170 PHYSIOLOGICAL REGULATIONS 



nished with homologous structures and paths. What can be 

 counted as alimentary water in Arbacia egg, as urinary output in 

 leucocytes'? (3) Even when organs are supposedly or actually 

 homologous, their performances may be diverse. Thus, frog's 

 urine is not known to be hypertonic to blood plasma under any 

 circumstances ; dog's urine is ; both urines are produced by kidneys. 

 In earthworms the output from nephridia may or may not be 

 counted as urine, and as homologous with the renal output in dogs. 



Three complete equilibration diagrams were constructed for 

 the dog (fig. 38), all of which are equally valid in representing this 

 species in comparisons of water relations. In fact, at least six 

 types of modification of water content were recognized; some in 

 excess and some in deficit; and there was no unique connection 

 between the particular excess and the particular deficit associated 

 in any one equilibration diagram. Among such diagrams, three 

 or more time relations were utilized: initial (of diverse durations), 

 steady, and maximal rates of water exchange. 



In comparisons among living units, whether species or parts of 

 individuals, these several distinctions at least may be maintained. 

 Each of them avoids one kind of confusion. 



§ 70. Some paeameters comparing exchanges 



Measurable gain was contrasted with measurable loss of water 

 at each load. What methods of evaluating the contrasts between 

 the two, and of exhibiting the modifications of each exchange sepa- 

 rately, are useful? 



(1) Economy quotients, the ratios of gain to loss, characterize 

 the responses to water increments in the several species (table 9). 

 The fastest recovery from water excess means suppression of gain 

 while loss is maximal ; and the fastest recovery from water deficit 

 presupposes the reverse. The economy quotient evaluates how 

 closely this possibly optimal relation is approached. 



The dog's exchanges show higher quotients at every negative 

 load than the rat's, which in deficits surpass those of man, frog, 

 and earthworm. In positive loads, values departing most from 1 

 might be sought. The quotients of dog and man are not most 

 extreme in short periods of time, for in water excesses the frog's 

 diuresis has less lag than the mammal's. By choosing diverse 

 modes of comparison, a case can be made for ''superiority" of the 

 quotient in almost any species. Economy quotients are available 



