substitution

The act or an instance of substituting.

The state of being substituted.

One that is substituted; a replacement.

In biology: The assumption by one organ of a function which was at one time performed by another organ. Thus the swim-bladder in fishes shows “that an organ originally constructed for one purpose, namely, notation, may be converted into one for a wholly different purpose, namely respiration.”.

The acquisition by an organ of a secondary function which, at first performed incidentally, may gradually become the chief function if the primary function becomes useless or is performed by another organ. Thus “the little folds of skin which originally served as ovigerous frena, but which, like-wise, very slightly aided the act of respiration, have been gradually converted by natural selection into branchiæ, simply through an increase in their size and the obliteration of their adhesive glands.”.