This book, written by Motoo Kimura (1924-94), is a classic in evolutionary biology. In 1968, Kimura proposed the "neutral theory of molecular evolution", which became the theoretical basis of modern evolutionary studies. After publishing his work in 1983 in the book "Neutral Theory of Molecular Evolution", Kimura wrote this book in 1988 for the general public. It was originally written in Japanese and is translated here for the first time.