INCUBATION 
The influence of air currents on evaporation is to increase it. 
directly proportional with the rate of air movement. Thus, 10 
cubic feet of air per hour passing through an egg chamber 
would remove twice as much moisture as would 5 cubic feet. 
If the percentage of water in any living body be changed a 
relatively small amount, serious disturbances of the physio- 
logical processes and ultimately death will result. The mature 
animal can, by drinking, take considerable excess of water 
without danger, for the surplus will be speedily removed by 
perspiration and by the secretion from the kidneys. But the 
percentage of water in the actual tissues of the body can vary 
only within a narrow range of not more than three or four 
per cent. The chick in the shell is not provided with means 
of increasing its water content by drinking or diminishing it 
by excretion, but the fresh egg is provided with more moisture 
than the hatched chick will require, and the surplus is grad- 
ually lost by evaporation. This places the water content of 
the chick’s body at the mercies of the evaporating power of 
the air that surrounds the egg during incubation. 
To assume that these risks of uncertain rates of evaporation 
is desirable, is as absurd as to assume that the risks of rain- 
fall are desirable for plant life. As the plants of a certain cli- 
mate have become adapted to the amount of soil moisture 
which the climate is likely to provide, so the egg has by nat- 
ural selection been formed with about as much excess of water 
as will be lost in an average season under the natural condi- 
tions of incubation. Plant life suffers in drought or flood, and 
likewise bird life suffers in seasons of abnormal evaporative 
conditions. This view is substantiated by the fact that the 
eggs of water fowl which are in nature incubated in damper 
places, have a lower water content than the eggs of land birds. 
The per cent. of water contained in the contents of fresh 
eges is about 74 per cent., or about 65.5 per cent. based on the 
weight, shell included. Unfortunately no investigations have 
been made concerning the per cent. of water present in the 
newly hatched chick. 
Upon the subject of the loss of water for the whole period 
of incubation, valuable data has been collected at the Utah, 
Oregon and Ontario Experiment Stations. 
In these tests we find that as a rule the evaporation of eggs 
under hens is less than in incubators. With both hens and 
incubators, the rate of evaporation is greatest at the Utah 
Station, which one would naturally expect from the climate. 
The eggs under hens at the Ontario Station averaged about 12 
per cent. loss in weight, and those at the Utah Station about 
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