2 ANIMAL LIFE IN DESERTS 



unsound, because it looks not to the forces which 

 produce the desert, but to the result of their action; 

 not to the weather and soil, but to the plants and 

 animals which can grow in the desert in spite of 

 its physical conditions. I use the word " desert " 

 to include places in which the climate is hostile 

 to animals and plants, in which normal agriculture 

 is impossible, and in which nearly all the existent 

 forms of animal and plant hfe are modified to endure 

 life in their pecuHar environment. I use the word 

 " semi-desert," without exact definition, to describe 

 country of which the climate is less hostile, and 

 the flora and fauna less specialized, than that of 

 a desert ; semi-deserts support cultivation at cer- 

 tain seasons, and provide grazing, though it is 

 often bad grazing, at all seasons. Let us also bear 

 in mind that deserts grade quite imperceptibly 

 into semi-deserts, and these into savannas, steppes, 

 and downs, and that a country may be extreme 

 desert at one season and covered with lush vegetation 

 at another. Furthermore, deserts are not neces- 

 sarily permanent or very ancient ; the buried 

 civilization of Eastern Persia and Turkestan and 

 the petrified forest near Cairo are sufficient evidence 

 of this. 



Most of the extensive deserts lie a little to the 

 north of the northern tropic, or a little to the south 

 of the southern tropic ; they do not lie close to the 

 Equator. Most of them lie in the northern hemi- 

 sphere and are bounded northwards by steppe, 

 that is to say, arid country with a covering of 

 grass and other perennials and an abundant flora 



