Darwin begins by explaining how he formed his theory of natural selection. As a naturalist aboard the H.M.S. Beagle (1831–1836), he observed, and recorded data on, the various species in South America and its neighboring islands. These observations led him to think more about the mystery of the origin of species, and upon his return to Great Britain in 1837, he started sketching out his theory. He had been working on synthesizing the arguments in his manuscript since that time. Although his work was not quite done, Darwin explains that two factors were forcing him to publish this abstract of his theory now: his failing health and the fact that naturalist Alfred Russel Wallace had come near to discovering the very theory that Darwin had been working to complete. Darwin recognized that subsequent publications would be necessary to fill in the details missing from the conclusions he draws here.
Darwin also notes that he is certainly not the first naturalist to observe similarities between organic species and conclude that species were not been created independently, but rather descended from one another. However, while contemporary naturalists assumed that external conditions, such as climate and food, are the sole cause of variation in species, Darwin argues that something more than the environment must be at work in creating beings so perfect in their structure and ability to adapt. While the anonymous author of Vestiges of Creation presents a theory of descent, he fails to account for the remarkable co-adaptation of beings to one another and to their specific physical conditions. Darwin argues that his theory, developed by a careful study of domesticated animals and cultivated plants, as well as natural organisms, will provide a better explanation for the origin of species.
Finally, Darwin provides an overview of the arguments in his origin theory. He previews the concept of natural selection, which states that beings with advantageous variations will be “naturally selected” over others and have the best chance of survival. He argues that his theory will shed light not only on the origin of species but also on the future successes and modifications of all the earth’s inhabitants. Species, he argues, are susceptible to change, and it is natural selection that causes these changes to occur.
In the introduction, Darwin introduces a few of his recurring themes. Foremost among them is the importance of using scientific reasoning to understand nature. Darwin is careful to phrase his methodology in terms of scientific experimentation. He grounds his theory in the two key principles of scientific research: experimentation and observation. He argues that the origin of species can be understood only through careful observation, the kind he did aboard the H.M.S. Beagle and later in his study of domesticated plants and animals. Darwin’s research method relies on inductive reasoning, which means the use of specific examples to draw broader conclusions. Implicitly, this research method gives authority to science and its power of knowledge and discovery.
Although Darwin applauds scientific methodology, he also hints at the uncertainties that plague scientific discovery. The tension between scientific authority and scientific uncertainty comes up often in Darwin’s writings. His awareness of the theoretical nature of his natural selection argument prevents him from presenting his theory with total confidence. By expressing doubt about the completeness of his own theory, Darwin allows us to question his conclusions. And by expressing doubt about other people’s scientific theories, Darwin admits to the uncertainty inherent in any scientific theory or process. Although science, according to Darwin, has the power to discover the origin of species, this power will never be absolute.
Another pioneering scientist named Alfred Russel Wallace devised his own theory of evolution, which closely resembles Darwin’s. This coexisting theory, outlined in Wallace’s essay On the Tendency of Varieties to Depart Indefinitely From the Original Type, prompted one of the main controversies surrounding Darwin’s work and consequent celebrity. Because Darwin had read Wallace’s manuscript prior to publishing the Origin of Species, many accused him of stealing Wallace’s theory. Darwin was urged to rush his findings into publication to beat Wallace to the punch and, as a result, received the credit from the scientific community for the theory of natural selection.