The modern discipline of biology was formed in the 20th century from roots deep in the natural-history tradition, which dates from Aristotle. Not surprisingly, therefore, ecology can also be traced to natural history, especially its 19th-century tradition emphasizing the adaptive nature of organisms to their environment. During the 20th century, ecology has developed and matured from pioneering work on successional stages to mathematically rich work on ecosystem energetics. By the end of the century, ecology has made a return to its natural-history heritage, emphasizing the importance of the integrity of ecosystems in considering human interactions with the environment.