PROTECTION FROM TOXIC REAGENTS 221 



In general the experiments on the group resistance of Planaria 

 to ultra-violet radiation forward our understanding of mass rela- 

 tions of individuals in two ways: First, the results emphasize the 

 fact that the phenomena of possible protection of individuals by 

 chemical exudates, which has been demonstrated for many animals 

 exposed to different situations, is not universal. Second, they show 

 that even when the massed animals are known to produce chemical 

 exudates which are harmful, the massing may still have survival 

 .value through the changed physical conditions which it produces. 



The work reviewed in the present chapter makes clear that group 

 protection from toxic agents may operate in diverse ways. There 

 may be group protection solely as a result of the distribution of 

 toxic material among so many individuals that no one receives a 

 lethal dose, or the toxic substance may be adsorbed on' the slime 

 which is often produced in copious quantities under the stimulation 

 of the abnormal situation. The survival value of the group may be 

 due to the depressed physiological condition obtaining among its 

 numbers, which favors increased survival when the animals are 

 exposed to strongly toxic solutions which kill actively metabolizing 

 animals more rapidly than those whose rate of metabolism is lower. 

 The group may act in a purely physical manner by altering the 

 electrical conditions, or against light and ultra-violet radiations by 

 a simple shading phenomenon. The evidence to date does not ex- 

 clude the possibility of protection from active toxic conditions by 

 the group conditioning the medium through some protective secre- 

 tion, although this kind of explanation should be adopted only on 

 positive proof. While it is clear that group protection is a fact, it is 

 also certain, as might have been expected, that there are many ways 

 by which groups accomplish this protection, and that more than one 

 may be acting in the same case. 



