It is a privilege of age and stature in a field to review the development of major concepts and summarize where we are; Lord and Lady Medawar do just that with style, intelligence, and compassion. Gifted scientists and specialists in immunology, they are also gifted writers. In this rich smorgasbord they present key ideas in biology in brief chapters requiring little background yet sophisticated enough to stimulate scholarly thought on human behavior. Evolution, Lamarckism, immunology, microbiology, demography, eugenics are some of the topics put in perspective. In addition, there are straightforward expositions on bodily constituents, circulatory systems, the nervous system, and so on. The Medawars give due praise to the innovative minds who discovered genetic transformations in bacteria; to those who developed immunological concepts like tolerance, or the notion that a kind of Darwinian randomness allows for the diversity of lymphocytes in recognizing antigens. They are refreshingly acerbic in recounting false or nonsensical notions: ""No modern biologist would ever speak of the 'inheritance of intelligence,'"" they say; or, apropos of population-control by limiting parents to two offspring--""this proposal has the air of commonsensical rightness about it which is invariably a symptom of some aberration of reasoning."" And when, in the course of simple exposition they say that a virus is ""simply a piece of bad news wrapped up in a protein,"" you know you've got to keep reading. That's how good they are.