ANIMALS— PHYSICAL ENVIRONMENT 93 



ii. Fauna op Waters in Deserts 



Conditions of life in a permanent well or spring 

 in the desert, or in a river which runs for part of its 

 course through desert, are not in any way different 

 from those which prevail in similar situations 

 elsewhere, and the animals of these springs and rivers 

 do not concern us. But there are other and more 

 temporary water-places in deserts, and those of 

 them which are liable to be filled with water at 

 fairly regular intervals support a very interesting 

 fauna. 



The inhabitants of the temporary pools are exposed 

 either to death or else to the full rigour of the desert 

 climate for many months on end, while the pools 

 are dry ; and even when they are j&Ued with water 

 the position of the animals is precarious, because the 

 period which they will be able to devote to active 

 life, to feeding and to reproduction, is a short one. 

 Moreover, during the gradual drying up of their 

 pool the saltness of the water increases : so that the 

 animals are faced with three separate problems — the 

 ordinary problems of Hf e in fresh water, the gradually 

 increasing salinity of their water, and its eventual 

 disappearance. 



Certain groups of animals, abundant in the fresh 

 water of other climates, are entirely unable to solve 

 these problems. The Newts, for instance, are found 

 all over Northern Europe and Asia, but they are 

 limited in a southward direction by the northern 

 edge of the Great Palsearctic Desert, and no newt 

 appears to have penetrated or crossed it. It would 



