232 



L. B. SLOBODKIN 



Fig. 7. — Yield per unit food per four days and F {Hydra oUgactis). 



population was subjected to F = 50 per cent and in this population H. 

 Uttoralis alone persisted. In a population with F = 90 per cent both species 

 persisted. Two mixed species populations were estabHshed in the dark. One 

 of these was not subjected to predation and in this one the brown hydrid 

 alone survived. In the other, subjected to F = 90 per cent both species 

 survived. 



