ference, since it is essential to all forms of life. It follows that the only absolute 

 desert from the biological point of view is that which has no reserve of water, no 

 means by which the rare rainfall can escape from instant evaporation. From that as- 

 pect the boulder- strewn rocky surfaces of the Sahara or of the Australian Central 

 Desert are more nearly absolute in their aridity than the moving sand-dunes, for the 

 latter will at least store rain within their mass, letting it our slowly to keep alive '■ 

 those scant bushes in the troughs between the dunes. 



This brings us to what, as a geographer interested in the practical application of 

 scientific knowledge, I regard as the most important consideration to come before the 

 participants in this conference. Even though deserts are, at the very best, but mar- 

 ginal land for the use of man, it behoves us to make what use we can of them. That 

 use, as we know, depends on the available reserves of water, but the investigation of 

 what reserves there are, is at present, a very expensive business. 



Yet we know there must be some close relation between the available water and 

 those plants which are permanent occupants of an arid area, and they must therefore be 

 indicators in some degree of the underground water. The ecologist and the physical 

 geographer have the duty of finding out how far one can trust indicator plants, which, 

 after all, are best qualified to give us the information once we have wit to interpret 

 their message- 

 In the past when searching for sub- surface water in arid areas we have been ac- 

 customed to send water- engineers and geologists and even physical geographers. In 

 my opinion the ecologist is the more appropriate scientist for such a mission, since he 

 should be able to ask the question of the plants, which really do know the answer, 

 whereas the rocks and the sand carry no visible prool that there is water below the 

 surface. 



To conclude on a still more practical note, I would suggest that the biologists 

 equip themselves more fully for this new duty by field- work directed especially to find- 

 ing out more about such indicator plants, particularly in those semi- arid regions which 

 could be put to better use than they are at present. I should like to quote the particu- 

 lar case of my own favourite 'desert', so miscalled, the Kalahari. 



Even in the worst parts of it there is a cover of bushes and small trees which sur- 

 vive one- or two-year droughts. To prove by drilling that there are underground water 

 supplies would be a costly and haphazard means of investigation. I would rather em- 

 ploy a field ecologist who would make it his chief if not his sole purpose to establish 

 a relationship between sub- surface supplies of water and perennial plants. 



The whole secret of life in arid regions is movement, a readiness and a freedom 

 to migrate. This is obvious enough for man and the larger animals who somehow find 

 out where the casual storms have occurred and move to the pastures so benefited. For 

 the less mobile small fry and the immobile plants it is a case of adapting themselves 

 to endure long dry periods in a state of dormant or suspended animation. This they do 

 in a myriad different ways, and perhaps the best examples of adaptation to environ- 

 ment are to be found in deserts. These adaptations however are in delicate balance 

 and it is for the biologist to study how far it is safe for man to interfere with them for 



