 Biologists generally agree that lifeforms are self-organizing systems which regulate their internal environments as to maintain this organized state, metabolism serves to provide energy, and reproduction causes life to continue over a span of multiple generations.Typically, organisms are responsive to stimuli and genetic information changes from generation to generation, resulting in adaptation through evolution; this optimizes the chances of survival for the individual organism and its descendants respectively.Non-cellular replicating agents, notably viruses, are generally not considered to be organisms because they are incapable of independent reproduction or metabolism.