Expansion of Functions

a principle of the functional evolution of organs according to which an organ acquires functions that it did not possess previously. For example, in bivalve mollusks the gill cavity also became a brood pouch, and the ventral fins of Elasmobranchii also became organs of copulation. The principle of expansion of functions explains the increase in organs’ multiplicity of function as the structure of organisms became differentiated in the course of evolution.