In the late 1830s, Darwin was surrounded by a number of British thinkers who provided information to correspond with his raw data from the Beagle voyage. Charles Lyell was the most important; Darwin had taken the first volume of his Principles of Geology along on his trip around the world. In this work, Lyell outlined his uniformitarian beliefs, in that changes were brought about by slow, constant processes over time. Based on this belief, the earth was much older than had originally been predicted (6000 years), and the same forces present at that time could be applied to the world today. From his observations in the Galápagos, Darwin was able to assert the validity of Lyell's theory and recognize the relative youth of the islands compared with mainland South America; thus, the large variety of life present struck him as even more fascinating.

         After his return from the Beagle, Darwin began to read extensively concerning domesticated plants and animals, and the ability of man to artificially select for or against certain characteristics in the breeding of a population. However, his confusion regarding how nature could utilize a similar means of selection was apparent until he decided to read the work Essay on the Principle of Population by the economist Thomas Robert Malthus. In this essay, Malthus described that in the case of overpopulation, humans would struggle to survive based on a lack of adequate food production. Thus, only those with certain characteristics (such as a reduced need for food) would move on. Suddenly, the idea of the evolution of species began to make sense; only those that could survive under a given set of conditions would produce offspring, while those that could not adapt would die out.

         In 1842, Darwin began to produce an abstract on the subject of natural selection, and expanded the treatise to a manuscript of 230 pages in 1844. At this point in his career, though, there is a gap; The Origin of Species was not published until 1859. Part of the reason was Darwin's illness, which he had contracted during his worldwide travels. Additionally, his father passed away in 1847; partially out of grief and partially due to physical weakness, Darwin retreated to an eight-year study of the Cirripedes, or barnacles. While many would question the time spent on such a subject, the work is regarded today as a prominent manuscript in that particular area of study, and the time lapse was used to regain his youthful strength and energy.

         "On occasion, and it was true of Darwin, a scientist concludes that in order to present his new ideas for debate in the most effective manner it is necessary to undertake a sweeping reconstruction of existing knowledge." (Ospovat 3) Thus, in 1854 Darwin began to prepare his manuscript on natural selection via an extensive examination of his pre-existing collection of notes, including those on the Galápagos. At this point, the importance of the islands came to light; in the descent of multiple species from a common ancestor, it was necessary for each variety to become "adapted to many and highly diversified places in the economy of nature." (Autobiography 72) Hence, the realization of the methodology of speciation of the finches became the classic example of Darwinian evolution. Darwin was still preparing his manuscript in the summer of 1858, when he received a copy of the essay On the Tendency of Varieties to Depart Indefinitely from the Original Type, which was written by Alfred Russel Wallace from Malaysia. Essentially containing the same ideas that Darwin had begun formulating twenty years before, Darwin rushed to joint-publish with Wallace in the Journal of the Proceedings of the Linnean Society. Thirteen months later, The Origin of Species was published.

         Possibly the most controversial book ever published, all 1250 copies of the first edition of The Origin of Species were sold on the very first day. In addition to the obvious conflict with the direct account of creation in the book of Genesis, controversy surrounded the work's classification of humans as animals subject to the same forces as the rest of the world's species. The increased role of chance in the development of species also angered some, including men of science like Louis Agassiz, who maintained his criticism of Darwinism until his death in 1873. The work was even controversial within Darwin's own family: his oldest daughter Henrietta threatened on legal grounds to stop publication of the work, as it (supposedly) did not accurately reflect her father's views. Even Robert FitzRoy could not agree with the evolutionary theories, and his inability to prevent Darwin from establishing such a line of thinking may have led to his suicide in 1865. Despite the opposition, however, the treatise began to be embraced by young professional scientists looking for not only a way to jump-start their careers, but also a way to distinguish between the conflict between certain aspects of science and religion. In addition, the rational line of thought involved began to appeal to a growing number of older scientists, and his fluid writing style made the book accessible for the general public. Thus, the fame of Charles Darwin was assured, and his theory of evolution by natural selection began to grow in popularity.