To assume that these risks of uncertain rates of evaporation is desirable, is as absurd as to assume that the risks of rainfall are desirable for plant life.  As the plants of a certain climate have become adapted to the amount of soil moisture which the climate is likely to provide, so the egg has by natural selection been formed with about as much excess of water as will be lost in an average season under the natural conditions 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 eggs 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 15 per cent.  At both stations, incubators without moisture ran several per cent. higher evaporation than eggs under hens.  The conclusions at all stations were that the addition of moisture to incubators was a material aid to good hatches of livable chicks.

At Ontario the average evaporation ran from as low as 7 per cent.  At Utah it reached as high as 24 per cent.  Now as the entire loss of weight is loss of water, the solid contents remaining the same, and as the original per cent, of water contained in the egg (shell included) is only 65.5, the chicks of the two lots with the same amount of solid substance would contain water in the proportion of 58.5 to 41.5.  Based on the weight of the chick, this would make a difference of water content of over 25 per cent.

That human beings or other animals could not exist with such differences in the chemical composition of the body, is at once apparent.  In fact I do not believe that the chick can live under such remarkable circumstances.  As I have picked the extreme cases in the series given, it is possible that these extremes were experimental errors, and as in the Utah data, no information is given as what happened to the chicks, I have no proof that they did live.  But from the large number of hatches that were recorded below 9 per cent, and above 15 per cent., giving a variation of the actual water content in the chick’s body of about 10 per cent., it is evident that chicks do hatch under remarkable physiological difficulties.  One explanation that suggests itself is, that as there is considerable variation in evaporation of individual eggs due to the amount of shell porosity, and the chicks that hatch in either case may be the ones whose individual variations threw them nearer the normal.