Charles Darwin’s On the Origin of Species belongs to that category of books that almost every educated person knows by title and subject but has never read. Yet probably few other books have had so powerful an influence on nineteenth and twentieth century thought. Darwin’s report on his biological investigations came to have far-reaching importance beyond the field of biology, for the evidence and implications he presented eventually influenced psychology, sociology, law, theology, educational theory, philosophy, literature, and other branches of intellectual endeavor.

The ideas in this work were not entirely new in Western culture, as Darwin himself realized. It was he, however, who gave theory a definitive form that caught the public’s attention, so that in the public mind his book and his name came to represent an empirical, positivistic approach to problems and their study.

Scholarly opinion is somewhat divided as to Darwin’s contribution to biological science. He built on the researches of his predecessors, as all scientists do, but he brought immense labor of his own to the topic. More than twenty years before the publication of On the Origin of Species, he had first contemplated the theory that species were not immutable. He had spent five years in scientific study as a naturalist on the voyage of HMS Beagle. During that time he had unprecedented opportunities to observe flora and fauna around the globe. Those observations led him to believe that species did change, and what he observed led him to see the probability of common descent for all living organisms. As early as 1837 he had begun a systematic study to determine whether such hypotheses were correct, and by 1842 he had a rough draft of his theory of evolution. Wishing to secure his data with optimally exhaustive investigations, he postponed publication. In 1858, a manuscript came to him from A. R. Wallace, who, working independently, had come to similar conclusions. Darwin thereupon felt compelled to publish his work, which he began to do in July, 1858, at a meeting of the Linnaean Society. On the Origin of Species appeared a little more than a year later. His later books—among them Variation of Animals and Plants Under Domestication (1868) and The Descent of Man (1871)—elaborated particular aspects of the general theory promulgated in On the Origin of Species.

The possibility of evolution of species goes back in the history of thought to classical times; even Aristotle hinted at it in his writings. Darwin opened his book with an account of previous thinking on the theory of evolution in which he outlined earlier statements, beginning with Georges-Louis Leclerc, Georges Buffon, in modern times, and noting such men as Jean-Baptiste Lamarck, Geoffroy Saint-Hilaire, Leopold von Buch, Robert Chambers, the author of Vestiges of the Natural History of Creation (1833), and others. In his introduction, Darwin also exercised care in warning the reader what to expect. He wrote:

I can here give only the general conclusions at which I have arrived, with a few facts in illustration, but which, I hope, in most cases will suffice. No one can feel more sensible than I do of the necessity of hereafter publishing in detail all the facts, with references, on which any conclusions have been grounded.

In his final chapter, entitled “Recapitulation and Conclusion,” Darwin declared that the book is “one long argument” in favor of the theory of mutability and evolution of species in the plant and animal worlds. He pointed out that all the evidence had not been gathered and that even the likelihood of someday gathering all the evidence was so slight as to be inconceivable. He pointed out that there have been too many gradations, especially among broken and failing groups of organisms, including those that have in past eras become extinct.

Darwin laid a vast amount of information before the readers of On the Origin of Species, of which the chapter headings give an indication. There are chapters on variation in nature and under domestication, on the struggle for existence, on natural selection, on the principles of variation, on instinct, on hybridism, and on the geographical distribution of flora and fauna. There are also chapters on various objections to the general theory of evolution, on the mutual affinities of organic beings, and on the imperfections of the geological record of the succession of organisms. The individual chapters, which are to some extent interdependent and therefore not suited for separate study, cast light on all aspects of the theory of progressive evolution of species. Darwin’s organization of his book is as complex as it is lucid, an example of scientific writing at its best.