However, in some species, individual organisms are both male and female. Hermaphroditism is when one organism has both sexes. Earthworms and garden snails always have both male and female organs, and when, for example, two earthworms mate, they fertilize each other. A special variation on the theme is sequential hermaphroditism , in which an organism changes sex during its life. While most higher animals reproduce sexually, there are some species in which the females can, under certain conditions, produce offspring without mating.
Parthenogenesis is the ability of an unfertilized egg to develop and hatch.
This seems to be especially prevalent among insects. Some of the giant walkingsticks at the Zoo are females who, without mating, lay eggs that hatch into more females generation after generation. Other insects, like some aphids, have complicated life cycles that involve sexually-reproducing generations alternating with parthenogenically produced generations. In sexual reproduction, there must be some way of getting the sperm to the egg. Since sperm and eggs are designed to be in a watery environment, aquatic animals can make use of the water in which they live, but terrestrial animals must, in some way, provide the wet environment neeeded for the sperm to swim to the egg.