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.