THE park system of New York City is the product of more than a century of active expansion. In 1976, there are approximately 23,000 acres of park land under the City’s jurisdiction; 13,000 additional acres formerly in City possession are now part of the federally administered Gateway National Recreation Area. More than 17 per cent of the total acreage of the city is public park land-an acre of open space for every 210 residents.  The policy of constant acquisition was a result of valid planning assumptions based on optimistic estimates of long-term trends.  For generations New York’s population had been increasing. Each year additional acres of open space succumbed to development as urbanization swept across the five boroughs. Planners sought to provide facilities both in congested areas and in regions that soon would be developed. In the Depression era, because of a reduced rate of housing construction, park development was often far in advance of need.