The prevailing view in British scientific and theological communities when Charles Darwin began his inquiries was that species were fixed, immutable types, created by God. The complex and intricate design of organisms was taken as evidence of creation by a Designer. William Paley, a natural theologian, developed the argument, which Darwin studied during his education at Cambridge.

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However, the idea that species are not immutable but can be transmuted had many advocates before Darwin. They can be found as far back as the pre-Socratics, but the view was greatly advanced in early nineteenth century scientific discussions by Jean-Baptiste-Pierre-Antoine de Monet de Lamarck in his Philosophie zoologique: Ou, Exposition des considérations relative à l’histoire naturelle des animaux (1809; Zoological Philosophy: An Exposition with Regard to the Natural History of Animals, 1914). What Lamarck and other early evolutionary theorists lacked was an adequate mechanism to explain how evolution occurred.

Charles Lyell, in his Principles of Geology (1830-1833), argued that geological formations had been shaped by the same physical forces that acted on them today, a doctrine known as uniformitarianism. Although Lyell was not a transmutationist until after On the Origin of Species by Means of Natural Selection, he did acknowledge in the volumes that Darwin read that changing environments could lead species to undergo accommodations, migrations, and extinctions.

By Darwin’s own testimony, the idea of natural selection as the mechanism of evolutionary change occurred to him upon rereading in 1838 An Essay on the Principle of Population, As It Affects the Future Improvement of Society (1798) by Thomas Malthus, a clergyman and political economist. “The doctrine of Malthus applied with manifold force to the whole animal and vegetable kingdoms” is how Darwin described his theory of descent through modification by natural selection. Malthus asserted that because populations grow geometrically while crops grow arithmetically, there would be a struggle among creatures for survival. The fittest would prevail.

The thesis of On the Origin of Species by Means of Natural Selection is that species are created not by special acts of a Designer but by the process of evolution through natural selection. In each generation, more organisms are produced than can live to the point of reproducing. Those organisms that possess even a slight advantage in adapting to the demands of climate, food supply, and competition over fellow organisms (and other species of organisms) will have higher reproduction rates, will survive long enough to reproduce, and thus will pass their favored structures and instincts on to their offspring. Adaptations will accumulate over generations; internal organs, external structures, and behavior will be altered. New varieties, “incipient species” as Darwin called them, will appear and, over many generations, will come to be recognized as new species. Darwin does not explain the origin of life by his theory but rather how the diversity and complexity that is life came to be.

To speak persuasively for his theory, Darwin constructed what he described as “one long argument.” The book consists of arguments and evidence. The evidence is numerous illustrations of geological and biological facts about rocks, animals, and plants. The arguments and inferences support the superiority of natural selection and other natural causes of evolution as opposed to special creation as an explanation of the facts. The facts were not just those of the vast diversity of species, but also the innumerable details of their structures, behaviors, development, and interrelationships—facts that Darwin’s theory could integrate in a logical manner.

Darwin begins by discussing the variability that...