“An adult person made to remain naked in sun in a hot day in Sahara desert will die before the night expires.”

The above observation illustrates the intimate importance of the environment for a living organism.

Life manifests itself through a body. The body is sustained by its environment. It is as it should be, because a living cell is such a complex and delicate organism which needs a constant and uninterrupted expenditure of energy to maintain it. In addition the vital functions that characterize a life require energy.  Environment is the source of energy and matter for the organism. So the environment is vital to its survival. An healthy environment possesses all that the organism need in such forms as may be accessed by the organism. Additionally a healthy environment has the wherewithal for the processing and disposal of the metabolic waste.

The following example very well illustrates the interrelationships of organism and environment. Few drops of rain water or dew may get stuck in the crevices on the surface of a bare rock exposed to sun. These few drops may initiate a process which leads to the conversion of the rock to soil particles eventually. Water drops may be absorbed by the fungal component of crustose lichenes by virtue of its hygroscopic nature while the algal component manufactures organic matter by photosynthesis. With time colonies of crustose lichene are able to grow there. Crevices on the rock surface and organic content of the substratum increase as also its water retaining capacity. Now the surface is capable of holding foliose lichenes followed with fructose ones which have larger bodies and greater water requirement. The invaders cast shade over the previous occupants, which are now deprived of sunlight and are unable to compete for water. This results in their eventual disappearance. The process continues. Simultaneously decomposing bacteria appear. Transformation of the substratum is hastened.  Rock is replaced by soil. Lichenes are replaced by higher forms of plants and associated insects. Many animals too find it suitable to adopt it as their habitat.

Process of transformation of a bare rock into soil exemplifies the mutual relationships between organisms and environment. The metabolic activities of organisms bring about changes in the environment; the changed environment may become less favourable to the pre-existing communities and more favourable for pioneering the advent of fresh communities. Subsequently the pre-existing communities may be totally replaced by the newer communities. Side by side the surroundings undergo irreversible change.

Constant adaptation is required because the surroundings keep on changing. Organisms change structurally as well as physiologically by the process of adaptation so that its communication with the changed surroundings is not disrupted. Change is inevitable. Organism itself is the agent of change. Environment and the organism simultaneously pass through the phenomenon of evolution.
 
An healthy environment possesses all that the organisms need, in such forms as may be accessed by the organism. Additionally a healthy environment has the wherewithal for the processing and disposal of the metabolic wastes. Environment is said to be polluted when it is deficient in such capabilities and threatens the survival of the existing organisms. In the biosphere there must be compatibility between organisms and environment so that uninturrupted flow of energy and matter between the two continues unhindered.
 
It may appear incredible that the appearance of oxygen in the atmosphere was a life threatening pollution in the then biospere. But this is true. There was no oxygen in the primitive atmosphere. The organisms respired anaerobically. Oxygen appeared in the atmosphere as a cosequence of photosynthesis by some of these anaerobically respiring microbes. The event created a crisis for them. There was rapid and uncontrlled oxidation of the living matter resulting in mass burning of the organisms. Threat of being wiped out loomed large over the bioshere. But as we learn from the principle of distribution all the members of a population are not uniform in any trait. Traits of various members are distributed over a range of values. This was true for tolerance of free oxygen for the then microbes too. Limited number with higher tolerance level survived the disaster. Their presence in the population was very little before the appearance of oxygen in the atmosphere. But now all the resources of the system was available to them without competition. With time new populations evolved by reproduction. These populaions were well adapted to aerobic conditions. It was not possible for them to survive in absence of oxygen. So now the presence of oxygen became established as a compulsary condition for survival and its deficiency an indicator of pollution.
 
The above illustrates that mutual relationship between organism and environment gives rise to chains of changes. And that change is an inevitable and continuous phenomenon.