The Fourth Edition of Invitation to Biology is in part a shorter version of Biology, Fourth Edition, in part a revision of Invitation to Biology, Third Edition, and in part a wholly new book. The entire book has been reconsidered, sentence by sentence and illustration by illustration, rewritten to a large extent, and once again thoroughly reviewed. The Introduction begins with a discussion of the signs of life, accompanied by a new selection of photographs to whet the student’s appetite for knowledge of the marvelous organization, functioning, and interactions of living organisms. This is followed by a brief consideration of the enormous diversity of living things, of evolution as the thread that ties together the diversity of life, and of science as a particular human activity, with characteristics that distinguish it from other human activities.