Darwin's evidence for evolution: Rudimentary ("vestigial") organs
I. Unusable organs
A. Some "organs" have no apparent nor predictable function (based on the behavior of the species (or sex) in which they are found, or the anatomy of the particular feature): e.g., male mammae, rudiments of pelvis and hind limbs in snakes, wings on many flightless birds.
B. Organs that serve dual purposes (such as flowers: production of gametes and attraction of pollinators) may be functional in one part and rudimentary in the other.
C. Rudimentary organs can often be detected in the embryo, but are lost later during development (e.g., teeth in the upper jaws of embryos of whales and ruminants).
II. How do the other hypotheses "explain" rudimentary organs?
(That they are there "for the sake of symmetry" or to "complete the scheme of nature" are not explanatory statements.)
III. How does descent explain their existence?
A. Each feature in an organism tends to be inherited, even if they are not used by descendents.
B. Explains why such features are of great importance in classification (which thereby reflect inheritance)
C. Although Darwin suggests "Disuse" as the main mechanism of the degradation of rudimentary organs, he also invokes Natural Selection as a possible mechanism:
An organ that is useful under some circumstances may be injurious under others (e.g., insects with wings on small exposed islands often have fused elytra).
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