In a little book of 155 pages the late John Scot Haldane gave the world his final message. Much as his friends and admirers must regret his recent death, we may rejoice that in these few pages he has succeeded in presenting in clear and unmistakable fashion the philosophy which, throughout his long life of highly successful detailed research in physiology combined with equally effective and untiring application of his findings to practical problems, he slowly developed into the outlines of a comprehensive and rounded system.