30 THE WANDERINGS OF ANIMALS [OH. 



to the retention of the pouch in correlation with the 

 arboreal life which the immediate ancestors of all the 

 recent marsupials are known to have led. Arborealism 

 may have been forced upon the primitive, indigenous 

 fauna as a means of escaping competition against the 

 coming higher placental mammals. 



Tropical forests teach two impressive lessons : 

 the awe-inspiring competition into which plants and 

 animals alike are forced in their struggle for life, and 

 the fact that the fight is so fierce, because the physical 

 conditions plenty of warmth, water and food are 

 so favourable to all. Every living thing is modelled 

 by adaptation to the prevailing surroundings, coupled 

 with the cumulative inheritance of the characters 

 acquired. 



Deserts. 



A characteristic essential feature of deserts is the 

 scarcity of rain. An annual amount of rainfall of less 

 than eight inches inclines a country towards desert 

 conditions. Much depends upon the distribution of 

 the rain, whether the few inches come down in the 

 shape of halt a dozen or more days of good rain, at 

 various times of the year in little doses, or whether 

 there is a drought of twelve months, or even longer, 

 and the whole scanty allowance pours down in one 

 dose, most of which is thus squandered since there is 

 no time for it to soak in. There are in fact various 



