The source of this book goes back 46 years to a biology class at Lowell High School in San Francisco. The first and second semesters were divided into botanical and zoological studies, respectively. In the second semester, the course was organized by phyla to emphasize the evolution of various organs and physiological processes. It became apparent to me that by the time we reached the humble earthworm, all of the human organs and processes, except the skeleton (and liver), had been developed, at least in rudimentary form. Evolution from that point forward was an improvement on these mechanics rather than a radical departure from them. I had no way of figuring out why, but the question lodged itself in my mind and would not let loose.