Philosophers of existentialist outlook, although obsessed with the human condition, have neglected the fact that men have bodies and arc members of the animal kingdom. Because of this neglect, two opportunities have been missed. Existentialists have failed to draw upon the knowledge amassed by biology about living things and about man's place in the organic world. They have also failed to take a look at biological phenomena in the light of their own conceptual scheme. As a result, they have had a one-sided view of what the human condition is and have misconstrued the nature of man. It is therefore an event worthy of notice when a philosopher sympathetic to existentialism deals seriously with these matters, as Professor Jonas does in his interesting book.