scopeofzoology

oth cases, we take implied analogies from man-made machines which have both structure
and function. However, further examination of living things has made these classical
viewpoints of biology much less clear than they have seemed in the past.
This simple view fails when we ask: What is the life of an animal? What is passed on from
generation to generation to provide continuity? What is it that changes through time by the
process we call evolution?
The answer to these questions cannot be given by either the anatomist or the physiologist. It
has gradually become apparent that the body is not a fixed, definite structure, as it appears to
casual observation or when dissected. In life, there is ceaseless activity and change going on
within the apparently constant framework of the body. The essence of life is not a particular
substance or substances, but a particular kind of dynamic organisation. Sometimes, this
belief is expressed by likening the organism to a candle-flame. Just as in a quiet atmosphere,
the flame keeps its shape despite the fact that new particles of wax are being constantly fed
into it while the burned remains of the wax are leaving the flame as smoke, so the living
organism keeps its shape in spite of the constant replacement by synthesis and degradation
of the molecules that compose it. The continuous movement of blood was one of the earliest
examples of this activity, examined by Harvey in the 17th century. We now know of
innumerable others. Skin is continually being renewed by growth from below and many other
types of tissue are similarly replaced.
This phenomenon is described as the process of turnover and is a reflection of events at the
chemical level. The concept of chemical turnover is now the mainstay of biology.
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