The organism, a physico-chemical entity, co-exists in delicate balance with its environment. This is manifested not only by external, morphological modifications, but also by structural and physiological changes expressed at tissue and cellular levels. These changes may be divided into two categories:- the first represents a gradual process such as, for example, a unicellular organism evolving into a multicellular one (increase in efficiency); while the second represents a more rapid form of “evolution” whereby an organism adapts to its environment by modifications of preexisting structures (adaptive radiation). With respect to vision, light (radiation) is the single-most important factor contributing to retinal changes. These changes may, in some instances be rapid enough to be manifested during the life time of the organism, although they may be temporary and revert to the original form once the stimulant is removed.
KeywordsRetinal Pigment Epithelium Turbid Water Adaptive Radiation Visual Pigment Pigment Granule
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