Short answers, pulled from the story.
On the 24th of November 1859, a small book titled On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life, was released into a world that was not ready for its contents. The first edition consisted of only 1,250 copies, yet every single one was sold out by the time the book was offered to booksellers on the 22nd of November, just two days before its official publication.
The theory of natural selection was not born in a single moment of inspiration but was forged over a period of twenty years of silence and solitary work. Darwin began his notebooks on transmutation of species in mid-July 1837, and by the 5th of September 1857, he had sent a detailed abstract of his ideas to Asa Gray, yet he did not publish his full theory until 1859.
The publication of Alfred Russel Wallace's paper in 1858, which described a similar mechanism of natural selection, finally forced Darwin's hand. Wallace, who was staying on the Maluku Islands, had sent Darwin a manuscript that described an evolutionary mechanism identical to Darwin's own, leading to a joint presentation of their papers at the Linnean Society on the 1st of July 1858.
The evidence that Darwin presented in his book was drawn from a wide range of fields, including biogeography, geology, and embryology. He noted that the species found in one area of a continent were more closely allied with species found in other regions of that same continent than to species found on other continents, and he discussed how a volcanic island formed a few hundred miles from a continent might be colonized by a few species from that continent.
One of the most significant objections was the lack of intermediate forms between closely related species, which Darwin attributed to the competition between different forms, combined with the small number of individuals of intermediate forms, often leading to the extinction of such forms. He also addressed the absence or rarity of transitional varieties in time by noting that the geological record was extremely imperfect.
Within two decades, there was widespread scientific agreement that evolution, with a branching pattern of common descent, had occurred, although scientists were slow to give natural selection the significance that Darwin thought appropriate. The book has since become the unifying concept of the life sciences, and its ideas have been applied to a wide range of fields, from genetics to ecology.