Africa, of course, has most of its population living in tropical climes. The study explains this significant tropical zone shortfall in terms of the greater prevalence of disease, poor soils, typhoons and other natural calamities in the tropics. Surprisingly, the study seems oblivious of W. A. Lewis' (1969; 1978) pioneering work seeking to explain economic performance in the tropics as a result of falling terms of trade. Lewis emphasized however that not all tropical countries responded to this increased demand for exports. As the exports in greater demand were largely water-intensive, only those areas with enough water - or minerals - to substantially increase their exports were able to take advantage of the new opportunities.