84 THE SOCIAL LIFE OF ANIMALS 



If a current catches it, such sperm can travel many 

 times thirty centimeters, but even in sea water the 

 sexes must be relatively aggregated if fertilization 

 is to be successful. In fresh water, the life of shed 

 gametes is much shorter. After ten minutes, eggs of 

 the pike lose the power to be fertilized, (102) and 

 the longevity of sperm of certain fresh-water fishes 

 is said to be less than a minute, so that in fresh 

 water the aggregation is even more essential. With 

 animals that require internal impregnation the 

 necessity for close co-operation between at least 

 two individuals is obvious. Such considerations must 

 be fundamental for the long-recognized breeding 

 aggregations of animals, especially of those that shed 

 eggs and sperm into surrounding water. 



Mass relationships may be even more important 

 sexually, and here I come to the new suggestion: 

 perhaps they had a hand in shaping sex itself. Pre- 

 sumably sexual evolution started, as it does today in 

 plants, with a time when all gametes of any one spe- 

 cies were similar. Under these conditions a first step 

 toward the union of two reproductive elements 

 could be supplied by the greater well-being fos- 

 tered by the presence of more than one gamete 

 within a limited area, as even the simpler proto- 

 zoans are stimulated to asexual division today by the 

 near-by presence of another of the same species. In 



I 



