The Origin of Species · Chapter
On the Affinities of Extinct Species to Each Other, and to Living Forms explained
Darwin explains how extinct species, when combined with living forms, fill gaps between existing groups, supporting the theory of descent with modification.
What happens
Darwin explains how extinct species, when combined with living forms, fill gaps between existing groups, supporting the theory of descent with modification. He uses a diagram to illustrate how ancient forms are intermediate between their ancestors and descendants, and cites examples from paleontology, such as the Macrauchenia, Hipparion, and Archeopteryx, to show that older fossils often bridge distinct groups. He also addresses objections and emphasizes that the imperfection of the geological record limits our ability to see all transitional links.
Themes in this chapter
Common Descent and the Tree of Life
Central theme: extinct and living species are linked by descent from common ancestors, forming a branching tree of life.
Extinction and the Succession of Forms
Explored through the idea that extinct forms fill gaps between living groups, and that faunas of successive geological periods are intermediate in character.
Imperfection of the Geological Record
Acknowledged as a limitation: the record is broken and incomplete, so we cannot expect to find all transitional links.
Characters to notice
- Charles Darwin
Author presenting the theory of descent with modification to explain affinities between extinct and living species.
- Professor Owen
Cited for his concept of 'generalised forms' and for reclassifying Pachyderms and Ruminants based on fossil links.
- Agassiz
Mentioned for his terms 'prophetic or synthetic types' referring to intermediate fossil forms.
- Dr. Falconer
Referenced for his arrangement of mastodon and elephant species, showing that extreme forms are not always oldest or most recent.
- Professor Huxley
Noted for considering Zeuglodon and Squalodon as cetaceans connecting to aquatic carnivora, and for bridging birds and reptiles.
- M. Barrande
Cited as a high authority asserting that paleozoic groups were less distinctly separated than modern ones.
Key passages
“Thus, on the theory of descent with modification, the main facts with respect to the mutual affinities of the extinct forms of life to each other and to living forms, are explained in a satisfactory manner. And they are wholly inexplicable on any other view.”
Darwin asserts that only his theory of descent with modification can account for the observed relationships between extinct and living species, and that no alternative explanation works.
Key summary statement of the chapter's argument.
“If we confine our attention either to the living or to the extinct species of the same class, the series is far less perfect than if we combine both into one general system.”
Looking at only living or only extinct species gives an incomplete picture; combining both reveals a more continuous series of forms.
Highlights the importance of fossils in completing the natural system.
“It is a common belief that the more ancient a form is, by so much the more it tends to connect by some of its characters groups now widely separated from each other.”
Older fossils are often more intermediate in character, linking groups that are now very distinct.
General principle about the intermediate nature of ancient forms.