n. The ability and tendency of certain systems to maintain a relatively constant internal state in spite of changes in external conditions; this ability is achieved by the presence of feedback mechanisms which can adjust the state of the system to compensate for changes in the state caused by the external environment. It is exemplified in homeothermal biological systems, such as animals which maintain relatively constant blood temperature and composition in spite of variations in external temperature or the composition of the food ingested. [ PJC ]