78 VICTOR E. SHELFORD. 



perature and illumination, but mot closely correlated with 

 relative humidity. From the standpoint of including many 

 factors, the evaporating power of the air is by far the most in- 

 clusive and is therefore by far the best index of physical conditions 

 surrounding animals wholly or partly exposed to the atmosphere. 

 It is not however to be expected that it will hold good for all 

 the factors under all climatic conditions, and for this reason, 

 records of light, temperature, pressure, carbon dioxid, etc., 

 should be made. 



(a) Effect of Evaporation upon Animals. In the case of man 

 some observations have been made. According to Pettenkofer 

 and Voit (fide Hann), an adult man eliminates 900 grams of water 

 from his skin and lungs daily. Of this amount 60 per cent, or 

 540 grami come from the skin alone and changes in relative 

 humidity of only I per cent, cause perceptible changes in the 

 amount of evaporation from the skin. If evaporation from the 

 skin and lungs is diminished, the amount of urine is increased, 

 as in many cases are also the secretions of the intestines. Sudden 

 changes in humidity make themselves felt in sudden increased or 

 decreased blood pressure. The less dilute blood of dry climates 

 operates as a stimulant and increases the functions of the nervous 

 system. The consequences are excitement and sleeplessness 

 (Hann, pp. 56-57). 



Lie tie has been done on the physiological effect of evaporation 

 or desiccation upon animals. Various writers have found a loss 

 of water associated with hibernation. Greeley obtained the same 

 results with desiccation as with freezing (Greeley, 'oi ; Bachmet- 

 jew, '99; Semper, '79, pp. 182-188). The reactions of animals 

 to an atmospheric humidity gradient has probably never been 

 studied. The chief conclusion to be drawn from the literature 

 is that a high rate of evaporation is advantageous to some animals 

 and decidedly detrimental to others. Attempts to keep insects 

 and spiders which live exposed on the prairie vegetation, near 

 Chicago, in the laboratory in screen cages containing vegetation, 

 usually result in the death of the animals within a few hours. 

 On the other hand, the same species will live in glass jars covered 

 or partially covered with glass plates, long after the vegetation 

 which was placed in with them has turned brown and has soured 



