VITAL LIMIT OF EXSICCATION. 49 



The migration of the amphibians from water to land is regarded 

 as a very important event in the evolution of animals. Closely 

 associated with the adoption of a terrestial mode of existence by 

 the amphibians is a change in the water content (Oswald. 1917). 

 Before the frog takes to land, for instance, it consists of ninety- 

 three per cent, water. When it becomes a land animal much of 

 its water is lost. During its terrestial life a frog lives in moist 

 places and when subjected to extreme dryness loses much of its 

 water. In the writer's experiments as much as forty-one per 

 cent, of the body weight, or fifty per cent, of the water contained 

 in the body, was given up without loss of vitality. As water is 

 given up so readily, areas where the evaporation rate is high 

 would be generally unfavorable for frogs and for amphibians in 

 general. Shelford (1913) has shown that amphibians, and 

 other animals as well, react to evaporation gradients and con- 

 cludes that the evaporating power of the air is probably the best 

 index of the suitability of conditions for land animals. 



Water does not evaporate from animals as it does from a surface 

 of free water. Even those animals in which the integument per- 

 mits little apparent evaporation, are able in some way to retard 

 evaporation of water. To test this point a small stender dish 

 containing 30 cc. of distilled water was subjected to the same con- 

 ditions as a salamander during exsiccation, The area of surface 

 exposed including skin, mouth, and lungs of a salamander weigh- 

 ing thirty-five grams, was estimated to be 121 square centimeters. 

 The average amount of water lost from the dish having an area 

 of 22.9 square centimeters was 4.520 grams per day. The aver- 

 age loss from salamanders was 0.770 grams per day. Of the 

 animals used in the experiments described in this paper the sala- 

 mander showed the most rapid rate of evaporation, yet the rate 

 of evaporation from an exposed surface of free water was thirty 



times as great. 



GENERAL SUMMARY. 



1. Animals in general are very susceptible to loss of water in 

 atmospheres of low relative humidity. 



2. Animals may lose a very large percentage of the water con- 

 tained in their bodies without loss of vitalitv. 



