This is a reasonably cautious book with a firm delineation between the ascertained and the speculative. Actually everything that touches directly on the title subject belongs in the latter category. There is an admission that there are appearances of primitive life on Mars, but the claims of evidences of extraterrestrial life on earth (flying saucer sighting, the peculiar explosion in Siberia in 1908) are held dubious. More encouraging to the science fiction hopefuls are the author's theoretical views. Following an overly lengthy (almost half the book) description of the vast limits of the universe, hope is held out that there may be several billions of planets capable of supporting life. There is some interesting detail about Just what the requirements are for this to develop and a few hints as to variations that could occur. The final chapters deal with the various methods of getting into communications with advanced civilizations (there has already been one futile attempt to locate radio signals) and what the implications would be for interplanetary , intergalactic relations. The book is short, but tantalizing in its suggestions.