The Origin of Species · Chapter
XIV: Mutual Affinities of Organic Beings: Morphology—Embryology—Rudimentary Organs—Classification explained
Darwin explains that the natural classification of organic beings into groups under groups is not arbitrary but reflects genealogical descent with modification.
What happens
Darwin explains that the natural classification of organic beings into groups under groups is not arbitrary but reflects genealogical descent with modification. He argues that natural selection, divergence of character, and extinction produce the hierarchical arrangement seen in nature. He discusses morphology, embryology, and rudimentary organs as evidence for common descent, and emphasizes that the Natural System is essentially genealogical.
Themes in this chapter
Common Descent and the Tree of Life
Central theme: Darwin argues that all organic beings are descended from common parents, and that classification should reflect genealogical relationships.
Natural Selection and the Struggle for Existence
Natural selection is presented as the driving force behind divergence of character and the hierarchical arrangement of groups.
Adaptation and the Utilitarian Doctrine
Discussed in the context of analogical vs. homologous characters and the use of rudimentary organs in classification.
Characters to notice
- Milne Edwards
Cited for his emphasis on the importance of looking to types in classification.
- Professor Haeckel
Mentioned for his work on phylogeny and use of embryological characters in classification.
- Professor Flower
Quoted regarding the conformity of limb structure in marsupials, suggesting inheritance from a common ancestor.
Key passages
“We use the element of descent in classing the individuals of both sexes and of all ages under one species, although they may have but few characters in common; we use descent in classing acknowledged varieties, however different they may be from their parents; and I believe that this element of descent is the hidden bond of connection which naturalists have sought under the term of the Natural System.”
Descent is the hidden link that naturalists have been looking for in the Natural System; we already use it to classify individuals and varieties.
Darwin asserts that genealogical descent is the foundation of natural classification.
“What can be more curious than that the hand of a man, formed for grasping, that of a mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pattern, and should include similar bones, in the same relative positions?”
It is remarkable that such different limbs as a human hand, a mole's digging claw, a horse's leg, a porpoise's flipper, and a bat's wing all share the same bone structure.
Darwin uses homologous structures as evidence for common descent.
“On the principle of successive slight variations, not necessarily or generally supervening at a very early period of life, and being inherited at a corresponding period, we can understand the leading facts in embryology; namely, the close resemblance in the individual embryo of the parts which are homologous, and which when matured become widely different in structure and function.”
Embryonic similarities are explained by slight variations inherited at corresponding life stages, even though adult forms may differ greatly.
Darwin links embryology to his theory of descent with modification.