Play, playfulness, or playlike behavior has not proven easy to define or study. But identify ing it as a “real” phenomenon is another mat ter. Play has been noted in books on animal be havior and comparative psychology for many years, as in the 19th-century writings of Thompson (1851), Darwin (1874), Lindsay (1879), Buchner (1880), and Romanes (1892). For these writers a definition seemed almost unnecessary: play was behavior that was not serious (in the sense of being directed to ac complishing some immediate task), was often highly energetic and most characteristic of healthy individuals, and was probably pleasur able. But then as now, play was an oddly dis continuous category: prominent in some spe cies (especially mammals) and absent or rare in most (especially invertebrates and poikilothermic vertebrates). Thus play was, and still is, rarely incorporated into any general system of comparative psychology.