The primary purpose of a flower is reproduction. Flowers are the reproductive organs and mediate the joining of the sperm contained within pollen to the ovules, normally from one plant to another but many plants also can pollinate their own flowers. The fertilized ovules produce seeds that are the next generation. Sexual reproduction produces genetically unique offspring, allowing for adaptation. Flowers have specific designs which encourages the transfer of pollen from one plant to another of the same species. Many plants are dependent upon external factors to move pollen between flowers, including the wind and animals, especially insects. 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