PHYSIOLOGICAL EFFECTS 129 



discussion. The factors involved in this interest- 

 ing demonstration that the minimum population 

 density is not the most favorable for rapid rate 

 of reproduction in early stages of population 

 growth, still await discovery. The problem is 

 being investigated in at least one laboratory at 

 the present time. 



Another type of group protection deserves 

 comment. In the cases given above, the group 

 was able to protect the various individuals of 

 which it was composed by removing some harm- 

 ful substance or by so distributing the poisonous 

 material that no single individual received a full 

 death-dealing dose. Group protection may also 

 be given when the lack of necessary elements in 

 the environment is causing death. This condi- 

 tion is found when marine animals are put into 

 fresh water. Ordinarily they are accustomed to 

 living in sea water with a salt concentration of 

 about 3.5 per cent. Many die rapidly when 

 transferred to fresh water. 



Two different types of simple animals known 

 as flatworms have been shown to survive longer 

 if many are present than if but few individuals 

 are placed together in the fresh water. These 

 experiments have been carried on by different 

 investigators on both sides of the Atlantic. In 

 one lot of these experiments the amount of salts 



