ABSTRACT
Aridity is a relative condition since the amount of water available for organisms is a function of a number of interacting variables, such as precipitation, temperature, slope and aspect of the terrain, and soil texture. Endemism is low and almost all existing families have their centres of origin in contiguous semi-arid regions. The only reliable sources of water in deserts are those supplied by coastal fog, dew and ground-water at a depth that can be tapped by organisms. The low-latitude, intertropical deserts with low cloud cover experience the highest global intensity of annual solar radiation, except in the fog-deserts where relative humidity is comparatively high and mean annual temperature is low. Organisms which inhabit the hot deserts of the world have to be able to cope with excessive heat and with drought to ensure that neither internal temperatures nor tissue dehydration attain lethal levels.
