periods at extremely high temperatures, — exceeding in fact those of the tropical 

 regions. These high temperatures are paralleled as one goes further north-east or 

 as the elevation increases, by extremes of low temperature, — falling in the Iranian 

 Plateau to freezing point, in the Turkestan Deserts to figures well below freezing 

 point, and in the Gobi Desert of Mongolia to figures below zero degrees Fahrenheit. 

 It is clear therefore that animals inhabiting desert regions must exhibit marked 

 tolerance to heat, and in some areas, equally marked tolerance to cold. It is not, of 

 course, possible to give strictly comparable figures for solar radiation, but they may 

 be assumed to run roughly parallel with temperature. 



Heat tolerance is, however, also affected by humidity, whether the animal loses 

 heat by sweating, by transudation or by respiration. Variations in humidity are most 

 conveniently shown by means of climographs, in which the dry bulb temperature is 

 plotted against the relative humidity. From a study of such climographs, one would 

 expect the need for heat tolerance to be low in the temperate areas of Europe; in a 

 tropical desert like the Sahara there will need to be a high heat tolerance, but there 

 will be opportunities for heat regulation by evaporative loss at the low relative 

 humidities ; in wet tropical areas, where high temperature and high humidity are 

 combined, the dissipation of heat will however be especially difficult. Desert areas 

 in intermediate climates show strikingly wide variations in environmental conditions, 

 from hot dry in the summer months to cold wet in the winter; clearly the animals in 

 such areas will have to possess wide adaptations to the direct effects of climate. 

 It is perhaps relevant at this point to note that, while few reliable records of air 

 movement are available, desert areas may normally be taken to involve free air 

 movement, while wet tropical jungle conditions are usually characterised by rela- 

 tively still air. 



Turning to the indirect effects of climate (i.e. those concerned with vegetation 

 and water supplies), these are best reflected in precipitation curves. VThether rain- 

 fall follows the continental or the Mediterranean pattern, the outstanding features of 

 all desert areas are not merely their low total precipitation, but the long periods 

 during which there is virtually a complete absence of rainfall, — extending for as 

 much as six months out of the year. The only difference between semi- arid areas 

 and the more typical desert areas is the higher total precipitation in the former 

 during the rainy season, which will affect both the type of vegetation and the water 

 resources. But all are characterized by long rainless periods, with consequent vio- 

 lent fluctuations in herbage growth and therefore in facilities for natural grazing. 



May I turn now to actual illustrations of the climatic effects on animal mor- 

 phology and adaptation. If in doing so I seem to rely too largely on teleological 

 argument, I hope it will be realised that this is inevitable in view of our present 

 meagre and non- quantitative knowledge of the subject. 



The direct effects of climate on the size and conformation of animals have been 

 summarized in Bergmann's and Allen's Rules. The former postulates a larger body 

 size in the colder climates, — the effect of the larger body size being to reduce the 

 surface area available for the dissipation of heat. The latter postulates a parallel 

 lessening of the extremities in the colder climates. Put concisely, one would expect 



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