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Hopefully, this book will be taken off of the shelf frequently to be studied carefully over many years. More than 40 researchers were involved in this project, which examines respiration, circulation, and metabolism from?sh to the land vertebrates, including human beings. A breathable and stable atmosphere?rst appeared about 500 million years ago. Oxygen levels are not stable in aquatic environments and exclusively water-breathing?sh must still cope with the ever-changing levels of O 2 and with large temperature changes. This is re?ected in their sophisticated count- current systems, with high O extraction and internal and external O receptors. 2 2 The conquest for the terrestrial environment took place in the late Devonian period (355–359 million years ago), and recent discoveries portray the gradual transitional evolution of land vertebrates. The oxygen-rich and relatively stable atmospheric conditionsimpliedthatoxygen-sensingmechanismswererelativelysimpleandl- gain compared with acid–base regulation. Recently, physiology has expanded into related?elds such as biochemistry, molecular biology, morphology and anatomy. In the light of the work in these?elds, the introduction of DNA-based cladograms, which can be used to evaluate the likelihood of land vertebrates and lung?sh as a sister group, could explain why their cardio-respiratory control systems are similar. The diffusing capacity of a duck lung is 40 times higher than that of a toad or lung?sh. Certainly, some animals have evolved to rich high-performance levels.