PHYSIOLOGICAL EFFECTS 119 



tion and grouping, the more so since one cannot 

 tell the sexes apart by simple inspection. 



Similarly with the water fleas widely known as 

 Daphnia, the grouped animals consume less oxygen 

 than do similar isolated individuals. On analysis 

 it appears that the lower rate of use of oxygen by 

 these water fleas is due to the accumulation of 

 carbon dioxide which acts as a depressing agent 

 and so lessens the consumption of oxygen. Under 

 certain conditions such a state of depression 

 turns out to be of benefit to the grouped animals. 

 If they are exposed to fairly severe concentrations 

 of various salts, the grouped individuals will sur- 

 vive longer than if they are isolated. Chemical 

 analyses show that with salts, the grouped Daph- 

 nias do not remove the salts from solution but 

 that their increased resistance is due to the fact 

 that they do not respire so rapidly as do their 

 isolated fellows; hence they are making smaller 

 demands on their environment and so are able 

 to live longer under adverse conditions. 



On the other hand, if the concentrations of salt 

 solutions were made very low so that the most 

 vigorous animals could become adjusted to their 

 presence while the less vigorous ones are unable 

 to so acclimate themselves, then the isolated 

 animals with the higher rate of general metabolism 

 lived longer. 



