GENERAL FEATURES OF WATER EXCHANGES 169 



sions of reference are available. Occasionally the absolute volumes 

 exchanged are described. Or, the exchanges are expressed as frac- 

 tions of the volume and the water content of the body, whereby 

 other objectives are gained. Again, time is measured relative to 

 some physiological process (physiological times) instead of by the 

 same clock in all species. 



Diversity of conditions and states in each species constitutes a 

 severe limitation to comparisons ; I therefore compare while insist- 

 ing upon the tentative nature of the results. Particularly, com- 

 parisons of water exchanges by isolated tissues with those of tis- 

 sues in situ may seem fantastic. Probably this opinion is the safer 

 one ; perhaps the same view is preferable throughout comparative 

 biology and in all analogies to non-living systems. It is always 

 a propos to say : in this seemingly neat comparison the diversities 

 of surface area, or the presence or absence of a circulation, or the 

 amenities of vitality, are disregarded. So they are, and I think 

 something is disregarded of the hundreds of possible factors, in 

 any conceivable comparison. Hence the investigator chooses be- 

 tween making provisional comparisons and making none; if the 

 former, it is invidious to say that one mode of comparison is better 

 than another; it may rather be said that it is different. Quanti- 

 tative comparisons have the probable advantages of being im- 

 plicitly provisional, and of being guided by dimensions and other 

 criteria away from some varieties of confusions. I may confi- 

 dently expect that some of the features coming to light are inde- 

 pendent of what the observer imposes. The utmost judgment will 

 not insure like conditions for two individuals or species, or for two 

 days. Diversity of states in one individual may or may not rep- 

 resent 'inherent" variability, so long as there is no possibility of 

 robbing the organism of all environment. 



Why not subject all species to the same conditions, it may be 

 asked? Only in the abstract, might it be ideal to use identical 

 means and criteria of water load in all the organisms that are to 

 be studied comparatively. (1) Not all organisms can survive like 

 treatment or like environment. Desiccations of Ameha and of 

 Thamnophis by evaporation, though both technically possible, do 

 not attain the same water deficits in a uniform time. Recoveries 

 of dog immersed in water, Ameha in air, rabbit blood in distilled 

 water, are equally unsuitable. (2) Not all organisms are fur- 



