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, Ppp- 
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- 
