Comparative physiology and the challenge of design

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Abstract
Analysis of the complexity of biological organization commonly takes us through an initial and necessary stage of over-simplification towards the ultimate goal of an appreciation of the subtle intricacy of nature. One example of this is that recent advances in studies of the phosphagens have failed to confirm earlier hopes that the distribution of these substances could be used to confirm phylogenetic relationships. It is now suggested that a previously unrecognized factor influencing this distribution may be competing demands for arginine in the development of the testes. Another illustration is the belief that the presence of iodine in ascidians may provide a clue to the origin of thyroidal biosynthesis. This belief is probably sound, but the presence of abundant iodine in the tunic may well be a false trail. More significant is the presence of much smaller amounts of bound iodine in the endostyle, where its presence is probably correlated with specialized protein metabolism associated with the ciliary feeding mechanism. A third illustration, discussed in more detail, is provided by recent recognition that the gut is a major component of the vertebrate endocrine system. This lays open attractive fields of research, ranging from problems of molecular and hormonal evolution to aspects of fish farming and the physiology of gut parasites. The need is thus plain for breadth of perspective in comparative physiological research, and for recognition that the ultimate concern of the biologist is with the life and relationships of the whole organism.

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