52 PRESIDENTIAL ADDKESS SECTIOX C. 



These cliaiig-es include the evolution of the flora of South 

 Africa as it is going- on now — the evolution of the flora as a 

 whole, and of parts of it — of regions, districts and areas ; the 

 reflex action of that changed flora on the climate, and again 

 of changed climate on the flora — evolution which, though 

 insidious, can be seen and recognised during a lifetime, even 

 without written records, and all bearing a trend which is 

 unmistakable. That trend is not necessarily the evolution of 

 new species, but rather the gradual disappearance of what 

 were climax types and the substitution of species of a more 

 xerophytic nature. 



Man and his actioiis play an important part in producing 

 this change, for not only does he bring with him the weeds 

 and survivors of cultivation, but through cultivation he 

 destroys the natural herbage, and gives these aliens oppor- 

 tunity for naturalisation, of which some of them have freely 

 availed themselves, sometimes to the exclusion of native 

 species. 



Cultivation is, however, a necessity, and its attendant 

 troubles are more or less inevitable. AYe have nothing to say 

 against good cultivation in which the original flora is displaced 

 by cultures yielding permanently returns of more value than 

 the original, without haviiig other detrimental effect. 



But it is rather with pastoral agriculture and with bush- 

 cutting, their methods and their results, that I wish to deal, 

 especially in reference to how these are affected by natural 

 conditions, and how in return the surrounding conditions are 

 affected by them. 



In order to get a clear grasp of these reflex actions, it is 

 necessary to review separately several rather disconnected 

 subjects, and then bring their bearings together as a final 

 clause. So far I have suggested man as an important factor, 

 and the advent of civilised man as a starting ])oint for the more 

 rapid evolution of South Africa's flora and climate. 



Xatural Causes. 



But there are also natural causes at work, far beyond man's 

 control, though still some of their results may be influenced 

 by what man does. I refer particularly to the phenomena con- 

 nected with climatology — phenomena whose influences are the 

 final factors in plant life and in plant distribution, and on 

 that account I ask the indulgence of Section A of this Associa- 

 tion, if I deal at some length with a subject in which Section A 

 and Section C overlap, as a necessary introduction to what 

 follows. 



The flora agrees with the climate in every case; a change 

 in the climate brings in its trail a change in the flora, whether 

 that change take the form of adaptation of existing species, 

 increase or decrease in the representation of existing species, 

 or their substitution bv other species. 



Bolus and Wolley-Dod (1903), p. 231, find that grass burn- 

 ing and bush-fires tend to destruction of si)ecies and consequent 

 greater uniformity, but not necessarily greater usefulness of 

 the vegetation, and that the tendency is slowly and gradually 



