the condition known as 'anhidrotic heat exhaustion' there is evidence of actual 

 damage to the sweat glands as a result in many cases of pre- existing prickly 

 heat. It has also been claimed that injury to sweat glands is likely in the early 

 stages of sunburn. Such damage brings about a greatly reduced capacity to carry 

 out active work in the heat for the lack of sweating makes exertion very unplea- 

 sant and inefficient and circulatory failure is easily induced. This condition and 

 certainly prickly heat are both far more common in hot humid climates than in 

 desert conditions and could only occur in closed spaces with extremely poor 

 ventilation, 



6. Desert and 'racial' selection 



It has earlier been pointed out that representative groups of all the varieties of 

 mankind (whether these are classified serologically or morphologically) are to be 

 found in deserts, so pointing to the adaptiveness of mankind as a whole. Indeed, 

 laboratory tests have shown that short term acclimatization phenomena are similar in 

 Europeans, Asians and Africans. Nevertheless, this does by no means preclude the 

 possibility that these desert sub-groups have undergone distinctive changes as a 

 result of continuous residence in the desert. Evidence has accumulated that in re- 

 gions of high mean annual temperature some differences in bodily development and in 

 physique are encountered. D. F. Roberts in this laboratory has shown that peoples of 

 hot climates are of lower body weight and often in addition exhibit an elongation, 

 relative to trunk, of either upper or lower limbs or both, as compared with people in 

 temperate and cold climates. Probably 50% of the variance in these characteristics 

 is attributable to differences in mean annual temperature. These modifications in 

 physique will be recognised as being in line with Bergmann's and Allen's Rules and 

 it could be argued on physical grounds that they represent advantageous adaptations 

 to hot climates. However, the existing records relate overwhelmingly to peoples in 

 the hot humid regions and it is by no means clear whether desert peoples universally 

 exhibit these physical characters. Nor is it certain that such changes are neces- 

 sarily genotypic. Another effect of hot humid climates (which may be merely pheno- 

 typic) is a slowing down of the rate of skeletal maturation and sexual maturity. Again, 

 data for desert peoples is lacking. 



The most striking of all 'racial' characters associated with desert people is of 

 course the steatopygia of the Kalahari bushmen. While there has been some specu- 

 lative discussion as to the role of this fat deposit as a fuel and water store, no 

 direct study has yet been made. Some of the Andamanese pygmy negritoes show 

 similar female steatopygia proving that the condition is to be found in humid tropics 

 as well as in deserts. 



Yet another problem is the significance to be allotted to melanin pigmentation of 

 the skin and here again, owing to lack of adequate technique, no objective compari- 

 sons of skin colour amongst desert people can be quoted though one can be certain 

 that a great range of skin colour must exist. The general opinion that melanin depo- 

 sition is protective against ultra-violet light and of value in hot climates is streng- 

 thened by some recent experiments of Thomson (1951) in which he showed that in 

 Europeans sunburn can be associated with damage to the sweat glands. Another 



198 



