Observations in the Welwitschia Desert. 117 



S. W. Africa, especially the distribution of the rainfall, have, inr 

 their main features, been permanent for an enormously long period. 

 Although now so distinct, the flora is probablv derived from the 

 same stock as the Acacia-formation which flourishes to the east of 

 it and in a former period mav perhaps have extended con- 

 siderably to the west of its present limit. 



