The purpose of this review is to summarize the present status of preparative zonal centrifugation and to indicate where the remaining problems, in the reviewer's opinion, may lie. The field is sufficiently new for interest in practical separartions to be uppermost. However, a number of theoretical problems relating to gradient capacity, zonal rotor configuration and particle—particle interactions remain to be fully explored.

Since the earliest intelligent history man has tried to understand the origin of life. Some remarkable scientific advances have been made. The concept of natural selection is firmly established as the basis of organic evolution. Advances in the genetic mechanism of inheritance have shed light on the processes and levels at which evolutionary changes occur. Evolution is understood to be a dynamic process, and the organisms present at any given time are those best suited to the environment by virtue of the history of their predecessors. But a deeper question arises: how free is evolution? Are the particular biomolecular structures and processes in existence now in any way uniquely predetermined, or are they the result of a sequence of accidental occurrences?