The primary purpose of a flower is reproduction by the joining of pollen of one plant with the ovules of another (or in some cases its own ovules) in order to form seed which grows into the next generation of plants. Sexual reproduction produces genetically unique off spring, allowing for adaptation to occur. As such, each flower has a specific design which best encourages the transfer of this pollen. Many flowers are dependent upon the wind to move pollen between flowers of the same species. Others rely on animals (especially insects) to accomplish this feat. Even large animals such as birds, bats, and pygmy possums can be employed. The period of time during which this process can take place (the flower is fully expanded and functional) is called anthesis.