BEGINNINGS OF CO-OPERATION 83 



plicated problems such as these that deal with the 

 rate of population growth are controlled by more 

 than one mechanism. 



The suggestions from the simpler protozoans, 

 taken together with other aspects of the mass physi- 

 ology of protozoa which have been only partially 

 reviewed here, and with the acceleration of devel- 

 opment demonstrated for sea-urchin eggs, encourage 

 me to renew a suggestion made some years ago, (3) 

 which has, so far as I am aware, been overlooked 

 to date. 



Let us go back to consider the case of external 

 fertilization among aquatic animals. When sperma- 

 tozoa and eggs are shed into sea water by sea- 

 urchins or other marine animals, their length of 

 life is distinctly limited. If a sperm fails to contact 

 an egg during the fertilizable period, death results 

 probably from starvation for the spermatozoa, per- 

 haps from suffocation for the egg. This means that 

 the animals of the two sexes must be fairly close 

 together if there is to be a union of the shed sexual 

 products. The most vigorous sperm of the sea- 

 urchin Arhacia can travel in still water about 

 thirty centimeters, that is, about one foot and two 

 inches. (55) Spermatozoa of these animals diluted 

 a few thousands of times can survive from three to 

 twelve hours; the majority succumb by seven hours. 



