WATER RELATIONS IN HIBERNATION. 24! 



series, the sharp changes in the line of survival of hibernation 

 and in elimination during hibernation that occur between the 

 introduction and the first hibernating period, at the end of the 

 second generation. 



In hibernation temperature and humidity in the soil are the 

 two chief factors concerned with these animals, and their action 

 upon the organisms is difficult of isolation. In the two environ- 

 ments chosen for the experiments, the conspicuous difference 

 is one of humidity, not of temperature; soil conditions at Chicago 

 during the period of hibernation having a high and rather 

 constant water content, whereas at Tucson the relation is one 

 of continuously diminishing humidity. At the close of the 

 summer rains at Tucson, the intense desiccation accompanying 

 the high temperatures of the dry after summer, rapidly lowers the 

 water content in the soils until the onset of the winter rains in 

 December, and again in the dry fore summer from the end of 

 the winter rains in March to the beginning of the summer rains 

 in July, -the water content is progressively lowered until at the 

 end of the period it may be as low as 5 per cent, or even less in 

 the upper layers of the soil. It is this latter period in which 

 the most intensive elimination of the cultures at Tucson takes 

 place. 



In desert organisms, in general, some mechanism is present by 

 which the water content of the resting organisms is conserved 

 during the dry and resting periods in the year, otherwise the 

 plants or animals are not able to survive the hostile conditions 

 of these rigorous portions of the year that are so characteristic 

 of every desert environmental complex, and which may be pro- 

 longed, irregular, and highly variable in their manifestation in 

 different years. Consequently, surviving desert types must 

 meet, not only the average extreme and hostile conditions in 

 their existence, but irregular and extreme manifestations of the 

 environic complexes, often of prolonged duration. In other 

 words, a desert environment is not only a most rigorous one, 

 but also about the most variable habitat that an organism has to 

 meet. 



The response found in these experiments at Tucson suggests 

 at once that the alteration is one of water relations, one in 



