EVAPORATION 163 



daily. Of this amount 60 per cent or 540 gms. come from the skin alone 

 and changes in relative humidity of only 1 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 (125, pp. 56-57). 



Little has been done on the physiological effect of evaporation or 

 desiccation upon cold-blooded animals. Various writers have found a 

 loss of water associated with hibernation. Greeley (132) obtained the 

 same results with desiccation as with freezing (132; 133; 51, pp. 

 182-88). The reactions of animals to evaporation gradients have been 

 studied by the writer (134). A high rate of evaporation is advanta- 

 geous to some animals and decidedly detrimental to others. Animals 

 inhabiting dense woods turn back when they encounter air with a high 

 evaporating power. This is true of frogs, salamanders, insects, and 

 millipedes. The frogs and salamanders die in an hour or more in an 

 atmosphere of high evaporation power but centipedes and ground beetles 

 and other heavily armored animals do not die for many hours or even 

 days though they react negatively to the dry air when they encounter 

 it in a gradient. Animals from hot, dry, sand areas usually select air of 

 high evaporating power and die in air of high evaporating power only 

 after very long exposure. The results of a long series of experiments 

 may be summarized as follows: (1) the animals studied react to air of 

 a given high rate of evaporation whether the evaporation is due to 

 moisture, temperature, or rate of movement; (2) the sign and degree 

 of reaction to the given rate of evaporation are in accord with the com- 

 parative rates of evaporation in the habitats from which the animals 

 were collected; (3) the animals of a given habitat are in general agree- 

 ment in the matter of sign and degree of reaction; the minor differences 

 which occur are related to vertical conditions (see below) and kind of 

 integument; (4) there is a rough agreement between survival time in air 

 of high evaporating power, and kind of integument, but no agreement 

 between survival time and habitat when a number of members of a com- 

 munity are taken together. The relation of warm-blooded animals to 

 rate of evaporation has been sufficiently studied so that, when it is 

 taken with the work on cold-blooded animals, we are warranted in con- 



