a principle of organic evolutionary change whereby the main function of an organ gradually loses its importance and is replaced by a secondary function. The former main function either survives as a secondary function or completely disappears. A. Dohrn established the principle in 1875.
An example of change of function accompanied by the loss of a previous main function is the development of the wing of a bird from the pentadactyl extremity of reptiles. The previous main function, support on the ground, was completely lost and replaced by a new function, movement in the air. An example of change of function whereby the former main function becomes a secondary function is the development of the talpid anterior extremity. The support function during crawling is preserved, but digging has become the main function. Change of function occurs as a result of environmental change and is made possible by the multiplicity of function of organs.
A. S. SEVERTSOV
a principle of evolutionary change in organisms whereby a function that is lost—usually with an accompanying reduction of the organ that performed it—is replaced by another, biologically equivalent function that is performed by a different organ. The principle was determined by the Soviet biologist A. N. Severtsov. An example of change of function can be seen in snakes: their reduced extremities were once their ancestors’ organs of locomotion, a function that is now effected by a flexing of the snake’s spinal column.