Life has evolved into a multitude of sizes. Over the course of three billion years, life has taken on many forms--from an .02-micrometer-long bacteria to the 110-foot-long blue whale. Scientists are learning how the struggle for survival has led some animals to become small and others to get huge. Understanding the evolution of size tells us why giant dinosaurs went extinct while the first tiny mammals thrived; gives us answers to why mammoths evolved into pygmies when restricted to islands; and why carnivorous mammals have never grown to weigh more than a ton. Whether it's the speed of movement or population numbers, the biological world revolves--and "evolves"--around size.