(N/A) Organisms are often bothered by the highly fluctuating external environment. It is expected that during the millions of years of their existence,many species have evolved a relatively constant internal (within the body) environment that permits all biochemical reactions and physiological functions to proceed with maximal efficiency and thus,enhance the overall fitness of the species.
This constancy could be in terms of optimal temperature and osmotic concentration of body fluids. Ideally,an organism should try to maintain the constancy of its internal environment (a process called homeostasis) despite varying external environmental conditions that tend to upset its homeostasis.
Let us take an analogy to clarify this important concept. Suppose a person is able to perform his/her best when the temperature is $25^o C$ and wishes to maintain it so,even when it scorchingly hot or freezingly cold outside. It could be achieved at home,in the car while travelling,and at workplace by using an air conditioner in summer and heater in winter. Then his/her performance would be always maximal regardless of the weather around him/her. Here the person's homeostasis is accomplished,not by physiological,but by artificial means.