COREELATIVES OF WATER CONTENT AND EXCHANGES 273 



(a) It is recognized that the exchanges of water as described 

 depend upon absorbing and excreting surfaces, conveying and 

 circulating bodies, coordinating pathways, and many other parts. 

 The presence of probable oversize (factor of safety) of each, is 

 itself related to body size among species. Rapid movement of 

 internal fluids is often absent in small species, such as eggs, certain 

 worms, and some sessile organisms ; without convection, the move- 

 ment of water is often presumed to be limited by processes of 

 molecular ''diffusion." With convection inside the body, the in- 

 tegument may be so thick and immobile as to limit osmosis. In gen- 

 eral, how large might an organism be before osmosis through its 

 permeable tissues would limit its water intake or output? Compu- 

 tations might be predicated ; the only experiments seem to be on the 

 frog (of 32 grams weight). When blood circulation was suddenly 

 stopped, water output ceased, but intake of water from a dilute salt 

 solution was unchanged in rate (Adolph, '31b). When stopped 

 ("death") by previous desiccation, even maximal water intake was 

 undiminished in initial rate (fig. 66). 



(b) Among terrestrial animals it is widely recognized that turn- 

 over of water is directly related to turnover of food. Sometimes 

 it is specified that the nitrogenous fraction of the food is a correla- 

 tive (Babcock, '12) ; at other times that rate of energy transfor- 

 mation is important. In mammals most water is regularly drunk 

 with or following food ingestion. 



For mammals the rate of water intake during daily or longer 

 periods is correlated with the potential energy of the food taken 

 during the same periods. The ratio of the two increases with body 

 size (fig. 141) ; possibly the ratio should be between the logarithms 

 of the two intakes, making this relation also allometric. The two 

 species having least ratios are inhabitants of deserts. Since water 

 formed by oxidation is nearly the same per potential Calorie of all 

 foodstuffs, the diversity among species is wholly in the portion of 

 the water ingested as such. 



(c) For many of the same species of mammals the water output 

 may be partitioned by paths (fig. 142), the total output being taken 

 as equal to the intake. The fractions put out in fecal, urinary, and 

 evaporative channels are those indicated. The evaporative frac- 

 tion is nearly constant per unit of potential energy at all body 

 weights ; this is often spoken of as the proportionality of rate of 

 vaporization to rate of energy transformation. Its range of vari- 



