If a group avoids the sunlight for several generations they will tend to evolve pale skin, and an aversion to sunlight. The eyes may become huge, to catch what little light there is. In areas of total darkness the eyes may become tiny and blind, perhaps permanently closed, or even absent altogether. To compensate, the ears may grow larger, or the feet evolve a new sensitivity to vibrations.
Lack of sunlight causes melancholy and sickness, and we do not survive well in darkness. Thus those who live underground are usually driven to it by dire circumstances. The main reasons are as follows:
- Safety: Those who are harried on the surface may be innocent, such as refugees and escaped slaves, or guilty, such as criminals, murderous cults, or insane scientists who wish to perform unwholesome experiments with impunity.
- Shame: An individual may punish themselves for some crime by never seeing the sun. Likewise those whose bodies are changed by magic may wish to avoid the sight of others.
- Mania: Some forms of madness may lead the sufferer to seek darkness; for example those who falsely believe themselves to be vampires.
- Fell Influence: Many creatures which exercise a destructive magical influence on the foolish or unwary live underground, and they may wish their followers to gather close to them.
- Accident: As when a city is swallowed by the earth, or a group of miners are cut off from the surface, and there are survivors who find a source of food.