The "medium" is the substance or material that carries a mechanical wave. One of the most important things to remember about waves is that they transport energy, not matter. This makes them different from other phenomenon in physics.
For simple definitions, visit the following website:
https://www.ducksters.com/science/physics/waves.php
The process which wherein an Ecosystem may renew itself is known as Ecological Succession. Ecological succession may be achieve over a period of time. The time scale may be decades or a million of years after a mass extinction.
that there once lived humans on mars
