With this sixth edition, the publishers have faced a number of challenges. The explosive growth of knowledge in the broad fields covered by this monumental work has forced expansion to two volumes. Four younger editors have coordinated the efforts of 218 contributors, while the three senior editors from the previous editions serve, in the interest of preserving some continuity, as consulting editors. Of course, many of the chapters benefit from having at least some of the same writers. It seems that all have wisely approached their tasks with a blend of freshness and due regard for a classic that soon enters its fourth decade of reliable service.
The first edition appeared in 1960, while I was in college studying zoology. Later, in medical school and fellowship, I was impressed with the early editions as they focused on a variety of concerns that were united by the early genetic investigations. How