Darwin's evidence for evolution: Embryology
I. The "facts of embryology"
A. There is usually a difference in form between embryo and adult.
B. Serial structures (e.g., segments, limbs) are usually identical in the embryo, but specialized and diverged in the adult.
C. Different species in the same Class (e.g., mammals) often have very similar embryos, even if the adult forms are quite different.
D. Embryonic structure is unrelated to "conditions of existence", unless the embryo is active (e.g., feeding)
E. Embryos are sometimes more highly "organized" than adults (e.g., some parasitic forms).
II. The "principles" that explain these facts
A. Adult variations supervene at a rather late stage
B. Variations in adult features are inherited (expressed by progeny) at correspondingly late stages
C. These "principles" are generally but not universally true (an exception was the tumbler pigeon) when one looks at empirical evidence from domesticated varieties
III. How do these principles explain the facts?
A. Breeders select on adult characters, regardless of the juvenile characters
B. Given this, and the two principles, the young of a new variety and the parental stock will tend to be more similar than the adults
C. Darwin extrapolates this to larger groups: For example, the forelimbs might be legs in an ancestral species, but would be modified as flippers, arms, wings, etc. at a late stage in development; but the pattern in the embryonic stage would remain similar if not unchanged.
D. In some cases, variation may occur at an early developmental stage, and new basic patterns might thus be produced (e.g., the tumbler pigeon); these types of exceptions to the "principles of embryology" obviously do not affect the power of descent to explain similarities in embryonic patterns among related species.
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