269 HUMAN BIOLOGY 



effect of sunlight depends on how much radiation we receive 

 from the red end of the spectrum with its long heat waves and 

 how much from the blue end with its short-waved, highly 

 active ultraviolet hght whose chemical effects upon rickets 

 and the hke have recently been much discussed. 



No study of climatic factors is complete unless it includes 

 atmospheric pressure and electricity, but we shall mention 

 them only to dismiss them. Time and again inexperienced 

 investigators think that they detect a close relationship 

 between barometric pressure and physiological activities. 

 Such a relationship is, indeed, very evident when the 

 low pressure on high mountains is compared with the 

 normal pressure at sea level, but thus far the most pains- 

 taking investigators have had little success in isolating any 

 clear-cut effects of the barometric variations at any one 

 place. Supposed effects of this kind appear to be due almost 

 wholly to the accompanying changes in temperature, 

 relative humidity, winds and sunlight. As to atmospheric 

 electricity, many little scraps of evidence suggest that it 

 may exert an important influence upon human well-being. 

 People appear to feel stimulated after thunder showers, or 

 in factories where electric sparks are active, but no one yet 

 knows whether the supposed effects are really electrical or 

 are due to special combinations of temperature, humidity, 

 and wind. 



THE LAW OF CLIMATIC LIMITS 



The physiological effects of temperature, humidity, 

 atmospheric movement, and sunlight can best be understood 

 in the light of two fundamental laws, those of climatic 

 limits and climatic optima. Although both laws are almost 

 self-evident, they are rarely understood or consciously 

 used as the basis of adjusting mankind to his environment. 



The law of limits may be stated thus: Almost every 

 environmental factor may be so extreme that it is fatal to the 

 individual, or prevents reproduction and is thus fatal to the 

 species. Sometimes there are both upper and lower limits, 

 and sometimes only one. Every form of life is subject to two 

 limits of temperature. A rise of 100° in the temperature at 



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