Alzarius is a swamp planet located in the pocket universe known as E-Space. Its coordinates are given as -10 0 -11 00 by 0 -2; negative due to being located in E-Space rather than regular N-Space. Coincidently, it possesses the same coordinates in E-Space as Gallifrey does in N-Space. The planet is most notable for its unique biosphere, in which native species naturally undergo periods of rapid evolution and are able to mutate into different species in response to environmental conditions.
The Alzarian life cycle seems to flow in a continuous evolution. During the epoch known as Mistfall (which happens roughly every fifty years) the riverfruits that fall from the trees hatch into newborn Alzarian Spiders. Those may give rise to Marshmen; which in turn may give rise to Alzarians. However, each of these species also reproduces on their own and generates more of their own kind, thus guaranteeing the continuing co-existence of all evolutionary stages. This amazing evolutionary cycle seems to be tied in to the ability displayed by Alzarian life forms to rapidly mutate in response to their environment; for instance, developing the ability to breathe the new gases released during the Mistfall. The life forms native to Alzarius also display strong regenerative abilities, being able to heal injuries quickly.
Ironically, the Alzarians (at the least the one civilization that has been observed) are completely unaware of their origins, believing themselves to be descendants from the survivors of a crashed Terradonian spaceship.
- Doctor Who season 18 - Full Circle (1980)