196 



PHYSIOLOGICAL REGULATIONS 



(table 12). No uniformity of conditions can be prescribed for all 

 kinds, "natural" environments not often being suitable for obtain- 

 ing the data required. Man might be best compared at 24-hour 

 intervals, but no one would recommend that interval for Arhacia 

 eggs (Goldforb, '35). Variabilities of weight among individuals 

 are of little use in studies of water relations except as water con- 

 tent is correlated with absolute size (or with age, or other correla- 

 tives). In single individuals, high variability is found with high 



TABLE 18 

 Water contents of various species of terrestrial vertebrates when in water 'balance 



rates of turnover and with small body sizes. With the exception 

 of Helix, multicellular species that have been observed maintain 

 their water contents within ± 3 % of Bo over a period of 24 hours. 

 Rates of turnover of water are in each species the total amounts 

 gained and lost again in a unit period of time. Sometimes this 

 quantity is termed water requirement, a recognition that turnover 

 occurs in the maintained state of the organism. Among the 

 diverse species represented (tables 20 and 21) there are great 

 inequalities of physiological factors under which data are obtained. 

 To put dog, cow, rat, and frog, on the same diet, and in the same 

 atmosphere, would be equally unsuitable. While each species is 

 measured (with respect to mean and to variability) in a state where 



