Substitution of Organs

a principle of evolutionary change whereby an organ that performs a certain function in an ancestral organism, having disappeared in the descendants, has been replaced by another organ that performs the same function. The principle was formulated in 1886 by the German scientist N. Kleinenberg. An example of organ substitution is the replacement of the chorda, a feature peculiar to the lower chordates, by the spinal column of the higher chordates.