18 president's address. 



tracks but by incidentally or purposely removing obstacles to 

 erosion and water-transport in river channels.** A large part of 

 these effects of human occupation is unavoidable, and only the 

 increasing value of land which accompanies a growing population 

 will compel men to take proper care of it. 



Occasionally one comes across a striking piece of evidence of 

 the adaptation of the higher animals to the varying conditions in 

 their surroundings, and one isuch was the discovery by Mr. Marais 

 in 1913 of a crocodile and many fish living but buried deep in 

 the dry bed of the Magalakwin." The habits of the Protopterus 

 of the Zambesi basin are another example. Such instances prove 

 that the ancestral experience of these animals had forced the 

 survivors amongst their descendants to learn to cope with the 

 severest droughts affecting their habitat. The}^ are also proof 

 that any particular drought during which the animals are found 

 alive in their refuge is not necessarily the worst yet experienced, 

 otherwise they w^ould not be found alive. 



The gradual changes in the distribution of plants afford 

 evidence of changes of climate, or of conditions of which climate is 

 a factor. The level of the ground-water, for instance, depends 

 upon climate, the nature of the ground and the facilities for sub- 

 soil drainage ; if the latter be greatly increased by the development 

 of dongas or by the multiplication of wells, the level of the ground- 

 water falls, and this will affect the distribution of plants, for those 

 with short roots may no longer be able to maintain their footing. 

 Changes of this kind may well be of importance in determining 

 the limit between predominantly grass and bush veld in a region 

 like the north-eastern Karroo, where there is a very fluctuating 

 rainfall and the conditions are favourable to the erosion of dongas. 

 In such a region, as in the Great Karroo, the establishment of 

 hundreds of farms, each with one or more wells or boreholes 

 from which stock are watered throughout the year, whereas before 

 the advent of the white farmer pools or streams served the 

 antelopes and other wild animals which migrated elsewhere when 

 the veld failed them, must have a lowering effect on the ground- 

 water level apart from the influence of dongas and the artificially 

 increased erosion of the stream beds. 



An interesting description of the increasing drought between 

 1904 and 1916 in the drainage basin of the Swakop and Khan 

 Elvers has been written by Professor F. Jaeger,** who states that 

 the desert conditions of the Namib gradually stretched inland 

 beyond Karibib to Johann-Albrechts Hohe, but that from 1917 the 

 reverse change took place, a great part of the Namib itself becom- 

 ing covered with grass. No doubt the work done for the newly 

 instituted Botanical Survey, for the permanence and success of 

 which all members of this Association will hope, will discover 

 in the course of a few decades changes of various kinds in the 

 veld flora, and may n:iake it possible to assign certain changes to 

 altered physical conditions. When we think how valuable it 

 would be to have now an accurate account of the plants and 

 their approximate relative abundance in any half-dozen areas of 



