The book traces the ways in which terrestrial animals have evolved from aquatic ancestors and discusses the means by which they are adapted to life on land. The most important physiological adaptations are those involving salt and water balance, the excretion of nitrogen, reproductive mechanisms and the sense organ and these are given priority. Evidence from fossil history is combined with that from the ecology and physiology of present-day species to assess the probable routes along which various evolutionary lines had moved on to land. Individual chapters are concerned with specific animal groups and emphasis is placed on comparisons of physiological mechanisms between closely related animals before attempting wider generalisations. The book closes with a brief account of the recolonisation of the sea and fresh waters by terrestrial animals.