The Origin of Species was published in 1859. In retrospect that date can perhaps be seen as the beginning of the modern world. Darwin now has the status of a founding prophet. His book and all that followed from it led the way to a new set of values - the values of the twentieth century - and to profound social and political changes in which science and medicine inherited much of the previous influence of the Church.
Darwin is now often regarded as the discoverer of evolution. He was not. Evolution - or transmutation as it was then usually called - was already a familiar idea to educated people before the Origin was written. It made sense if one looked at the fossils in the rocks and it had received wide publicity in the Vestiges. What Darwin did was to make evolution scientifically respectable, by providing a plausible explanation for it - natural selection - which gave scientists the strength and confidence to decisively challenge the biblical doctrine of divinely created species. The full title of his book was "On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life." It was a book about natural selection. Evolution followed naturally; it was already a familiar idea to his readers; but their acceptance of it had been obstructed by the Church's insistence that species were immutable and only divine design, at the time of a one-off creation, could explain the perfect adaptations of the natural world.
Darwinism, as it very soon came to be called, had three components which are often confused but which need to be distinguished from one another: the theory of evolution, which convincingly claimed that all living things have developed from common origins; the theory of natural selection, which suggested an automatic mechanism to explain this development of life; and a new conception of man, as an animal related to other animals, and particularly to the apes, which followed from evolution but had a special significance of its own. Darwin did not write a book about evolution; he left that to others. His first great book tackled natural selection; his second, the Descent of Man; these were the two difficult issues; they still are.
The basis of the theory of natural selection was not experimental evidence or predictive success but simple logic. Give the very high rate of pre-reproductive mortality in all wild species, given the known incidence of variation, logic suggested that favourable variations would persist and be reproduced in subsequent generations, and this effect would be cumulative. This logical argument was supported by the analogy of the selective processes by which mankind has developed new varieties (but not species) of domesticated animals and cultivated plants. But there was nothing in the theory to say that it was the only thing which had caused evolution to take place. Indeed experience of human theories in general suggests that this is unlikely. Life is complex. For Darwin himself, natural selection was not an exclusive explanation for evolution, still less was it a dogma. It was the alternative argument he needed to defeat the argument of divine design. It was only the twentieth century which made natural selection into a dogma.
This website takes the basic model of evolution (or as it was called before Darwin "transmutation") for granted. It looks at the historical background of the Darwinian revolution, relates it to nineteenth century attitudes to religious issues, examines the meaning of natural selection and suggests how it relates to what Darwin called "the origin of species" and we call speciation. Throughout, I have used non-technical language wherever possible.