BEGINNINGS OF CO-OPERATION 69 



their ability to fertilize eggs much sooner than if 

 they are present in greater concentration. It is rou- 

 tine laboratory practice in experimenting with such 

 animals as the common sea-urchin, Arbacia, to keep 

 sperm in a cool place, densely massed outside the 

 body, for hours. Small drops can be withdrawn as 

 needed for experimentation, greatly diluted and 

 used almost immediately to fertilize eggs. When such 

 dilute suspensions have long since lost their fer- 

 tilizing power the sperm in the original dense mass 

 are still potentially as active as ever. 



So far we have been considering mass effects, the 

 survival value of which, if any, was shown by in- 

 creased length of life, often under adverse circum- 

 stances. Under many different conditions and for a 

 variety of organisms, the presence of numbers of 

 forms relatively near each other confers protection 

 on a part of those grouped together or even on all 

 present. 



It is possible to go a step farther and demonstrate 

 a more actively positive effect of numbers of or- 

 ganisms upon each other when they are collected to- 

 gether. Again I select a fresh case for close scrutiny; 

 that of crowding upon the rate of development in 

 sea-urchin eggs. 



Arbacia, mentioned above, is the common sea- 

 urchin of coastal waters south of Cape Cod (Fig- 



