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.