340 



These "rivers" of vegetation composed of xerophytes of a 

 much less severe type than those of the plateau, running through 

 so pronounced a desert as the Namib, are very remarkable. Two 

 obvmus hypotheses suggest themselves by way of explanation. 

 At the first glance it seems possible that during exceptionally wet 

 seasons seeds are washed down from the upper parts of the river 

 basins and are able to establish themselves in the sandv beds 

 when the flow has ceased. That they are washed down is certain 

 tor in 1904 I saw numbers of seeds of Acacia albida and 

 A. giraffae and of other up-country plants among the refuse 

 strewn along the beach at the mouth of the Swakop. But on the 

 other hand some of these Haikamchab species occur not onlv in 

 the mam bed of the river but also up to 200 or 300 ft. above it 

 m the lateral valleys. Conspicuous among these is Acacia 

 giraffae, a species with massive pods and heavy seeds which, in 

 these situations, cannot have been brought down from higher 

 levels by flowing water (for the species does not occur on the 

 plateau) and no natural agency seems sufficient to have carried 

 them up from below. An alternative view is that the present 

 vegetation of the Namib river-beds is the relic of a flora essentially 

 identical with the Acacia formation now occurring on the higher 

 lands to the east, which in former times flourished on the plateau 

 much to the west of its present limits, perhaps almost to the sea. 

 •Evidence for the Namib having once enjoyed a moister climate 

 has already been stated ; the gradual retreat to the ea-t of the 

 seaward boundary of the Acacia formation would follow the 

 diminishing rainfall and as it retreated descendants would be 

 left behind wherever life was still possible to plants of this type, 

 ■this condition is now only realised in the main river-beds. It 

 would follow that the species at present found on the desert 

 plateau are descended' from constituents of the same flora which 

 were sufficiently plastic to admit of adaptation to the altered 

 environment, the severity of which has now become so pro- 

 nounced. The fact that most of the genera and some of the 

 species now found on the Namib are constituents of the Acacia 

 formation lends support to this hypothesis. 



Mirage and Night Fogs. -Other features of this niosr inter- ng 

 desert-region, the Namib, which force themselves upon the notice 

 of the traveller are the mirage and the night-fogs. Of the former, 

 which has been so frequently described, there is nothing new to 

 be said. Every bush, every hill more than a few hundred yards 

 distant loses its stability and is raised above its proper level and 

 surrounded by a sea of blue rippling air, whose resemblance to 

 water is extraordinarily close. The severely barren plain between 

 Welwitsch and Pforte, which has already been described, seen 

 about 4 o'clock in the afternoon, forcibly suggested the agitated 

 surface of an extensive lake. The night-fogs in so arid a region 

 are hardly less remarkable. About 8 o'clock in the evening ot 

 January 24, as we crossed the dry bed of the Tubas river a cloud 

 appeared in the west. The stars were gradually obliterated, and 

 by 10 o'clock we were shrouded in a cold ' Scotch mist. After 

 seeping on the ground from midnight until 4 o clock on the 

 following morning I was able to wring the water out of my top- 

 covering, a woollen rug. As soon as daylight came, the ground 



