WATEK BALANCES AND EXCHANGES 281 



grouped into total, partial, and net. They are either gains or 

 losses. 



Acceleration of intake or output (table 14) might be a measure 

 of the readiness with which the living unit responds to loads. 

 Wherever latent periods are absent, accelerations are, of course, 

 enormous. In other instances (all of which are water outputs) 

 accelerations are less, and occupy varying periods of time up to 

 2 hours, whether gross or net exchanges be considered. 



Decelerations fall into similar classes, for rapid decelerations 

 usually follow rapid and prompt accelerations ; where accelerations 

 are less, decelerations are also less (table 14). 



Maximal rates of exchange are, within limits, direct functions 

 of water increments. Actually, few species show any limits to the 

 increase of net intake or of net output. It might be said that the 

 ''bottle necks" of water exchange, if any exist, are never com- 

 pletely crowded. 



Equilibration diagrams for water (SW/At at various ± AW) 

 are of one type, provided maximal rates of exchange at any one 

 load are chosen for correlation. 



(2) Species and parts (U). For present purposes it is often 

 sufficient to group species as is done by taxonomists, and parts as 

 is done by anatomists. Wherever physiological similarities ap- 

 pear, however, alternative bases for classifying are indicated. 

 Thus, among insect species, larvae often belong to utterly different 

 categories with respect to water balances and exchanges from 

 either embryos or adults. 



Innumerable bases could be found for lumping individuals, or 

 organs, or cells, measurements upon which shall be averaged. At 

 present many body builds, many muscles, many red blood cells are 

 studied as though they constituted a homogeneous population, in 

 spite of the fact that in the future, distinctions between gastro- 

 enemii and tibiales antici will probably become significant. In the 

 same arbitrary way, I separate parts in situ from parts isolated. 

 Fifty years ago it was usual to mix observations on anesthetized 

 rabbits and on unanesthetized men. Tomorrow, correlations may 

 be further limited to one age, litter mates, one individual, one diet, 

 one cage. 



(3) Paths of water exchange (p) may first be identified anatom- 

 ically (table 21). In many species and even phyla none is known. 

 None appears to be separately distinguishable in the exchanges of 



