2i6 ANIMAL AGGREGATIONS 



of the animals in the groups became the same as that of the isolated 

 animals. When he re-examined his original data in the light of 

 these findings, it was discovered that the cases in his early work in 

 which the singles lived for a longer time than the group were those 

 in which the dilution of the chemical was such that acclimatization, 

 rather than direct resistance, was an important factor. In such 

 cases, as Child has repeatedly pointed out (e.g. 191 5), the animals 

 with the higher rate of general metabolism have the greater power of 

 acclimatization, and hence of survival. On the other hand, whenever 

 the individuals tend to produce conditions which depress their rate 

 of metabolism and are exposed to solution strengths lethal within a 

 relatively short time before acclimatization becomes a factor, such 

 depression has survival value. 



Fowler's work also demonstrated that when Daphnia are exposed 

 in dilute solutions of sodium or potassium hydroxide, the grouped 

 individuals lived significantly longer than did similar isolated ani- 

 mals, and that under these conditions the hydroxide in the solution 

 surrounding the group was reduced in strength as compared with 

 that surrounding relatively isolated animals. Eight experiments 

 with NaOH showed that 2 animals in 20 cc. solution in 3 hours 

 reduced the hydroxide from a mean concentration of 0.00094 N to 

 0.000764 N. Under the same conditions 20 Daphnia reduced the 

 same amount of the same solution to 0.000606 N, a difference of al- 

 most 30 per cent, with a statistical probability of 0.002. Similarly 

 significant results were obtained with KOH solutions. Here the car- 

 bon dioxide given off by the animals reacts with the hydroxides to 

 form carbonates and water, thus weakening the toxic concentration 

 of the medium. The groups produce carbon dioxide more rapidly 

 than do animals isolated into the same volume of medium, and hence 

 decrease the toxicity more rapidly and survive longer. 



The problem of the number of animals present in relation to their 

 survival when exposed to toxic agents presents a somewhat different 

 aspect when applied to insects placed in containers into which toxic 

 gases have been introduced in concentration just sufficient to kill 

 only a part of the isolated individuals with the exposures employed. 

 Here the production of autoprotective materials presents a distinctly 



