ful' ant species required conditions in which agriculture was possible while the 

 hunting Cataglyphis were not so dependent on well-established vegetation and 

 were therefore to be expected on the desert fringes and gave way to more social 

 species in agricultural areas. It was not however true to say that the ants which 

 protected plant- sucking insects were invariably harmful. The activities of ants in 

 turning over and aerating the soil might be of special importance in cultivated areas 

 near deserts. In Brazil for example where there were no earthworms, it had been 

 calculated that ants brought to the surface nearly a third as much soil again as that 

 brought up by earthworms in Europe. 



Mr R. M. Elton referred to the importance of insects as human food and men- 

 tioned that he himself had sampled 43 species in Africa and Australia. Many such 

 as the witchetty grub contained a large proportion of moisture and their high salt 

 and glucose content enabled the natives to travel long distances on this diet in hot, 

 dry deserts. 



Professor A. Balachowsky said that crows were never to be seen feeding on 

 date-palms in the Sahara, but that in their search for ticks they sometimes injured 

 camels and were therefore shot when seen on the backs of these animals. 



Professor F. S. Bodenheimer emphasized the vulnerability of crops in oases 

 both to insects that changed their food habits, and to all the animals which attacked 

 plants for the sake of moisture. 



Dr C, B. Williams pointed out that the North African desert was one of the 

 routes by which insect migrants travelled to Europe and that they bred along the 

 fringes of the desert. This area was also the main breeding- ground of many insect 

 pests whose numbers varied according to the rainfall. Similar conditions occurred 

 in North America. 



Mr H. Green suggested that Professor L, Emberger had not sufficiently empha- 

 sized the skill required by an ecologist before he could safely interpret his obser- 

 vations and make them a guide in land use. Conditions were radically altered by 

 irrigation and fencing, pest control and the use of fertilisers or trace elements. 

 Consequently the ecologist's inferences involved a large subjective element of 

 skill and experience and an appreciable chance of error. 



Dr H. Boyko also said that the interpretation of ecology to agriculture was 

 skilled work and often man obtained less from the land than it could support natu- 

 rally. 



Session IV 

 ECONOMIC ASPECTS 



Chairman Dr H.G. Thornton F.R.S. 



In reply to a challenge for evidence that forest clearance resulted in reduced 

 rainfall. Professor E. P. Stebbing said that during their advance into India, Alex- 

 ander and his army had marched through vast areas of virgin forest where now only 

 desert was to be found. Professor J. F. V. Phillips added that during the last 150 



221 



