On the Origin of Species | Study Guide

Charles Darwin

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On the Origin of Species | Main Ideas


Natural Selection

While Charles Darwin does not exclude the possibility of alternative methods of species formation, he argues natural selection is the main mechanism by which new species are formed. Darwin bases his argument largely on observations of geologic and natural processes he found to be occurring in the natural world. For instance, he notes the similarity between fossil species and modern species found in the same location. Throughout this work, he argues the idea of independent creation fails to explain the natural processes occurring on the planet. In contrast, the theory of natural selection provides explanations for all of these processes. While Darwin is the first to fully articulate natural selection as a means to create new species, he fails to provide a mechanism for how this might occur. It was not until Gregor Mendel's work on genetic inheritance was rediscovered that a mechanism would be identified.

Common Ancestors

A natural extension of the theory of natural selection is the idea species, genera, and higher classes are defined by their common ancestors. These ancestral species explain why numerous species are similar to one another, why vestigial organs (body parts with no function) exist, and why island and mainland island species can have similar and distinct characteristics. This concept provides the basis for Darwin's argument the classification of organisms should be based on their ancestral relationships in a phylogenetic system. The idea of common ancestors is valid only if the slowness of species formation is accounted for, and recognizing the geological record is imperfect. These are both facts Darwin points out numerous times in his work. This concept laid the foundation for the phylogenetic system still in use today. However, it was also the source of much controversy. The logical extension laid out by Darwin in a later work, The Descent of Man, was that humans had also descended from a common ancestor. While this has since proven to be true (based on the work of anthropologists through the discovery of intermediate fossils connecting humans to an ancestral hominid species), at the time it was extremely controversial.


Charles Lyell's Principles of Geology largely informs Darwin's work. The principle of uniformitarianism stated the same principles that shaped the earth in antiquity still take place today. Darwin applied the geological principle to biological processes as well, specifically to his theory of natural selection. He argues the same processes that resulted in the formation of past species—seen in the fossil record—also result in the formation of species today. Using this method, Darwin can account for the slow change of species over time, rather than their sudden creation, either by catastrophic forces, or through the special act of a creator. He argues the sudden creation of some species rather than others—in contrast to the idea of slow change over time—does not explain the variation present in the natural world.

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