Two of the pioneers of the modern version of panspermia - the theory that comets disperse microbial life throughout the cosmos - trace the development of their ideas through a sequence of key papers. A logical progression of thought is shown to lead up to the currently accepted viewpoint that at least the biochemical building blocks of life must have derived from comets. The authors go further, however, to argue that not just the chemicals of life, but fully-fledged microbial cells have an origin that is external to the Earth. Such a theory of cosmic life, once established, would have profound scientific as well as sociological implications. The publication of this book is all the more timely now that we are on the threshold of verifying many of these ideas by direct space exploration of planets and comets.