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On the Origin of Species | Summary



Describing Natural Selection

Charles Darwin states the conclusions within this work are the result of observations he made during his travels as a naturalist aboard the HMS Beagle. He structures his argument by framing his ideas within the research of his predecessors and contemporaries, ending with the foundational thesis of the novel—species are modified through natural selection.

Darwin begins his argument with the premise that a large degree of the variability in species is due to humankind's "domestic production." He says species and varieties are ill defined, and natural selection occurs in wild species as well as domestic. He explains what he sees as the driving force behind the natural selection of wild animals—the struggle for existence. He defines natural selection as the preservation of adaptive traits at the expense of nonadaptive traits. In order to illustrate his ideas, Darwin refers to a drawing he uses to detail the evolution of organisms across space and time. After describing the theory of natural selection, he attempts to describe the "laws of variation," noting the role of the environment and sexual selection. However, he concludes little is known about these laws.

Addressing Oppositions

Darwin then addresses various objections against the theory of natural selection, such as the lack of transitional forms, difficulties of the emergence of unique forms, and the selection for complex organs. He also specifically addresses objections raised by specific scientists: paleontologist Heinrich Bronn, botanist Karl Wilhelm von Nägeli, and zoologist St. George Jackson Mivart, among others. From here he dedicates two chapters to a discussion of how natural selection informs not only physical characteristics, but also instinct, and why some of the interbreeding of two species will often produce infertile hybrids.

The Geological Record

Darwin moves on to a discussion of how natural selection is informed by the geological record. He addresses three important points for understanding natural selection using the geologic record: the record was formed over a great period of time; the record is imperfect due to the shifting of the crust and variations in how geological strata are formed; and not all animals are able to become fossils. These facts create biases in the record Darwin emphasizes must be accounted for. Even so, he finds the geologic record to be an apt record of transitional forms and of the slow change of species overtime.

Darwin then explains geological distribution, as it pertains to the extinct ancestors of many species and genera. He begins with three main points: the distribution of species across the globe cannot be accounted for when considering "climatal and other physical conditions" alone; the distribution of species is related to the presence of physical barriers; and there is a similarity of species across continents at different geographical points. These points refer both to terrestrial, oceanic, and island patterns.

Classification Based on Natural Selection

Darwin then transitions into a description of the flaws in the current species' classification system. He proposes the classification of organisms should be based on ancestral descent and sees this as the most obvious form of classification when considered part of the theory of natural selection. He ends the book by reiterating he does not find natural selection to be the only means by which variations in organisms occur. However, he posits it is the main form by which variation occurs. He notes the main reason why individuals may fail to consider the theory of natural selection is people are "slow in admitting great changes" when the process is not immediately apparent.

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