What is "alternation of generation"

Alternation \Al`ter*na"tion\, n. [L. alternatio: cf. F. alternation.]

1. The reciprocal succession of things in time or place; the act of following and being followed by turns; alternate succession, performance, or occurrence; as, the alternation of day and night, cold and heat, summer and winter, hope and fear.

2. (Math.) Permutation.

3. The response of the congregation speaking alternately with the minister.
   --Mason.

   Alternation of generation. See under Generation.