6 months ago


egion and another.

egion and another. Uniformity and stability are particularly evident under deep-sea conditions. Below about 300 fathoms, light and heat from the sun hardly penetrate. The main trend in the evolution of life has been for animals to move away from the relatively stable conditions of the oceans to inhabit first fresh-water and then terrestrial habitats, both of which are less stable and more varied than the sea. All freshwater and land animals show clear indications of an ultimate origin from marine ancestors and progressive adaptation to these secondary habitats has been accompanied by steadily increasing specialisation. A large number of animals have returned from both freshwater and land to a marine life. Whales, dolphins and porpoises all display a degree of differentiation in both structure and function, which no zoologist believes could have developed unless their ancestors had been terrestrial. A porpoise may, to the untrained eye, look like a fish, but in its lung structure, its nervous system, its method of reproduction, the way in which its young are nourished and so on, it affords clear proof of descent with appropriate modifications from land forms. Some of these features derived from terrestrial ancestors are very obvious, such as the hairy coat which is present from birth. All of these adaptations to a different habitat occur through inherited variations in morphology, physiology and biochemistry and it is the role of the zoologist to pursue these variations at any level and to make deductions as to the pathway and mechanism of evolution. Dynamic aspects Within a well-developed science, it should be possible to reduce the varied subject matter to order. In biology, this means we have to show that all differences can be understood to have arisen by the influence of specific factors operating to modify some original scheme. Nothing less than the establishment of a general scheme and simple set of factors to include many special circumstances should be the aim of biology as a science. That is to say, in order to know life, what it is, what it has been and what it will be, we must look beyond the details of individual ' lives and try to find the rules governing all our findings. In our efforts to elucidate the rules of life, we use relatively simple-minded concepts derived from man-made objects and processes which we can understand because we made them. Most obscurity in biology comes from the ill considered use of these analogies. We have a science of anatomy, which we are told is concerned with the 'structure' of animals. Physiology is the study of 'function'. In 26

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. 27

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