The Origin of Species · Chapter
Development and Embryology explained
Darwin explains how the principles of variation and inheritance at corresponding ages account for the similarities and differences observed in embryos and adults.
What happens
Darwin explains how the principles of variation and inheritance at corresponding ages account for the similarities and differences observed in embryos and adults. He uses examples from domestic breeds (pigeons, dogs, horses) to show that characteristic differences often appear later in life and are inherited at a corresponding age, which explains why embryos of different species resemble each other more closely than adults. He argues that community in embryonic structure reveals common descent, and that the embryo can be seen as a picture of the ancient progenitor of the group.
Themes in this chapter
Common Descent and the Tree of Life
Community in embryonic structure is presented as strong evidence for common descent, with the embryo revealing the ancient progenitor of the group.
Variation and Inheritance
The principle that variations appear at a not very early period of life and are inherited at a corresponding age is central to explaining embryological facts.
Characters to notice
- Charles Darwin
Author and narrator; presents the argument and evidence for embryological development as supporting common descent.
- Fritz Müller
Suggested that the absence of metamorphosis in terrestrial and freshwater animals is due to natural selection favoring earlier adult structure.
- Agassiz
Believed that ancient and extinct forms often resemble the embryos of existing species, a law Darwin discusses.
Key passages
“Thus, community in embryonic structure reveals community of descent; but dissimilarity in embryonic development does not prove discommunity of descent, for in one of two groups the developmental stages may have been suppressed, or may have been so greatly modified through adaptation to new habits of life as to be no longer recognisable.”
Similar embryos indicate a shared ancestor, but different embryos do not necessarily mean different ancestors, because development can be altered or lost.
Darwin clarifies that while embryonic similarity is strong evidence for common descent, dissimilarity does not disprove it.
“Embryology rises greatly in interest, when we look at the embryo as a picture, more or less obscured, of the progenitor, either in its adult or larval state, of all the members of the same great class.”
Embryology becomes fascinating when we see the embryo as a blurred image of the common ancestor of all species in a class.
Darwin emphasizes the value of embryology in revealing ancestral forms.