Using the published work of Nobel Laureate Niels Kaj Jerne, this book shows how he developd his ideas. The book is a compilation of his published work, but in fact it is much more than that. Whether the reader wants to read the book systematically, or only browse, it opens a fascinating world of hypotheses, theories, facts and vistas. His selection theory, his view of how immunological diversity is created, and his concept of lymphocytes interacting as a network, reveals Jerne's revolutionary spirit. The book ought to be a rich source of inspiration for everyone interested in science and how science is made.