n this book I have tried to bring together ideas from three different Istreams of thought with which I have been closely concerned during the last thirty years or so. The first is faunal history, usually regarded as a purely academic subject, but to some of whose events can be traced a number of the serious dislocations taking place in the world today. The second is ecology, particularly the structure and dynamics of populations. The third is conservation. I first published a few ideas about the signifi cance of invasions in 1943, in a war-time review called Polish Science and Learning, under the title of 'The changing realms of animal life'. Since then I have had the opportunity to think pretty hard about conservation, while taking part in the planning and development of the Nature Con servancy. In March 1957 I gave three broadcasts in the B. B. C. 's Third Programme, under the title of 'Balance and Barrier'. These were sub sequently printed in The Listener (1957, Vol. 57, pp. 514-15,556-7,596-7, and 600). The present book is essentially an expansion of these. I am extremely grateful to Mr James C. Thornton and Dr John Simons for in planning and giving these talks. advice and help In preparing this book I have had invaluable assistance from the staff of the Bureau of Animal Population. Miss C. M. Gibbs typed the fair copy. Miss M.