Biology is the study of life. Biologists originally studied the different forms and manifestations of life, the conditions and laws under which these phenomena occur, and the causes through which they have been effected. They introduced the doctrine of life. In the past, much of the work of biologists was to categorize all life forms on earth. Based on their (assumed) relations to other species, they were given names to bring them in relationship with other species. Most modern biology can be encompassed with five unifying principles: the cell theory, the evolution theory, genetics, homeostasis, and energy. The cell theory places emphasis on the passing on of hereditary information via DNA. The evolution theory explains why certain species are still alive today, while others are not, by concentrating on natural selection. Genetics focuses on DNA. Homeostasis is when living organisms regulate their internal environment by means of multiple dynamic adjustments of equilibrium. And energy explains that a continuous need for new energy exists and that some species draw energy from the sun, making it available for the entire following food chain (or web).