if the theory had been put forth by Wallace alone. Darwin was already well known for his account of the voyage of the Beagle and for his scientific work on that voyage. Furthermore, Darwin’s On the Origin of Species by Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life –– better known simply as The Origin of Species –– published November 24, 1859, is one long, persuasive argument for the theory, which convinced many biologists of its truth through many examples.

But before there was The Origin of Species or Wallace’s conclusions, biology had to progress to a point where the existence of evolution seemed reasonable and therefore deserving of a scientific explanation. For one thing, there was no point to explaining the origin of species until a species was defined. The modern concept of species began with John Ray’s 1686 definition, based on common descent (a point of view revived more recently as cladism). The definition was put into the form we still teach today by comte de Buffon in 1749: A species is a group of interbreeding individuals who cannot breed successfully outside the group.

In Darwin’s time, scientists considered species to be immutable. Even though dogs or cabbages could be bred to many different shapes, for example, no one could develop a breed of dog that is of the cat species or breed a cabbage into an onion. Ancient writers who had a less clear concept of species believed that wheat seed could sometimes sprout as millet.

Most scientists of the 17th and 18th centuries also believed in a doctrine called preformation. This concept existed in two much argued versions: In one, the adult existed in a tiny version, or was preformed, in the egg; in the other, the adult was preformed in the sperm. Preformation would seem to preclude one species giving birth to a member of another species. As the science of embryology became more precise, however, the doctrine of preformation –– a potential barrier to a theory of evolution –– was replaced by the recognition of epigenesis, the development of the embryo from undifferentiated tissues.

Another factor in creating the climate for evolution was the emerging understanding of fossils as the remains of living creatures and the realization that many fossils were of species that no longer existed.

Late in the 18th century, yet another precondition was met –– an understanding that the time required for evolution to occur was present in Earth’s history. Both Abraham Gottlob Werner and James Hutton proposed processes for the development of geological features that required great stretches of time for Earth to have existed. Large amounts of time meant that evolution could take place so slowly that it would not be observed among living creatures. This was a necessary precondition, since evolution at the species level is not observable during a lifetime.

With that background, many scientists and near scientists of the first half of the 19th century believed in some form of evolution. Lamarck, one of the first evolutionists, thought, correctly, that the environment causes species to evolve. But he also thought that acquired characteristics of the parent could be inherited by the offspring; this was greeted with much disbelief in Lamarck’s day and is not accepted today. Charles Darwin’s own grandfather, Erasmus Darwin (one of the near scientists), offered a general theory of evolution by generation; it included not only living organisms but also Earth itself.

In England the most influential evolutionist before Darwin was Robert Chambers. His 1844 Vestiges of Creation influenced both Darwin and Wallace. Indeed, Wallace recognized that Chambers had the fact of evolution but did not have an explanation for it, setting Wallace on his own search for an explanation. Chambers was not a careful scientist, however, and his errors put off many professional scientists.

After publication of Origin of Species many –– although not all –– scientists were converted both to evolution and to the Darwin-Wallace theory of natural selection. The public was less convinced at first, especially those who took Darwinism to be atheistic (which Darwin did not consider it for himself to be). Many people also rejected Darwin’s explicitly stated idea that humans and great apes share an ancestor. Two notable opponents of natural selection were Samuel Butler, the author of The Way of All Flesh, and George Bernard Shaw. Butler wanted to return to the ideas of Erasmus Darwin, and Shaw believed in a mystical life force. In the end, however, the theory of evolution and natural selection won all except those who continued to think that it contradicts the Biblical account of Creation.

The theory of evolution did not arise fully formed in 1859, however. Modern evolutionists continue to improve on Darwin and Wallace, and a few have even rejected their ideas. New ideas have emerged, such as the concept of mutation and the laws of heredity. These new ideas combined with natural selection to form the basis for neo-Darwinism, the prevalent theory of evolution during most of the 20th century. Since the 1940s, a theory of evolution has arisen that calls for rapid bursts of evolutionary change between long periods of species stability. This theory, “punctuated evolution,” has most recently been articulated by Stephen Jay Gould and Niles Eldredge. The discovery of how heredity works via DNA has more exactly explained the mechanisms of the theory of evolution as well.