ANIMALS— PHYSICAL ENVIRONMENT 83 



It is reasonable to suppose that just as some 

 animals obtain their small requirements of water 

 from the plants they eat, so others obtain their 

 supply from the ants, grasshoppers, etc., which are 

 their staple food : such cases are probably few be- 

 cause animals which are dependent upon insects for 

 their food and drink are uncommon in aU deserts : 

 this applies not only to the order Insectivora but 

 also to insect-eating members of other orders. 

 This rarity is due in all probabiHty to the great 

 seasonal fluctuation which occurs in the number 

 of insects, and that in turn follows the enormous 

 variations in climate to which all deserts are sub- 

 ject. In the Great Palsearctic Desert I know of 

 no purely insectivorous ^ mammal, except the bats. 

 Of the food of such resident birds as the Desert 

 Warbler {Sylvia nana), which is probably a pure 

 insectivora, we have no accurate information. In 

 the Australian deserts the ants are a dominant 

 group : it appears that they are the staple food of 

 Myrmecobius, a marsupial ant-eater, which devours 

 them above ground, and of Notoryctes, the marsupial 

 mole, which lives beneath the surface. 



Actual water-holes, springs, rivers, etc., provide 

 water to the creatures which live on their banks, 

 and to a number of others which come in from the 

 surrounding deserts to drink. Among them are 

 many birds, and bats, a few insects, and many 

 large terrestrial mammals. 



^The hedgehogs which inhabit many parts of this area (e.g. 

 Hemiechinus auritus), though '* Insectivora " in the technical 

 sense, are not an exception to this statement, for they are by 

 no means exclusively eaters of insects. 



