Some prey form groups (aggregate) in the presence of a predator. One benefit of aggregation might result from defensive action by the entire group, but another potential benefit lies in reducing the per-individual likelihood of death. Detailed studies of the behaviors of individual fish in schools show that fish are in constant motion. A fish on the outside of a school swims inward, leaving other fish on the edge. These new "edge" fish then move inward and the cycle repeats. 
describe how aggregation is likely to result in a greater likelihood of survival for schooling fish in the presence of a predator.and why predators and prey with many generations of interaction are likely to have pronounced behavioral responses to each other.