Natural selection is central to evolution and is thought to provide the mechanism for the development of new species. Pressure exerted by natural selection is thought to play a major role in the regulation of animal populations. If it could be demonstrated that natural selection is not needed for the maintenance of populations or that the harvest of prey species by predators is random, then evolution would be without a mechanism. Experimental evidence is presented indicating the freshwater planarian, Dugesia dorotocephala, regulates its own population density at healthy levels without need for starvation, disease or predation. Data also indicate worm density is a more important factor of reproduction rate than food under certain conditions. Additional work is needed with other species and under natural conditions.